activation energy

Meaning: Activation energy is the energy difference between the reactants and the transition state, the highest-energy point on the reaction coordinate diagram. Colliding molecules must have enough energy, and the right orientation, to climb this barrier. A higher Ea means fewer collisions succeed, so the reaction is slower. The forward Ea and reverse Ea differ by ΔH of the reaction.

Equation: k = Ae^(−Ea/RT), the Arrhenius equation. k is the rate constant (units depend on reaction order), A is the frequency factor (same units as k), Ea is the activation energy (J/mol), R is the gas constant (8.314 J/mol·K), and T is the absolute temperature (K).

On the MCAT: You may read a reaction coordinate diagram and measure Ea as the peak height above the reactants. You may compare catalyzed and uncatalyzed curves, or predict how k changes when T rises or Ea falls. Enzymes are the usual biological example of lowering Ea. A multistep diagram has several peaks, and the rate-determining step is the one with the highest Ea.

Common confusion: Students think a catalyst makes a reaction more favorable or shifts equilibrium. It does not. Ea is kinetic, while ΔG and Keq are thermodynamic. A catalyst lowers Ea for both directions equally, so equilibrium is reached faster but at the same position. Likewise, a large negative ΔG does not imply a small Ea.