# Nernst equation

The Nernst equation gives the cell potential when concentrations are not standard: E = E° − (RT/nF) ln Q. At 25 °C, E = E° − (0.0592/n) log Q. As the cell runs, Q grows, E falls, and at equilibrium E = 0.

Chemistry (university) · Part 08: Acids, bases and electrochemistry · Card 8 of 9 · Last updated 2026-10-19

## Analogy

A battery going flat: as products pile up, the push gets weaker until it's gone.

## Example

**Zn/Cu, [Zn²⁺] = 1.0 M, [Cu²⁺] = 0.010 M**

Q = 1.0 / 0.010 = 100, n = 2
E = 1.10 − (0.0592/2) × log 100
E = 1.10 − 0.0296 × 2
E = 1.04 V

## In practice

A pH meter is a cell whose voltage changes with [H⁺] according to the Nernst equation.

## Practice

### Flashcards

- **What does the Nernst equation correct for?** Non-standard concentrations.

## Questions people ask

### Why does a battery go flat?

As the cell reacts, reactants are used up and products build up, so Q increases. By the Nernst equation, E falls as Q rises. When Q reaches K, E becomes zero and the battery can do no more work. The reaction is at equilibrium.

### How are E° and K related?

At equilibrium, E = 0 and Q = K, so E° = (RT/nF) ln K. At 25 °C, this is E° = (0.0592/n) log K. A larger positive E° means a larger K. This ties electrochemistry to the equilibrium constant and to ΔG°.

## Related concepts

- [Galvanic cell](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-6)
- [Standard cell potential](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-7)
- [Electrolysis](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-9)

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