AP Chemistry / Unit 9: Thermodynamics and Electrochemistry
9.3 Gibbs Free Energy and Thermodynamic Favorability
Notes
$$\Delta G=\Delta H-T\Delta S$$
- $\Delta G<0$: thermodynamically favorable.
| $\Delta H$ | $\Delta S$ | Favorable? |
|---|---|---|
| − | + | always |
| + | − | never |
| − | − | at low $T$ |
| + | + | at high $T$ |
Practice
$\Delta H=+30$ kJ, $\Delta S=+0.1$ kJ/K. Favorable above:
$\displaystyle T=\frac{\Delta H}{\Delta S}$.
$\Delta H<0$ and $\Delta S>0$:
$\Delta G$ always negative.
$\Delta G<0$ means a reaction is:
Favorability, not speed.
Ready for more? Take the Unit 9 test.