Gibbs free energy change
Gibbs free energy change, ΔG
- Gibbs free energy decides whether a reaction can happen on its own.
- It combines enthalpy and entropy with temperature.
- A negative $\Delta G$ means the reaction is feasible.
The equation
$$\Delta G^{\ominus} = \Delta H^{\ominus} - T\Delta S^{\ominus}$$
- $T$ is the temperature in kelvin.
- A reaction is feasible (can happen) when $\Delta G \leq 0$.
Practice
The Gibbs free energy change is calculated as:
ΔG° = ΔH° − TΔS°, with T in kelvin.
Practice
A reaction is feasible when:
A reaction can happen on its own when ΔG ≤ 0.
Feasibility and temperature
| $\Delta H$ | $\Delta S$ | Feasible |
|---|---|---|
| negative | positive | at all temperatures |
| positive | positive | only at high temperature |
| negative | negative | only at low temperature |
| positive | negative | never |
- The changeover temperature is found by setting $\Delta G = 0$, giving $T = \dfrac{\Delta H}{\Delta S}$.
Practice
A reaction with negative ΔH and positive ΔS is feasible:
With −ΔH and +ΔS, ΔG = ΔH − TΔS is always negative, so it is feasible at all temperatures.
Practice
The changeover temperature (where feasibility begins) is found by setting:
At ΔG = 0 the reaction is on the verge of feasibility; rearranging gives T = ΔH/ΔS.
You've got it
Key idea
- $\Delta G^{\ominus} = \Delta H^{\ominus} - T\Delta S^{\ominus}$ (T in kelvin)
- a reaction is feasible when $\Delta G \leq 0$
- $-\Delta H$ & $+\Delta S$ → feasible at all T; $+\Delta H$ & $-\Delta S$ → never
- changeover temperature: set $\Delta G = 0$, so $T = \dfrac{\Delta H}{\Delta S}$