2 Electrochemistry Terminology #1 Oxidation – A process in which an element attains a more positive oxidation stateNa(s) Na+ + e-Reduction – A process in which an element attains a more negative oxidation stateCl2 + 2e- 2Cl-
3 Electrochemistry Terminology #2 An old memory device for oxidation and reduction goes like this…LEO says GERLose Electrons = OxidationGain Electrons = Reduction
4 Electrochemistry Terminology #3 Oxidizing agentThe substance that is reduced is the oxidizing agentReducing agentThe substance that is oxidized is the reducing agent
5 Electrochemistry Terminology #4 AnodeThe electrode where oxidation occursCathodeThe electrode where reduction occursMemory device:Reductionat theCathode
6 Table of Reduction Potentials Measured against the Standard Hydrogen Electrode
7 Measuring Standard Electrode Potential Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts.
9 Zn - Cu Galvanic Cell From a table of reduction potentials: Zn2+ + 2e- Zn E = -0.76VCu2+ + 2e- Cu E = +0.34V
10 Zn - Cu Galvanic Cell Zn Zn2+ + 2e- E = +0.76V The less positive, or more negative reduction potential becomes the oxidation…Cu2+ + 2e- Cu E = +0.34VZn Zn2+ + 2e E = +0.76VZn + Cu2+ Zn2+ + Cu E0 = V
11 Line Notation Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) | || | An abbreviated representation of an electrochemical cellZn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)AnodematerialAnodesolutionCathodesolutionCathodematerial||||
12 Calculating G0 for a Cell G0 = -nFE0n = moles of electrons in balanced redox equationF = Faraday constant = 96,485 coulombs/mol e-Zn + Cu2+ Zn2+ + Cu E0 = V
13 The Nernst Equation R = 8.31 J/(molK) T = Temperature in K Standard potentials assume a concentration of 1 M. The Nernst equation allows us to calculate potential when the two cells are not 1.0 M.R = 8.31 J/(molK)T = Temperature in Kn = moles of electrons in balanced redox equationF = Faraday constant = 96,485 coulombs/mol e-
14 Nernst Equation Simplified At 25 C (298 K) the Nernst Equation is simplified this way:
15 Equilibrium Constants and Cell Potential At equilibrium, forward and reverse reactions occur at equal rates, therefore:The battery is “dead”The cell potential, E, is zero voltsModifying the Nernst Equation (at 25 C):
16 Calculating an Equilibrium Constant from a Cell Potential Zn + Cu2+ Zn2+ + Cu E0 = V
17 Both sides have the same components but at different concentrations. ???Concentration CellBoth sides have the same components but at different concentrations.Step 1: Determine which side undergoes oxidation, and which side undergoes reduction.
18 Both sides have the same components but at different concentrations. ???Concentration CellBoth sides have the same components but at different concentrations.AnodeCathodeThe 1.0 M Zn2+ must decrease in concentration, and the 0.10 M Zn2+ must increase in concentrationZn2+ (1.0M) + 2e- Zn (reduction)Zn Zn2+ (0.10M) + 2e- (oxidation)Zn2+ (1.0M) Zn2+ (0.10M)
19 Both sides have the same components but at different concentrations. Concentration Cell???Concentration CellBoth sides have the same components but at different concentrations.AnodeCathodeStep 2: Calculate cell potential using the NernstEquation (assuming 25 C).Zn2+ (1.0M) Zn2+ (0.10M)
21 Electrolytic Processes Electrolytic processes are NOT spontaneous. They have:A negativecell potential, (-E0)A positive free energy change, (+G)
22 Electrolysis of Water In acidic solution Anode rxn: Cathode rxn: -1.23 VCathode rxn:-0.83 V-2.06 V
23 Electroplating of Silver Anode reaction:Ag Ag+ + e-Cathode reaction:Ag+ + e- AgElectroplating requirements:1. Solution of the plating metal2. Anode made of the plating metal3. Cathode with the object to be plated4. Source of current
24 Solving an Electroplating Problem Q: How many seconds will it take to plate out 5.0 grams of silver from a solution of AgNO3 using a 20.0 Ampere current?Ag+ + e- Ag5.0 g1 mol Ag1 mol e-C1 s1 mol e-20.0 Cg1 mol Ag= 2.2 x 102 s
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