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Electrochemistry and Redox
Oxidation-reduction:Oxidation-reduction: “Redox”“Redox”
Electrochemistry:Electrochemistry:
study of the interchange between chemical study of the interchange between chemical change and electrical workchange and electrical work
Electrochemical cells:Electrochemical cells:
systems utilizing a redox reaction to produce systems utilizing a redox reaction to produce or use electrical energyor use electrical energy
Redox Review
Redox reactions:Redox reactions: electron transfer processeselectron transfer processes
Oxidation:Oxidation: loss of 1 or more eloss of 1 or more e--
Reduction:Reduction: gain of 1 or more egain of 1 or more e--
Oxidation numbers:Oxidation numbers: imaginary chargesimaginary charges
(Balancing redox reactions)(Balancing redox reactions)
Oxidation Numbers (O.N.)
1.1. Pure elementPure element O.N. is zeroO.N. is zero
2.2. Monatomic ionMonatomic ion O.N.O.N. is chargeis charge
3.3. Neutral compoundNeutral compound:: sum of O.N. is zerosum of O.N. is zero
Polyatomic ionPolyatomic ion:: sum of O.N. is ion’s chargesum of O.N. is ion’s charge
*Negative O.N. generally assigned to more *Negative O.N. generally assigned to more electronegative elementelectronegative element
Oxidation Numbers (O.N.)
4.4. HydrogenHydrogen
assigned +1assigned +1
(metal hydrides, -1)(metal hydrides, -1)
5.5. OxygenOxygen
assigned -2assigned -2
(peroxides, -1; OF(peroxides, -1; OF22, +2), +2)
6.6. FluorineFluorine
always -1always -1
Oxidation-reduction
OxidationOxidation is loss of e is loss of e--
O.N. increases (more positive)O.N. increases (more positive)
ReductionReduction is gain of e is gain of e--
O.N. decreases (more negative)O.N. decreases (more negative)
Oxidation involves lossOxidation involves loss OILOIL
Reduction involves gainReduction involves gain RIGRIG
Redox
OxidationOxidation is loss of e is loss of e--
causes reductioncauses reduction
““reducing agentreducing agent””
ReductionReduction is gain of e is gain of e--
causes oxidationcauses oxidation
““oxidizing agentoxidizing agent””
Balancing Redox Reactions
1.1. Write separate equations (Write separate equations (half-reactionshalf-reactions) ) forfor oxidation and reductionoxidation and reduction
2.2. For For eacheach half-reaction half-reaction
a. a. Balance elements involved in eBalance elements involved in e-- transfer transfer
b.b. Balance number eBalance number e-- lost and gained lost and gained
3.3. To To balance ebalance e--
multiply each half-reaction by whole numbersmultiply each half-reaction by whole numbers
Balancing Redox Reactions: Acidic
4.4. AddAdd half-reactions/cancel like terms (e half-reactions/cancel like terms (e --))
5. 5. Acidic conditions:Acidic conditions:
Balance oxygen using HBalance oxygen using H22OO
Balance hydrogen using HBalance hydrogen using H++
Basic conditions:Basic conditions:
Balance oxygen using OHBalance oxygen using OH--
Balance hydrogen using HBalance hydrogen using H22OO
6.6. CheckCheck that all atoms and charges balance that all atoms and charges balance
Examples
Acidic conditions:Acidic conditions:
Basic conditions:Basic conditions:
3(aq)
2(aq)
2(aq)
-4(aq) FeMn FeMnO acid
2(aq)2(g)(aq)(s) Ag(CN)OCN Ag base
Types of cells
Voltaic (galvanic) cells:Voltaic (galvanic) cells:
a spontaneous reaction generates electrical energya spontaneous reaction generates electrical energy
Electrolytic cells:Electrolytic cells:
absorb free energy from an electrical source to absorb free energy from an electrical source to drive a nonspontaneous reactiondrive a nonspontaneous reaction
Common Components
Electrodes:Electrodes:conduct electricity between cell and conduct electricity between cell and
surroundingssurroundings
Electrolyte:Electrolyte:mixture of ions involved in reaction or mixture of ions involved in reaction or carrying chargecarrying charge
Salt bridge:Salt bridge:completes circuit (provides charge balance)completes circuit (provides charge balance)
ElectrodesAnode:Anode:
Oxidation occurs at the anodeOxidation occurs at the anode
Cathode:Cathode:
Reduction occurs at the cathodeReduction occurs at the cathode
Active electrodes:Active electrodes: participate in redoxparticipate in redox
Inactive:Inactive: sites of ox. and red.sites of ox. and red.
Voltaic (Galvanic) Cells
A device in which chemical energy A device in which chemical energy is changed to electrical energy.is changed to electrical energy.
Uses a spontaneous reaction.Uses a spontaneous reaction.
