Chemical kinetics and equilibrium

Chemical kinetics and equilibrium answer two different questions about a reaction. Kinetics asks how fast: reaction rates depend on reactant concentrations (captured in rate laws like rate = k[A]^x[B]^y, determined experimentally), temperature, surface area, and catalysts. According to collision theory, molecules must collide with enough energy — the activation energy — and the right orientation to react. Catalysts speed reactions by providing a pathway with lower activation energy, without being consumed.

Equilibrium asks where the reaction settles. In a reversible reaction, the forward rate slows as reactants deplete while the reverse rate grows, until the two rates are equal. At that point concentrations stop changing — but the reaction hasn't stopped; it's a dynamic equilibrium. The equilibrium constant Keq (products over reactants, each raised to its stoichiometric coefficient) describes the position: Keq much greater than 1 favors products, much less than 1 favors reactants.

Le Châtelier's principle predicts how equilibrium responds to stress: adding a reactant shifts the system toward products, raising pressure shifts a gas-phase reaction toward the side with fewer moles of gas, and heating shifts it in the endothermic direction. Crucially, kinetics and equilibrium are independent: a catalyst gets a system to equilibrium faster but never changes Keq, and a large Keq says nothing about speed — a thermodynamically favorable reaction can still be extremely slow.

On the MCAT, kinetics and equilibrium form a core chemistry topic: expect questions on determining rate laws from experimental data, activation energy and catalysts, interpreting Keq versus the reaction quotient Q, and applying Le Châtelier's principle — plus the classic distinction that kinetics and thermodynamics are separate questions.