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17.1/17.2 GALVANIC CELLS AND 17.1/17.2 GALVANIC CELLS AND STANDARD REDUCTION POTENTIALSSTANDARD REDUCTION POTENTIALS
Day 1
Electrochemistry Terminology Electrochemistry Terminology #1#1
OxidationOxidation – A process in which an element attains a more positive oxidation state
Na(s) Na+ + e-
ReductionReduction – A process in which an element attains a more negative oxidation state
Cl2 + 2e- 2Cl-
Electrochemistry Terminology Electrochemistry Terminology #2#2
GGainain EElectronslectrons == RReductioneduction
An old memory device for An old memory device for oxidation and reduction goes oxidation and reduction goes like this…like this…
LEOLEO sayssays GERGERLEOLEO sayssays GERGER
LLoseose EElectronslectrons = = OOxidationxidation
Electrochemistry Terminology Electrochemistry Terminology #3#3
Oxidizing agentOxidizing agentThe substance that is reduced
is the oxidizing agent Reducing agentReducing agent
The substance that is oxidized is the reducing agent
Electrochemistry Terminology Electrochemistry Terminology #4#4
Anode Anode The electrode
where oxidation occurs
CathodeCathodeThe electrode
where reduction occurs
Memory Memory device:device:
RedReductionuctionat the at the
CatCathodehode
Table of Table of Reduction Reduction PotentialsPotentials
Measured Measured against against the the StandardStandardHydrogenHydrogenElectrodeElectrode
Always written as REDUCTION potentials
Measuring Measuring Standard Standard Electrode Electrode PotentialPotential
Potentials are measured against a Potentials are measured against a hydrogen ion reduction reaction, which hydrogen ion reduction reaction, which is arbitrarily assigned a potential of is arbitrarily assigned a potential of zero zero voltsvolts..
Relative Strength of Oxidizing Relative Strength of Oxidizing Reducing AgentsReducing Agents
1. The more + the reduction The more + the reduction potential, the stronger the potential, the stronger the Oxidizing Agent, (easier to Oxidizing Agent, (easier to reduce)reduce)
– Compare the reduction potential of Compare the reduction potential of each substance each substance
– Whichever has the more positive Whichever has the more positive reduction potential is the better reduction potential is the better oxidizing agentoxidizing agent
Galvanic (Electrochemical) Galvanic (Electrochemical) CellsCells
Spontaneous redox processes have:
• A positive cell potential, E0
cell• A negative free energy change,
(G is negative)
CalculatioCalculation of Eºn of Eºcellcell
Zn2+ + 2e- Zn Eº = -0.76VCu2+ + 2e- Cu Eº = +0.34V
From a From a table of table of reduction reduction potentials:potentials:
Calculation of EºCalculation of Eºcellcell
EºEºcellcell = Eº = Eºredred+ Eº+ Eºoxidoxid
• Both values are written as reduction potentials in the table
• Determine what is oxidized• Flip the equation• Flip the sign• NeverNever multiply the potential by multiply the potential by
and integer, it is an and integer, it is an intensive intensive propertyproperty
Zn - Cu Zn - Cu Galvanic Galvanic
CellCell
Cu2+ + 2e- Cu Eº (red) = +0.34V
The less positive, The less positive, or more negative or more negative reduction reduction potential potential becomes the becomes the oxidation.oxidation.
Zn Zn2+ + 2e- Eº(oxid) = +0.76V
Cu2 + + Zn + Zn2+ + Cu Eºcell = +
1.10 V
Line Line NotationNotation
Zn(Zn(ss) | Zn) | Zn2+2+((aqaq) || Cu) || Cu2+2+((aqaq) | Cu() | Cu(ss))
An abbreviated An abbreviated representation representation of an of an electrochemical electrochemical cellcell
AnodeAnodesolutionsolution
AnodeAnodematerialmaterial
CathodeCathodesolutionsolution
CathodeCathodematerialmaterial|| ||||||
Complete description of a Complete description of a galvanic cellgalvanic cell
1. The cell potential Eºcell2. Drawing of the cell* 3. Direction of electron flow
a. Designation of anode and cathodeb. Composition of the electrode (Pt
or Graphite if no metals are in the half reactions)
c. All ions in the solution from the half reactions
*line notation is used if indicated
17.3 CELL POTENTIAL, 17.3 CELL POTENTIAL, ELECTRICAL WORK AND FREE ELECTRICAL WORK AND FREE ENERGYENERGY
Day 2
Cell Potential and WorkCell Potential and Work
CJ
VEcell charge
systemthebydonework max
Galvanic Cell: Ecell >0 and *work <0
qw
E maxcell
*The amount of work a cell can perform (by the system on the surroundings)
cellqEw max
Charge and Chemical Charge and Chemical ReactionsReactions
Charge (q): number of coulombs transferred during a redox reaction
nFq
n=number of moles of electrons transferredF= Faraday Constant (96485 C/mole e-)
Charge and Chemical Charge and Chemical ReactionsReactions
Combine
nFq
cellnFEw max
Need balanced reaction and cell potential
cellmax qEw
Calculating Calculating GG00 for a Cell for a Cell
GG00 = -nFE = -nFE00
nn = moles of electrons in balanced redox equation= moles of electrons in balanced redox equation
FF = = Faraday constant = 96,485 coulombs/mol eFaraday constant = 96,485 coulombs/mol e--
Zn + Cu2+ Zn2+ + Cu EE00 = + 1.10 V
17.4 DEPENDENCE OF CELL 17.4 DEPENDENCE OF CELL POTENTIAL ON CONCENTRATIONPOTENTIAL ON CONCENTRATION
Day 3
The Nernst EquationThe Nernst EquationStandard potentials assume a Standard potentials assume a concentration of 1 M. The Nernst concentration of 1 M. The Nernst equation allows us to calculate potential equation allows us to calculate potential when the two cells are not 1.0 M.when the two cells are not 1.0 M.
