NERNST EQUATION Chapter 20 To play the movies and simulations included, view the presentation in Slide Show Mode.

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. Electrical potential is usually measured in volts (V). Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Electrical Potential The standard cell potential (E0cell) 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. E0cell = E0red – E0oxid Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

E0cell = E0red – E0oxid 2.00 V = 0.34 V – (-1.66 V) Sample Problem 21.1 Cu2+(aq) + 2e– →Cu(s) E0Cu2+ = 0.34 V Al3+(aq) + 3e– → Al(s) E0Al3+ = -1.66 V E0cell = E0red – E0oxid 2.00 V = 0.34 V – (-1.66 V) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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

AP SHEET

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˚

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

X X Q = [ Zn2+] [Ag+]2 X

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AP Question

Is E˚ related to ∆G? YES!

Eo and ∆Go Eo is related to ∆Go, the free energy change for the reaction. ∆Go = - n F Eo where F = Faraday constant = 9.6485 x 104 J/V•mol and n is the number of moles of electrons transferred Michael Faraday 1791-1867

∆Go = - n F Eo For a product-favored reaction Reactants ----> Products ∆Go < 0 and so Eo > 0 Eo is positive For a reactant-favored reaction Reactants <---- Products ∆Go > 0 and so Eo < 0 Eo is negative

Eo = - 0.20 v ∆Go = - n F Eo

HW

Is E˚ related to K? YES!

AP SHEET

R= 8.31 J/(mol K)