reaction rates

Meaning: Reaction rate is how fast reactants are consumed or products formed, as change in concentration per unit time. Collision theory: molecules must collide with at least the activation energy and proper orientation. More concentration means more collisions; higher temperature raises k; a catalyst lowers the activation energy without being consumed. The slowest step of a mechanism, the rate-determining step, sets the rate law.

Equation: rate = k[A]^m[B]^n, where rate is in mol/(L·s), k is the rate constant (units depend on overall order, such as s⁻¹ for first order), [A] and [B] are reactant concentrations in mol/L, and m and n are the orders in A and B (unitless).

On the MCAT: Passages give a table of initial rates for several trials. Compare two trials where only one concentration changes: if doubling [A] doubles the rate, the reaction is first order in A; if it quadruples the rate, second order; if the rate does not change, zero order. You may also read concentration-versus-time graphs (first order shows a constant half-life) or energy diagrams where a catalyst lowers the barrier.

Common confusion: Orders come from experiment, not the balanced equation's coefficients, except for a single elementary step, where they match molecularity. Also, a catalyst speeds a reaction but leaves Keq and ΔG unchanged.