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Electrochemistry
Redox Equations and Stoichiometry
Dr Jorge L AlonsoMiami-Dade College ndash
Kendall CampusMiami FL
CHM 1045 General Chemistry and Qualitative Analysis
Textbook Reference bullChapter 11-4 to 11-6bullModule 7
Note that the 1st part of this lesson is a review of CHM 1045 Redox Rx New
Unit begins in slide 32
Electrochemistry
Redox Equations amp Oxidation Numbers
In order to keep track of what loses electrons and what gains them we assign oxidation numbers
In redox reactions electrons are transferred from one species to another
Electrochemistry
Oxidation and Reduction
bull A species is oxidized when it loses electronsHere zinc loses two electrons to go from neutral
zinc metal to the Zn2+ ion
Electrochemistry
Oxidation and Reduction
bull A species is reduced when it gains electronsHere each of the H+ gains an electron and they
combine to form H2
Electrochemistry
Oxidation and Reduction
bull What is reduced is the oxidizing agentH+ oxidizes Zn by taking electrons from it
bull What is oxidized is the reducing agentZn reduces H+ by giving it electrons
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Redox Equations amp Oxidation Numbers
In order to keep track of what loses electrons and what gains them we assign oxidation numbers
In redox reactions electrons are transferred from one species to another
Electrochemistry
Oxidation and Reduction
bull A species is oxidized when it loses electronsHere zinc loses two electrons to go from neutral
zinc metal to the Zn2+ ion
Electrochemistry
Oxidation and Reduction
bull A species is reduced when it gains electronsHere each of the H+ gains an electron and they
combine to form H2
Electrochemistry
Oxidation and Reduction
bull What is reduced is the oxidizing agentH+ oxidizes Zn by taking electrons from it
bull What is oxidized is the reducing agentZn reduces H+ by giving it electrons
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidation and Reduction
bull A species is oxidized when it loses electronsHere zinc loses two electrons to go from neutral
zinc metal to the Zn2+ ion
Electrochemistry
Oxidation and Reduction
bull A species is reduced when it gains electronsHere each of the H+ gains an electron and they
combine to form H2
Electrochemistry
Oxidation and Reduction
bull What is reduced is the oxidizing agentH+ oxidizes Zn by taking electrons from it
bull What is oxidized is the reducing agentZn reduces H+ by giving it electrons
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidation and Reduction
bull A species is reduced when it gains electronsHere each of the H+ gains an electron and they
combine to form H2
Electrochemistry
Oxidation and Reduction
bull What is reduced is the oxidizing agentH+ oxidizes Zn by taking electrons from it
bull What is oxidized is the reducing agentZn reduces H+ by giving it electrons
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidation and Reduction
bull What is reduced is the oxidizing agentH+ oxidizes Zn by taking electrons from it
bull What is oxidized is the reducing agentZn reduces H+ by giving it electrons
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Assigning Oxidation Numbers
1 Elements in their elemental form have an oxidation number of 0
2 The oxidation number of a monatomic ion is the same as its charge
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Assigning Oxidation Numbers
3 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Oxygen has an oxidation number of minus2 (except in the peroxide ion in which it has an oxidation number of minus1)
Hydrogen is minus1 when bonded to a metal (a metal hydride NaH) and +1 when bonded to a nonmetal H2O
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Assigning Oxidation Numbers
4 Nonmetals tend to have negative oxidation numbers although some are positive in certain compounds or ions
Fluorine always has an oxidation number of minus1
The other halogens have an oxidation number of minus1 when they are negative they can have positive oxidation numbers however most notably in oxyanions
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Assigning Oxidation Numbers
4 The sum of the oxidation numbers in a neutral compound is 0
NaHCO3
(+1)+ (+1)+ (x) + 3(-2) = 0
x = +4
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Assigning Oxidation Numbers
5 The sum of the oxidation numbers in a polyatomic ion is the charge on the ion
PO43-
(x) + 4(-2) = -3
x = +5
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Balancing Oxidation-Reduction Equations
