What is the effect of lowering the activation energy Ea on the rate of a catalyzed reaction at a fixed temperature?

• A. It increases the rate constant k and speeds up the reaction
• B. It decreases the rate constant k and slows the reaction
• C. It leaves the rate constant k unchanged
• D. It makes the reaction exothermic

Answer: (A)

Lowering the activation energy at a fixed temperature speeds up the reaction because it makes the kinetic barrier easier to overcome. The rate constant k follows the Arrhenius relation k = A exp(-Ea/RT). When Ea decreases, the exponential term becomes larger, so k increases and the reaction rate rises. A catalyst provides a lower-energy pathway, boosting k without changing the overall energy change of the reaction, so the reaction is faster but not thermodynamically different. Lower Ea does not change whether the process is exothermic or endothermic, nor does it leave k unchanged or reduce it.