9.4 Thermodynamic and Kinetic Control

Thermodynamic vs. kinetic control

A reaction under thermodynamic control forms the most stable product (lowest Gibbs free energy), favored at equilibrium or with enough time/energy to reach it. A reaction under kinetic control forms the product that forms fastest (lowest activation energy, Ea), often dominant at lower temperatures or shorter reaction times, even if it's less stable.

Reaction profiles and pathways

On an energy diagram, the kinetic product has a lower activation energy barrier but may sit at a higher energy (less stable) than the thermodynamic product, which has a higher Ea but ends up more stable. Which product dominates depends on whether the reaction is reversible and whether conditions allow the system to reach equilibrium.

Temperature and time matter

Low temperature and short reaction time favor the kinetic product because there isn't enough energy/time to overcome higher barriers or reverse to reach equilibrium. High temperature and long reaction time favor the thermodynamic product, since the system can surmount barriers and settle into the most stable, lowest-energy state.

Key terms

Thermodynamic control

Product distribution determined by stability (lowest free energy); favored with time/energy to reach equilibrium.

Kinetic control

Product distribution determined by rate (lowest activation energy); favored at low temperature/short time.

Kinetic product

The product that forms fastest due to a lower activation energy, even if less stable.

Thermodynamic product

The most stable, lowest-energy product, favored when the system can reach equilibrium.

Effect of temperature/time on product favored

High temp/long time → thermodynamic product; low temp/short time → kinetic product.

Try an AP-style question

Diamond → graphite has ΔG < 0, yet diamonds last for ages. Why?

1. The activation energy is very high, so it's extremely slow
2. Diamond is more stable than graphite
3. ΔG is actually positive for that conversion
4. The conversion is already at equilibrium