What is the effect of a catalyst on activation energy and the overall energy profile (ΔH) of a reaction?

• A. It has no effect on activation energy or ΔH
• B. It lowers ΔH; increases activation energy
• C. It increases activation energy; decreases ΔH
• D. It lowers activation energy; does not change ΔH

Answer: (D)

A catalyst changes the kinetics of a reaction, not its thermodynamics. It provides an alternate pathway with a lower energy barrier to reach the transition state, so the activation energy decreases. However, it does not alter the energies of the reactants or products themselves, so the overall energy change ΔH (the difference between products and reactants) remains the same. In other words, a catalyst makes the reaction go faster without changing how much energy is released or required overall, and it speeds up both the forward and reverse directions equally. This is why lowering the activation energy while leaving ΔH unchanged is the correct description.