The half-reaction method This involves treating (on paper only) the oxidation and reduction as two separate processes balancing these half reactions and then combining them to attain the balanced equation for the overall reaction
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
1 Assign oxidation numbers to determine what is oxidized and what is reduced
2 Write the oxidation and reduction half-reactions
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- Cr 3+
2(x)+7(-2)=-2 x = +6
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
3 Balance each half-reactiona Balance elements other than H and O
b Balance O by adding H2Oc Balance H by adding H+d Balance charge by adding electrons
In acidic medium
Oxidation Fe 2+ Fe 3+
Reduction Cr2O72- 2 Cr3+
14 H+ + 7 H2O
+ 1e-
6 e- +
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6+
Fe 2+ + Cr2O72- Fe 3+ + Cr 3+
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
4 Multiply the half-reactions by integers so that the electrons gained and lost are the same
In acidic medium
6(Fe 2+ Fe 3+ + 1e-)
Cr2O72- + 14 H+ + 6 e- 2Cr3+ + 7 H2O
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
6Fe 2+ 6 Fe 3+ + 6e-
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
5 Add the half-reactions subtracting things that appear on both sides
6 Make sure the equation is balanced according to mass
7 Make sure the equation is balanced according to charge
6Fe 2+ + Cr2O72- + 14H+ 6Fe 3+ + 2Cr 3+ + 7H20
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Half-Reaction Method
First we assign oxidation numbers
MnO4minus + C2O4
2- Mn2+ + CO2
+7 +3 +4+2
Since the manganese goes from +7 to +2 it is reduced
Since the carbon goes from +3 to +4 it is oxidized
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidation Half-Reaction
C2O42minus CO2
To balance the carbon we add a coefficient of 2
C2O42minus 2 CO2
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidation Half-Reaction
C2O42minus 2 CO2
The oxygen is now balanced as well To balance the charge we must add 2 electrons to the right side
C2O42minus 2 CO2 + 2 eminus
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+
The manganese is balanced to balance the oxygen we must add 4 waters to the right side
MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Reduction Half-Reaction
MnO4minus Mn2+ + 4 H2O
To balance the hydrogen we add 8 H+ to the left side
8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Reduction Half-Reaction
8 H+ + MnO4minus Mn2+ + 4 H2O
To balance the charge we add 5 eminus to the left side
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Combining the Half-Reactions
Now we evaluate the two half-reactions together
C2O42minus 2 CO2 + 2 eminus
5 eminus + 8 H+ + MnO4minus Mn2+ + 4 H2O
To attain the same number of electrons on each side we will multiply the first reaction by 5 and the second by 2
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Combining the Half-Reactions
5 C2O42minus 10 CO2 + 10 eminus
10 eminus + 16 H+ + 2 MnO4minus 2 Mn2+ + 8 H2O
When we add these together we get
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Combining the Half-Reactions
10 eminus + 16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2 +10 eminus
The only thing that appears on both sides are the electrons Subtracting them we are left with
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Balancing in Basic Solution
bull If a reaction occurs in basic solution one can balance it as if it occurred in acid
bull Once the equation is balanced add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull If this produces water on both sides you might have to subtract water from each side
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
Balancing in Basic Solution
8 H2O + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 16 OH- + 10 CO2
+16 OH- + 16 OH-
= 16 H2O
- 8 H2O - 8 H2O
bull add OHminus to each side to ldquoneutralizerdquo the H+ in the equation and create water in its place
bull subtract water from each side
bull balance it as if it occurred in acid
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Redox Stoichiometry
Consider the reaction between MnO4minus and C2O4
2minus
MnO4minus(aq) + C2O4
2minus(aq) Mn2+(aq) + CO2(aq)purple clear clear clear
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Indicators in Redox Titrations
(clear) (clear) (clear) (clear) (light green) (clear)
OH7 Cr2 Fe6 H14OCrFe6 2332
722