RR = 8.31 J/(mol= 8.31 J/(molK) K)
TT = Temperature in K = Temperature in K
nn = moles of electrons in balanced redox equation = moles of electrons in balanced redox equation
FF = Faraday constant = 96,485 coulombs/mol e = Faraday constant = 96,485 coulombs/mol e--
Nernst Equation SimplifiedNernst Equation Simplified
At 25 At 25 C (298 K) the Nernst Equation C (298 K) the Nernst Equation is simplified this way:is simplified this way:
Equilibrium Constants and Cell Equilibrium Constants and Cell PotentialPotential
At equilibriumequilibrium, forward and reverse reactions occur at equal rates, therefore:1.1. The battery is “dead”
2. The cell potential, E, is zero volts
Modifying the Nernst Equation (at 25 C):
Zn + Cu2+ Zn2+ + Cu EE00 = + 1.10 V
Calculating an Equilibrium Calculating an Equilibrium Constant from a Cell PotentialConstant from a Cell Potential
ConcentratioConcentration Celln Cell
Step 1: Determine which side undergoes oxidation, and which side undergoes reduction.
Both sides have the
same components
but at different
concentrations.
??????
ConcentratioConcentration Celln Cell
Both sides have the
same components
but at different
concentrations.
The 1.0 M Zn2+ must decrease in concentration, and the 0.10 M Zn2+ must increase in concentration
Zn2+ (1.0M) + 2e- Zn (reduction) Zn Zn2+ (0.10M) + 2e-
(oxidation)
??????
CathodeCathodeAnodeAnode
Zn2+ (1.0M) Zn2+
(0.10M)
Concentration CellConcentration Cell
Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C).
Both sides have the same
components but at different
concentrations.
??????
CathodeCathodeAnodeAnode
Zn2+ (1.0M) Zn2+
(0.10M)
ConcentratioConcentration Celln Cell
ElectrolytElectrolytic ic
ProcesseProcessess
A negative cell potential, (--EE00)
A positive free energy change, (++GG)
Electrolytic processes are NOTNOT spontaneous. They have:
Electrolysis of Electrolysis of WaterWater
In acidic solutionIn acidic solution
Anode rxn:Anode rxn:
Cathode rxn:Cathode rxn:-1.23 V-1.23 V
-0.83 V-0.83 V
-2.06 V-2.06 V
Electroplating Electroplating of Silverof Silver
Anode reactionAnode reaction::
Ag Ag Ag Ag++ + e + e--
Electroplating requirementsElectroplating requirements::1. Solution of the plating metal1. Solution of the plating metal
3. Cathode with the object to be plated3. Cathode with the object to be plated2. Anode made of the plating metal2. Anode made of the plating metal
4. Source of current4. Source of current
Cathode reactionCathode reaction::
AgAg++ + e + e-- Ag Ag
Solving an Electroplating Solving an Electroplating ProblemProblem
Q: How many seconds will it take to Q: How many seconds will it take to plate out 5.0 grams of silver from a plate out 5.0 grams of silver from a solution of AgNOsolution of AgNO33 using a 20.0 Ampere using a 20.0 Ampere current?current?
5.0 g
AgAg++ + e + e- - Ag Ag
1 mol Ag
107.87 g
1 mol e-
1 mol Ag
96 485 C
1 mol e-
1 s
20.0 C
= 2.2 x 10= 2.2 x 1022 s s
Uses of electrolysisUses of electrolysis
• Electrolysis of water– Produces H2 and O2 gas – 2H2O H2 + O2
•Production of aluminium, lithium, sodium, potassium, magnesium, calcium
•Production of chlorine and sodium hydroxide
Other usesOther uses
• Seperating mixtures of ions. – More positive reduction potential
means the reaction proceeds forward. – Ion with highest reduction potential
will plate out first or at the lowest voltage.