Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Thermodynamics (sign of ΔG°) tells you whether a process is favoured. Kinetics (Eₐ, rate) tells you how fast it goes. The two are independent.
- Many favoured processes are extremely slow: paper burning in air, hydrogen and oxygen forming water, diamond becoming graphite.
- A process that is favoured (ΔG° < 0) but does not occur at a measurable rate is under kinetic control.
- The usual cause is a high activation energy: very few collisions have enough energy to reach the transition state.
- No change ≠ equilibrium. At equilibrium, forward and reverse rates are equal and Q = K. Under kinetic control, both rates are nearly zero and Q ≪ K.
Key relationships
| Situation | ΔG° | Rate | Q compared with K | Catalyst added |
|---|---|---|---|---|
| Favoured and fast | < 0 | measurable | moves quickly to Q = K | reaches Q = K even faster |
| Kinetic control | < 0 | not measurable | Q ≪ K, stays there | reaction starts |
| At equilibrium | any | forward = reverse | Q = K | no change in amounts |
| Not favoured | > 0 | any | K < 1 | little product, ever |
A catalyst lowers Eₐ for both directions. It never changes ΔG°, ΔH° or K.
Assumptions
- “Not measurable” means too slow to detect on the time scale of the observation (hours, years), not exactly zero.
- ΔG° values refer to standard states at 298 K.
Mistakes to avoid
- Saying a reaction “is not favoured” when the data show ΔG° < 0. Say “favoured but under kinetic control”.
- Saying a stable mixture “is at equilibrium” because nothing changes. Compare Q with K.
- Claiming a catalyst makes ΔG° more negative.
- Explaining slowness without particles: say few collisions have enough energy to overcome Eₐ.
- Linking a large negative ΔG° to a fast rate. Size of ΔG° and size of Eₐ are unrelated.
Quick self-check
- ΔG° for 2H₂O₂(l) → 2H₂O(l) + O₂(g) is −233.6 kJ mol⁻¹. What is it per mole of H₂O₂? (−116.8 kJ)
- A favoured reaction shows no change for a year. Is it at equilibrium? (No: Q ≪ K; it is under kinetic control.)
- A catalyst is added to a mixture at equilibrium. What happens to the amounts? (Nothing: both rates rise equally.)
Next: practice questions.