The indicator used diphenylamine sulfonate indicator (DPAS Ind) It has a reduced form which is colorless in solution and an oxidized form is purple in solution
Excess Cr2O72- + DPAS Ind (red) DPAS Ind (ox) + Cr+3
(clear) (clear) (purple) (clear)Fe2+
Titrant = Cr2O7-2
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus
2 Mn2+ + 8 H2O + 10 CO2At end-point of the
titration ηox = ηred
(MV)ox (MV)red
x ox x ox
X = coefficient of oxidizing agent in balanced redox equationX = coefficient of reducing agent in balanced redox equation
ηox = moles of oxidizing agentηred = moles of reducing agentVox = volume of oxidizing agentVred = volume of reducing agentM = molarityg = gramsMW = molecular (formula) weightL = liters
=
ox
red
Subs
Subs
red
red
ox
ox
Subs
Subs
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Redox StoichiometryBalanced Redox Equation
16 H+ + 2 MnO4minus + 5 C2O4
2minus 2 Mn2+ + 8 H2O + 10 CO2
How many grams of Na2C2O4 (FW= 134) are required to completely react with 403 mL of a 0300 M KMnO4 (FW = 158) solution under acidic conditions
How many mL of a 122 M Na2C2O4 (FW= 134) are required to completely react with 263 mL of a 308 M KMnO4 (FW = 158) solution under acidic conditions
mL) 340(LKMnO 0300 CNa mL 4
422
O
mL 1000
L1
422 OCNa 1
g 134
= 405g
At end-point of the titration (MV)ox = (MV)red
2
mL 263 L 308
5
V L122
mL 166
L 122 2
mL 632L 308 5 V
4
422
KMnO 2
OCNa 5
X ox X red
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Unit 21Electrochemistry
Dr Jorge L Alonso
Miami-Dade College ndash Kendall Campus
Miami FL
Textbook Reference bullChapter 21bullModule 11
CHM 1046 General Chemistry and Qualitative Analysis
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
bull Coulomb (C) a quantity of electricity = 624 x 1018 electrons (amount of e- in 1 amperesec)
bull Faraday (F ) = a MOLE of electrons = 602 x 1023 e- = 96500 x C
Basic Electricity
e- flow
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Metallic Bonding Electron-Sea Modelbull Metals can be thought of as cations suspended in
ldquoseardquo of valence electronsbull Attractions hold electrons near cations but not so
tightly as to impede their flow This explains properties of metalsmdash Conductivity
of heat and electricity
Deformation
Metallic Bonding Electron-Sea Model
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Basic Electricity
bull Ampere (I) measure of the flow rate (volume) of electricity measured in coulombs second I = Cs
bull Volts (V) measure of electrical force (Joules) per unit of electricity
(coulomb) V = JC = JC-1
bull Watt (W) measure of electrical power W = V x I
bull Insulators substances which resist the flow of electrons Resistance (R) is measured in ohms (Ω)
High flow rate High force
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Corrosion of Metals = Oxidation-Reduction Reactions
bull Corrosion the degradation of metals as a result of electrochemical activity requires an anode and a cathode in order to occur
bull Two things are needed for corrosion (1) electrical contact (connection)
between the two substances or metals with oxidation potential difference and (2) the presence of an electrolyte (such as water) to conduct ions between them
Fe0 + frac12O2 Fe2+ + O2-
AnodeAnode
CathodeCathode
Fe
O2H2O
Fe2+ + frac12 O2 Fe3+ + O2-
bull The anode is the substance with a higher potential to oxidize (lose electrons) while the cathode is the substance with a higher potential for reduction (gaining of electrons)
Anode
Cathode
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced the
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode
CathodeFe0 + O2
0 Fe2+ + O2- Fe3+ + O2-
Calculate the voltage
produced by reaction
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
Corrosion of Iron
Fe2+ + 2e- rarr Fe -044 Fe rarr Fe2+ + 2e- 044
Fe3+ + e- rarr Fe2+ +077 Fe2+ rarr Fe3+ + e- -077
Reduction Reaction Eo (V) Oxidation Reaction Eo (V)
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
(2) Further oxidation of Fe 2+ Fe 3+ yields
O2 + 4H+ + 4e- rarr 2H2O +123 2H2O rarr O2 + 4H+ + 4e- -123
(1) Initial oxidation of Fe Fe 2+ yields ERx = Ered (cathode) minus Ered (anode)
ERx
ERx
= (123) ndash (- 044) = 167
= (123) ndash (+ 077) = 046
213
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Corrosion andhellip
(Fe does not corrode when pH gt 9)
Fe 2+(aq) + O2(g) + 4H2O + 2xH2O(l) 2Fe2O3 xH2O(s) + 8H+
(aq)
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
What happens if you place a half Zn half Fe metal bar in the presence of water and oxygen
hellipCorrosion Prevention
