Which statement best describes the activation energy Ea in the Arrhenius equation?

• A. Ea represents the energy barrier that must be overcome for reaction to occur; higher Ea lowers rate constant.
• B. Ea is the pre-exponential factor.
• C. Ea is the gas constant.
• D. Ea has no effect on the rate.

Answer: (A)

Activation energy is the energy hurdle that reactant molecules must gain to reach the transition state and form products. In the Arrhenius equation k = A exp(-Ea/(RT)), Ea sits in the exponent, so at a given temperature a larger Ea makes the exponential term smaller, reducing the rate constant k and slowing the reaction. Conversely, a smaller Ea makes it easier for molecules to overcome the barrier, increasing the rate. The pre-exponential factor A is a separate term that reflects how often collisions occur and how favorably they are oriented, while R is just the gas constant. So Ea truly represents the energy barrier and higher Ea lowers the rate.