thermodynamic control
Summary
Thermodynamic control means a reversible reaction reaches equilibrium, so the major product is the most stable one (lowest Gibbs free energy), not the one that forms fastest. Higher temperature and longer time favor it; the faster-forming product is the kinetic one.
Detail
Meaning: Two products can form from the same reactants. When the steps are reversible and the mixture reaches equilibrium, the more stable product (lower Gibbs free energy) predominates. That is thermodynamic control. Under kinetic control the product with the lower activation energy predominates, because the reverse reaction is too slow to matter. Higher temperature and longer time favor thermodynamic control.
Equation: ΔG° = −RT ln Keq, where ΔG° is the standard free energy change (J/mol), R is the gas constant (8.314 J/(mol·K)), T is absolute temperature (K), and Keq is the equilibrium constant (unitless).
On the MCAT: Passages give a reaction coordinate diagram with two pathways, or product ratios at different temperatures or times. Predict which product dominates: low temperature and short time favor the kinetic product, high temperature and long time the thermodynamic one. A classic case is HBr addition to 1,3-butadiene (1,2 product kinetic, 1,4 product thermodynamic).
Common confusion: Students link the thermodynamic product with the faster reaction. Activation energy sets rate; ΔG° sets stability. The product that forms fastest is often less stable, and it persists only if the reaction cannot reverse.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.