ERx = Ered (cathode) minus Ered (anode)
= Ered (123) minus Ered (-044) = 167
= Ered (123) minus Ered (-076) = 199
Fe + O2 Rx ERx
Zn + O2 Rx ERx
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
hellipCorrosion Prevention
e- e-
2H2O
4H+
Reduction Potential
Reduction Potential
- 076 V
- 044 V
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
hellipCorrosion Prevention
Oxidation
Mg Mg 2+
e-
e-
e-e-
4H+ + O2
2H2O
Fe
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
hellipCorrosion Prevention
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Redox Reactions
In spontaneous oxidation-reduction (redox) reactions electrons are transferred and energy is released
Now perform the same reaction but separate the Zn(s) from Cu2+
(aq) by wire
Anode (oxidation)
Cathode (reduction)
Cu2+
Zn
Cu2+
Zn2+
Cu
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic (Galvanic) Cells
bull Electrical energy is generated from spontaneous redox reactions
bull We can use that energy to do work if we make the electrons flow through an external device (wire) ZnCu
Cu2+
Zn2+
cathode anode
Beaker 2 Zn metal with Zn(NO3)2 solution
Beaker 1 Cu metal with Cu(NO3)2 solution
Which is the Anode and the Cathode
E0Rx = (034) ndash (- 076) = 110
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The first Voltaic (Galvanic) Cell 1799
Luigi Galvani (1737-1798)
Alessandro Giuseppe Antonio Anastasio Volta
(1745ndash1827)
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Alkaline Batteries4+ 3+
Lead Batteries
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
undergoes oxidation producing cations in solution
Voltaic Cells
Causes reduction of the cations in solution into solid metal
Salt Bridge
Beaker 1 Zn metal anode with Zn(NO3)2 solution
Beaker 2 Cu metal cathode with Cu(NO3)2 solution
||
Voltaic-Galvanic Cells
LemmonCells
e-
e- e- e- e-
e-
e-
e-
e-
e-e-
e-
e-
ZnCu
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic Cells amp Salt BridgesOnce electrons flow in external wire from the anode to the cathode the charges in each beaker would not be balanced and the flow of electrons would stop
Therefore we use a salt bridge that contains a salt solution to keep electrical (ionic) charges balanced
Cations in salt bridge move toward the cathode
Anions move toward the anode
Current Balancing in Salt Bridge
Salt Bridge
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic Cell
Notation
Zn | Zn2+(1M) || Fe3+ (1M) Fe2+ (08M) | Pt
Anode Compartment Cathode Compartment
Salt BridgePhase separator
Separator of Different Species of same phase
Ered = +12
Ered = - 076 Ered = + 077
Zn2+(aq) + 2e- Zn(s)
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Standard Reduction Potentials (Ered)
EMF is the maximum potential difference between two electrodes of a electrochemical cell It is also called cell potential (Ecell)
This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Standard Conditions
bull25degC
bull1 atm (760 torr)
bull1 Molar solution
deg
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Standard Hydrogen Electrodebull Their values are referenced to a standard hydrogen
electrode (SHE)bull By definition the reduction potential for hydrogen is 0 V
2 H+ (aq 1M) + 2 eminus H2 (g 1 atm)
Voltaic ZnH Cell
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry and Free EnergyThe greater the difference between the two the greater the voltage of the cell
G for a redox reaction can be found by using the equation
Under standard conditions
G = minusF Ecell
where is the number of moles of electrons transferred in balanced redox equation and F is a constant the Faraday1 F = 96485 coulombs mole of electrons
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Cell Potential under Non-Standard Conditions The Nernst Equation
Dividing both sides by minusnF we get the Nernst equation
E = E minusRTnF ln Q
or using base-10 logarithms
E = E minus2303 RT
nF log Q
][
][Q
Reactants
Products
bull Remember that
G = G + RT ln Qbull This means
minusnF E = minusnF E + RT ln Q
G = minusnF E
Effect that non-standard conc (Q) have on EO
]Reactants[
]Products[
Some Values for Q
M1
M1
M1
M10
M10
M1
0log 101 0
-1log 1010 1
1log 1010 1
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Nernst Equation
At room temperature (298 K)
Thus the equation becomes
E = E minus00592
nlog Q
2303 RTF
= 00592 V
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Construct voltaic cell for reaction with greatest voltage
2003B Q6
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Al3+ Cu2+
CuAl
