How do homogeneous and heterogeneous catalysts differ in their influence on Ea and the reaction rate?

• A. They do not affect Ea
• B. Both provide an alternative pathway with a lower Ea; thus they increase the rate constant k
• C. Only homogeneous catalysts affect Ea
• D. Only heterogeneous catalysts affect Ea

Answer: (B)

The main idea is that a catalyst provides an alternative reaction pathway with a lower activation energy, which makes the reaction easier to start and speeds it up. Because the rate constant k in the Arrhenius equation k = A exp(-Ea/RT) depends exponentially on Ea, lowering Ea increases k and thus the reaction rate at a given temperature.

Both homogeneous and heterogeneous catalysts achieve this lower-energy pathway, just in different ways. A homogeneous catalyst works in the same phase as the reactants and often forms intermediate species or cycles that lower the energy barrier. A heterogeneous catalyst provides active sites on a solid surface where reactants are adsorbed, oriented, and transformed through surface reactions, again stabilizing the transition state and reducing the barrier. In both cases the overall effect is a faster reaction, not a change to the uncatalyzed pathway.

So, the correct view is that catalysts—whether homogeneous or heterogeneous—lower the activation energy by offering an alternative, lower-energy route, which raises the rate constant and increases the reaction rate.