Entropy and free energy · Grade 12
Whether a reaction can happen by itself depends on two things: the enthalpy change, ΔH, and the entropy change, ΔS. Entropy measures disorder, the number of ways particles and their energy can be arranged. The Gibbs free energy change, ΔG = ΔH − TΔS, combines them: a reaction is feasible when ΔG is zero or negative.
Key facts
- Entropy, S, is measured in J K⁻¹ mol⁻¹. Gases have far more entropy than liquids, and liquids more than solids. A perfect crystal at 0 K has S = 0.
- ΔS = ΣS(products) − ΣS(reactants). ΔS is positive when a solid melts or dissolves, a liquid boils, or the number of moles of gas increases.
- ΔG = ΔH − TΔS, with T in kelvin. Change ΔS to kJ K⁻¹ mol⁻¹ (divide by 1000) to match ΔH in kJ/mol.
- A reaction is feasible when ΔG ≤ 0. Feasible does not mean fast: a high activation energy can make a feasible reaction too slow to see.
- ΔH negative and ΔS positive: always feasible. ΔH positive and ΔS negative: never feasible. When the signs are the same, the reaction changes over at T = ΔH ÷ ΔS.
- A graph of ΔG against T is a straight line with gradient −ΔS and intercept ΔH.