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2006B Q2
3 x ( )2 x ( )
n = 6 mol of e-
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
[108]+[008 ]
[] - []
From previous problem
PresentChange
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Anode (oxidation)
Cathode (reduction)
For Voltaic (Galvanic) Cells Spontaneous Reactions that produce electricity
Reactions are more spontaneous (more likely to occur) the further apart the chemicals are on this Table
G = minusnF E
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrolytic Cells Process by which electrical energy is used to force
non-spontaneous electrochemical reactions to occur Electrodes used are usually inert electrodes (they do
not participate in the reaction
Electrolysis of Water
Electrolysis Movie H2O H2 + frac12 O2
2 H2O + 2e- H2 + 2 OH-
H2O 2 H+ + frac12 O2 + 2 e-
Cathodic Rx
Anodic Rx
3 H2O H2 + frac12 O2 + 2 H+ + 2 OH-
Net Rx 2 H2O
frac12O2 + H2
H2O
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic vs Electrolytic Cells
Electrolytic cells
1 One container for the reaction mixture
2 Two electrodes immersed in the same reaction mixture
3 Requires a power supply of direct current
Types of electrolytic cells
1Molten salts2Solutions of salts (includes electroplating)
Voltaic (Galvanic) Cell Electrolytic Cell
vs
Spontaneous electricity produced
Non-spontaneous electricity required
1 NaCl(l)
2 NaCl(aq)
has water
which may
hydrolize
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Table of Standard Reduction Potentials
The substance with the more positive
reduction potential will become oxidized substances having
less positive reduction potential
become reduced
Electrolytic cells are forced to run in reverse from voltaic (galvanic) cells
Anode(oxidation)
Cathode (reduction)
For Electrolytic Cells Non-Spontaneous Reactions forced by electricity
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry2 NaCl (l) 2 Na(l ) + Cl2(g)
How do we get pure SodiumThe Electrolysis of Molten NaCl
elect
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Molten Potassium Chloride
(1) Draw cell(2) Write half equations for rx occurring at the anode amp cathode(3) Write a bal cell equation
bull In all electrolytic cells electrons are forced to flow from the positive electrode (anode) to the negative electrode (cathode)
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrolysis of Aqueous Salts
More possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
H2O H2 or frac12O2
Na+(aq) Cl -
(aq)
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Anion will oxidize instead of water
Will reduce when there is no water if it is melted
Will oxidize if anions are below
Will reduce if cations are below
Electrolysis of Aqueous Salts
Water oxidizing
Water reducing
Examples NaCl(aq) Fe(OH)2(aq) AgF(aq) K2SO4(aq)
Metals will reduce in water solutions
In all electrolytic cells the most easily oxidized species is oxidized and the most easily reduced species is reduced
Anode(oxidation)
Cathode (reduction)
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H O H2 Cl 2 reaction Cell
OH 2 H e 2 O H2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-
g2g22
-
-
g2
-
2
-
2(g)
-
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrolysis of Aqueous SaltsMore possibilities for electrolysis 1The ions of dissolved salts may electrolyze2Water may electrolyze (into O2 or H2)
Reduction at the cathode In an aqueous salt solution there are two possibilities Mn+ + n e- M
(all metal ions get reduced except IA+ IIA2+ or Al3+) 2 H2O + 2e- H2 + 2 OH-
(If metal is IA+ IIA2+ or Al3+ then water gets reduced)
Solution containing salts of Ni Fe amp Zn Which will become reduced
Will not reduce water will reduce
Will reduce in water solutions
Will reduced if ions are below
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrolysis of Aqueous SaltsOxidation at the anode In an aqueous salt solution there are two possibilities X- frac12 X2 + e-
(of the common anions only Cl- Br- or I- get oxidized) H2O 2 H+ + frac12 O2 + 2e-
(If anions is not Cl- Br- or I- then water gets oxidized)
bull In all electrolytic cells the most easily reduced species is reduced and the most easily oxidized species is oxidized
Solution containing salts of F Cl I Which will become oxidized
Will oxidize water will not
Will oxidize if ions are below
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
bull In this electrolytic cell hydrogen gas is produced at one electrode The aqueous solution becomes basic near this electrode What reaction is occurring at this electrode You do itYou do it
