Walther Nernst

Nernst Equation

This equation is named after the German physical chemist who first formulated it,Walther Nernst.

The Nernst equation is an equation that relates the equilibrium reduction potential of a half-cell in an electrochemical cell to the standard electrode potential, temperature, activity, and reaction quotient of the underlying reactions and species used.

Nernst Equation

Problem

Consider a galvanic cell that uses the reaction:

Cu(s) + 2Fe3+(aq) → Cu2+(aq) + 2Fe2+(aq)

What is the potential of a cell at 25 °C that has the following ion concentrations?

[Fe3+] = 1.0 x 10-4 M [Cu2+] = 0.25 M [Fe2+] = 0.20 M

Solution

Reaction quotient is given by:

Cell is not under standard conditions, so the Nernst Equation must be used:

Solution

Eocell = Eocathode – Eoanode

Eocell = 0.77 V – 0.34 V = 0.43 V

n = 2 since 2 electrons are transferred

Problem

Calculate the equilibrium constant of the reaction:

Cu(s) + 2Ag+(aq) → Cu2+ (aq) + 2Ag(s)

Eo cell = 0.46 V

Solution

Problem

The standard electrode potential for Daniell cellis 1.1V. Calculate the standard Gibbs energy for the reaction:

Zn(s) + Cu2+(aq) → Zn⎯ 2+(aq) + Cu(s)

Solution

n =2, F = 96487 C mol-1, Eocell = 1.1 V

ΔrGo = – nFEocell