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