bull Gaseous chlorine is produced at the other electrode What reaction is occurring at this electrode You do itYou do it
bull These experimental facts lead us to the following nonspontaneous electrode reactions
edelectrolyz isr that wateNote ionspectator a is K
OH 2Cl H OH 2 Cl 2 reaction Cell
OH 2 H e 2 OH 2 reaction Cathode
e 2 Cl Cl 2 reaction Anode
-g2g22
-
-g2
-2
-2(g)
-
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Standard Reduction Potentials
The substance with the more positive
reduction potential will become reduced
substances having less positive
reduction potential become oxidized
Reduction potentials for
many electrodes have been
measured and tabulated
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Chloride
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2Cl- Cl2 (g) + 2e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous KCl
Cl2 gasH2 gas
+ pole of battery- pole of battery
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
bull In this electrolysis hydrogen gas is produced at one electrode The solution becomes basic near this electrode What reaction is occurring at this electrode
You do it
bull Gaseous oxygen is produced at the other electrode The solution becomes acidic near this electrode What reaction is occurring at this electrode
You do it
bull These experimental facts lead us to the following electrode reactions
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
O 2H OH 2 isreaction overall The
OH 4 4H O H 2 OH 6reaction Cell
)2OH H2e- OH 2(2reaction Cathode
e 4 H 4 O OH 2 reaction Anode
2(g)2(g)2
OH 4
-2(g)2(g)2
-2(g)2
-2(g)2
2
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
The Electrolysis of Aqueous Potassium Sulfate
2 H2O + 2e- H2 (g) + 2 OH-
cathode reaction2H2O O2 (g) + 4H+ + 4e-
anode reaction
Cell diagram Battery a source of direct currente- flow
- electrode + electrode
e- flow
aqueous K2SO4
O2 gasH2 gas
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
ElectPlating H2Cr2O7 rarr Cr0
Electroplating
(6+)
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law - The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell
bull A faraday is the amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode
-23 e 106022y electricit offaraday 1
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Stoichiometry and Electrolysis
bull How much of a chemical reaction can happen with a certain current over a certain amount of time
bull Current is measured in amps (Ampere A 1 C s)bull Determine the number of moles of electrons involved by using
the Faraday (1 F = 96500 x C = 602 x 1023 e-)bull Use coefficients of balanced equation to determine moles and
grams of substances involved
Cu2+(aq) + 2e- Cu(s)
Ag+(aq) + e- Ag(s)
Ampere = coulombs per second (Cs) Faraday (F ) = 96500 x C = 1 mole of e-
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Examplebull How long must a current of 500 A be applied to a solution
of Ag+ ions to produce 105 g of Ag metal
Ag+(aq) + e- Ag(s)
gAg868107
molAg1gAg510e moles -
e mol1
C48596e mol10739min 2
bull How many moles of e- are requiredbull How many minutes does it take to produce that many moles of e-
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
e mol10739molAg1
e mol1 2
min331sec60
min1
C005
sec1
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Examplebull How many grams of Cu will be plated out when 100 A of
current is passed for 300 min through a Cu2+ solution
sec1
C 010
e mol2
molCu1
Cu2+(aq) + 2e- Cu(s)
Ampere = Cs Faraday (F ) = 96500 x C = 1 mole of e-
min1
sec60min030Cu grams x
C 48596
e mol1
gCu945molCu1
g54663
Strategy(1)From amperage and minutes coulombs(2)From coulombs get moles of e-(3)From moles of e- get moles and then grams of Cu
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull A coulomb is the amount of charge that passes a given point when a current of one ampere (A) flows for one second
bull 1 amp = 1 coulombsecondcoulombs 48796e 106022faraday 1 -23
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Faradayrsquos Law states that during electrolysis one faraday of electricity (96487 coulombs) reduces and oxidizes respectively one equivalent of the oxidizing agent and the reducing agent This corresponds to the passage of one mole of electrons through
the electrolytic cell
-23
-23
e 106022 of lossagent reducing of equivalent 1
e 106022 ofgain agent oxidizing of equivalent 1