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Porous disk
Reducingagent
Oxidizingagent
e –
e –
e – e –
e –
e –
CathodeAnode (b)(a)
Oxidation Reduction
Zn2+(aq) + Cu(s) Cu2+
(aq) + Zn(s)
Zn Zn gives up electrons to Cugives up electrons to Cu— ““pushes harder” on epushes harder” on e--
— greater potential energygreater potential energy— greater “electrical potential”greater “electrical potential”
Spontaneous reactionSpontaneous reaction due to due to — relative difference in metals’ abilities to give erelative difference in metals’ abilities to give e --
— ability of eability of e-- to flow to flow
Cell Potential
Cell PotentialCell Potential / / Electromotive ForceElectromotive Force (EMF): (EMF):
The “pull” or driving force on electronsThe “pull” or driving force on electrons
Measured voltage (potential difference)Measured voltage (potential difference)
VC
J
moved charge ofunit
energy potential electricalor work Ecell
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e–
e– e–
e–
Zn 2+
SO4 2–
Zn(s)
1.0 M Zn 2+
solution
Anode
1.0 M Cu 2+
solution
Cathode
Cu 2+
SO4 2–
Cu(s)
Ecell = +1.10 V
Cell Potential, E0cell
EE00cellcell
cell potential under standard conditionscell potential under standard conditions
elements in standard states (298 K)elements in standard states (298 K)
solutions:solutions: 1 M1 M
gases:gases: 1 atm1 atm
Standard Reduction Potentials
EE values for reduction half-reactions with values for reduction half-reactions with solutes at 1M and gases at 1 atmsolutes at 1M and gases at 1 atm
CuCu2+2+ + 2e + 2e Cu Cu
EE = 0.34 V vs. SHE = 0.34 V vs. SHE
SOSO4422 + 4H + 4H++ + 2e + 2e H H22SOSO33 + H + H22OO
EE = 0.20 V vs. SHE = 0.20 V vs. SHE
E0cell and G0
EE00cellcell > 0 > 0 GG00 < 0 < 0 SpontaneousSpontaneous
EE00cellcell < 0 < 0 GG00 > 0 > 0 NotNot
EE00cellcell = 0 = 0 GG00 = 0 = 0 EquilibriumEquilibrium
Calculating E0cell
EE00cellcell = E = E00
cathodecathode - E - E00anodeanode
BrBr2(aq)2(aq)+2V+2V3+3+ +2H +2H22OO(l)(l) 2VO 2VO2+2+(aq)(aq)+ 4H+ 4H++
(aq)(aq)+ 2Br+ 2Br--(aq)(aq)
Given:Given: EE00cellcell = +1.39 V = +1.39 V
EE00Br2Br2 = +1.07 V = +1.07 V
What isWhat is EE00V3+V3+ and is the reaction spontaneous? and is the reaction spontaneous?
E0 values
More positive:More positive:
Stronger oxidizing agentStronger oxidizing agent
More readily accepts eMore readily accepts e--
More negative:More negative:
Stronger reducing agentStronger reducing agent
More readily gives eMore readily gives e--
Stronger R.A. + O.A. Stronger R.A. + O.A. Weaker R.A. + O.A. Weaker R.A. + O.A.
Free Energy and Cell Potential
n:n: number of moles of enumber of moles of e--
F:F: Faraday’s constantFaraday’s constant
96485 C 96485 C
mol of emol of e--
00max nFEG w
G0, E0, and K
At equilibrium:At equilibrium: GG00 = 0 and K = Q = 0 and K = Q
At 298 K:At 298 K:
00 nFERTlnKG
lnKnF
RTE0 so
logKn
0.0592E0
Nernst Equation
Under nonstandard conditionsUnder nonstandard conditions
RTlnQnFEnFE
RTlnQGG0
0
lnQnF
RTEE 0
cell
lnQn
0.0592EE 0298K
cell
Concentration Cells
. . .. . . a cell in which both a cell in which both compartments have the compartments have the same same componentscomponents but at but at different different concentrationsconcentrations
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Reference solution ofdilute hydrochloric acid
Silver wire coated withsilver chloride
Thin-walled membrane
Batteries
A A batterybattery is a galvanic cell or, more is a galvanic cell or, more commonly, a commonly, a groupgroup of galvanic of galvanic cells connected in series.cells connected in series.
Fuel Cells
Galvanic cells Galvanic cells
Reactants are continuously supplied.Reactants are continuously supplied.
2H2H2(2(gg)) + O + O2(2(gg)) 2H 2H22OO((ll))
anodeanode: : 2H2H22 + 4OH + 4OH 4H 4H22O + 4eO + 4e
cathodecathode: : 4e4e + O + O2 2 + 2H + 2H22O O 4OH 4OH
Corrosion
Some metals, such as copper, gold, silver and Some metals, such as copper, gold, silver and platinum, are relatively difficult to oxidize.platinum, are relatively difficult to oxidize.
These are often called These are often called noble metalsnoble metals..
Electrolysis
ForcingForcing a current through a cell to a current through a cell to produce a chemical change for produce a chemical change for which the cell potential is negative.which the cell potential is negative.
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Moltenaluminum
Carbondioxideformed atthe anodes
Carbon-lined iron tank
Plug
Molten Al2O3/Na3AlF6mixture
Electrodes ofgraphite rods
To externalpower source
Stoichiometry
How much How much chemical change chemical change occurs with the flow occurs with the flow of a given current for a specified time?of a given current for a specified time?
current and time current and time quantity of charge quantity of charge moles of electrons moles of electrons moles of analyte moles of analyte grams of analytegrams of analyte
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