What is activation energy and how does it affect the rate of a reaction at a constant temperature?

• A. Higher Ea increases the rate at a given temperature.
• B. Ea is the energy released by the reaction.
• C. The energy required to reach the transition state; higher Ea lowers the rate at a given temperature.
• D. Lower Ea has no effect on rate constants.

Answer: (C)

Activation energy is the energy barrier that must be overcome for reactants to reach the transition state and form products. At a constant temperature, the rate depends on how many molecules have enough energy to get over that barrier. Because molecule energies follow a distribution, only a fraction have energy equal to or greater than Ea; a larger barrier means fewer molecules can cross it, so the reaction proceeds more slowly. This relationship is captured in the Arrhenius view, where the rate constant k scales as exp(-Ea/RT). Lowering the barrier (as a catalyst does) increases the rate at the same temperature. Activation energy is not the energy released by the reaction—that’s the enthalpy change—and lowering Ea does affect rate constants.