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-1 Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 320 amperes of current through a solution of palladium (II) sulfate for 300 minutes
g 106 2(96500) g 106
mol 1 mol 2 mol 1
Pd 2e +Pd Cathode 0-+2
Cathode Pd + 2e Pd
1 mol 2 mol 1 mol
106 g 2(96500) 106 g
320 amp = 320
g = 300 min60 smin
Cs
g Pd2 96500 C
g Pd
2+ - 0
Cs
3 20 106
316
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Counting Electrons Coulometry and Faradayrsquos Law of Electrolysis
bull Example 21-2 Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in example 21-1
C 965004 L 224
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
STP
-+g22
2STP2STP
2STP2STP
STP
-+g22
O mL 334or O L 3340
C 965004
O L 422
s
C 203
min
s 60min 300 O L
C 965004 224L
mol 4 mol 4 mol 1 mol 2
4e+ 4H + O OH 2 Anode
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Commercial Applications of Electrolytic Cells
Electrolytic Refining and Electroplating of Metals
bull Impure metallic copper can be purified electrolytically to 100 pure Cu The impurities commonly include some active metals
plus less active metals such as Ag Au and Pt
bull The cathode is a thin sheet of copper metal connected to the negative terminal of a direct current source
bull The anode is large impure bars of copper
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2007A Q3
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Hydrogen Fuel Cells
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2007B Q3
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Applications of Oxidation-Reduction
Reactions
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Batteries
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Alkaline Batteries
4+ 3+
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
pH Meters
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic Cellsbull In the cell then electrons leave the anode and flow through the wire to the
cathode
bull As the electrons leave the anode the cations formed dissolve into the solution in the anode compartment
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Voltaic Cellsbull As the electrons reach the cathode cations in the cathode are attracted to
the now negative cathodebull The electrons are taken by the cation and the neutral metal is deposited
on the cathode
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electromotive Force (emf)bull Water only spontaneously
flows one way in a waterfall
bull Likewise electrons only spontaneously flow one way in a redox reactionmdashfrom higher to lower potential energy
EMF is the maximum potential difference between two electrodes of a electrochemical cell This quantity is related to the tendency for a substance to acquire (ie gain) or release (loss) electrons
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electromotive Force (emf)
bull It is the electrical potential difference (voltage) between the anode and cathode in an electrochemical cell
bull It is also called the cell potential and is designated Ecell Cell potential is measured in Joules of energy per Coulomb of charge = volt (V)
1 V = 1 JC
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Oxidizing and Reducing Agents
bull The strongest oxidizers have the most positive reduction potentials
bull The strongest reducers have the most negative reduction potentials
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Corrosion andhellip
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2004A Q6
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2005 B electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2001
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2002
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2002 B
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2003 A
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2003 B
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2004 A
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2004 B
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
2005 A
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Anode oxidation where Anions (A-) go to loose electrons
Cathode reduction where Cations (Ca+) go to gain electrons
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
Electrochemistry
Concentration Cells
bull The Nernst equation implies that a cell could be created that has the same substance at both electrodes
bull For such a cell would be 0 but Q would notEcell
bull Therefore as long as the concentrations are different E will not be 0
E = E minus2303 RT
nF log Q
