All question types · Thermodynamics

K from ΔG°

UKChO Round 1 · UKChO Round 2 · IChO · unreviewed

Rearrange ΔG° = −RT ln K to K = exp(−ΔG°/RT), with ΔG° in J mol⁻¹ and T in kelvin.

Practise this with fresh numbers

Worked example

A reaction has a standard Gibbs energy change of 22.8 kJ mol⁻¹ at 441 K. What is K?

1. Standard Gibbs energy and K: solve for K
$K = \exp\!\left(-\dfrac{\Delta G^\circ}{RT}\right)$
= 0.001993

Answer: K = 0.00199

Method

Rearrange ΔG° = −RT ln K to K = exp(−ΔG°/RT), with ΔG° in J mol⁻¹ and T in kelvin.

Common mistakes

That is 10^(−ΔG°/RT). ΔG° = −RT ln K uses the natural log, so K = e^(−ΔG°/RT).

Formulas used

Standard Gibbs energy and K
$\Delta G^\circ = -RT\ln K$

Practise this with fresh numbers