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Nernst equation

UKChO Round 2 · IChO · unreviewed

E = E° − (RT/nF) ln Q.

Practise this with fresh numbers

Worked example

A cell has E° = 1.07 V with n = 3. Find E at 298 K when Q = 0.10.

1. Nernst equation: solve for E
$E = E^\circ - \dfrac{RT}{nF}\ln Q$
= 1.090 V

Answer: E = 1.09 V

Method

E = E° − (RT/nF) ln Q. At 298 K that is E° − (0.0592/n) log₁₀ Q.

Common mistakes

n seems to be missing: the correction is (RT/nF) ln Q.
Sign slip: E = E° − (RT/nF) ln Q, so a larger Q lowers E.

Formulas used

Nernst equation
$E = E^\circ - \dfrac{RT}{nF}\ln Q$

Practise this with fresh numbers

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