1 NERNST EQUATION Chapter 20

21.2 Half-Cells and Cell Potentials > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential What causes the electrical potential of an electrochemical cell? The electrical potential of a voltaic cell is a measure of the cell’s ability to produce an electric current.The electrical potential of a voltaic cell is a measure of the cell’s ability to produce an electric current. Electrical potential is usually measured in volts (V).Electrical potential is usually measured in volts (V).

21.2 Half-Cells and Cell Potentials > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Electrical Potential The standard cell potential (E 0 cell ) is the measured cell potential when the ion concentrations in the half-cells are 1M, any gases are at a pressure of 101 kPa, and the temperature is 25°C. E 0 cell = E 0 red – E 0 oxid

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5 RED = REDUCED BLACK = OXIDIZED e- Cu Al

21.2 Half-Cells and Cell Potentials > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 21.1 Cu 2+ (aq) + 2e – →Cu(s) E 0 Cu 2+ = 0.34 V Al 3+ (aq) + 3e – → Al(s) E 0 Al 3+ = V E 0 cell = E 0 red – E 0 oxid 2.00 V = 0.34 V – (-1.66 V)

7 If Al and Cu are separated work is done by the electrons. Voltmeter is used to measure the energy. What determines the measured value?

8 AP SHEET

9 NERNST EQUATION E at Nonstandard Conditions E = potential under nonstandard conditions n = no. of electrons exchanged ln = “natural log” If [P] and [R] = 1 mol/L, then E = E˚

10 NERNST EQUATION E at Nonstandard Conditions If [R] (bottom) > [P], Q <1, ln is neg then E is ______________ than E˚ greater less If [R] (bottom) 1, ln is pos then E is ______________ than E˚

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13 AP Question

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15 Q = [ Zn 2+ ] [Ag + ] 2 XX X

16 Is E˚ related to ∆G? YES!

17 E o and ∆G o E o is related to ∆G o, the free energy change for the reaction. ∆G o = - n F E o where F = Faraday constant = x 10 4 J/Vmol and n is the number of moles of electrons transferred Michael Faraday

18 ∆G o = - n F E o For a product-favored reaction Reactants ----> Products ∆G o 0 E o is positive For a reactant-favored reaction Reactants <---- Products ∆G o > 0 and so E o < 0 E o is negative

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20 ∆G o = - n F E o E o = v

21 HW

22 Is E˚ related to K? YES!

23 AP SHEET

24 R= 8.31 J/(mol K)

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