# Free energy and the equilibrium constant

The standard free energy change and K are linked: ΔG° = −RT ln K. A negative ΔG° means K > 1 and products are favoured. A positive ΔG° means K < 1. At equilibrium, ΔG = 0.

Chemistry (university) · Part 07: Chemical equilibrium and solubility · Card 7 of 9 · Last updated 2026-10-19

## Analogy

The depth of a valley: the deeper the products' valley, the more the mixture ends up there.

## Example

**K = 1×10⁵ at 298 K**

ΔG° = −RT ln K
= −8.314 × 298 × ln(10⁵)
= −2478 × 11.51
= −28.5 kJ/mol

## In practice

Chemists estimate K for a reaction from tables of thermodynamic data without doing the experiment.

## Practice

### Flashcards

- **Link ΔG° and K.** ΔG° = −RT ln K.

## Questions people ask

### What is the difference between ΔG and ΔG°?

ΔG° applies when all species are at standard conditions, 1 M or 1 bar. ΔG applies to the actual mixture at any time, and ΔG = ΔG° + RT ln Q. At equilibrium, Q = K and ΔG = 0, which gives ΔG° = −RT ln K.

### If ΔG° is positive, does the reaction never happen?

No. A positive ΔG° means K is below 1, so the equilibrium mixture has more reactants than products. But some product still forms. If the product is removed, the reaction continues. Biochemistry uses this by coupling an unfavourable step to a favourable one.

## Related concepts

- [Dynamic equilibrium](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-1)
- [Equilibrium constant](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-2)
- [Reaction quotient](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-3)
- [Kp and Kc](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-4)

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