Which statement best describes how a catalyst affects activation energy and the overall enthalpy change ΔH?

Which statement best describes how a catalyst affects activation energy and the overall enthalpy change ΔH?

• A. A catalyst increases ΔH but lowers activation energy.
• B. A catalyst changes the overall enthalpy ΔH but not the activation energy.
• C. A catalyst does not affect activation energy or ΔH.
• D. A catalyst lowers the activation energy, providing an alternative pathway, and does not change ΔH.

Answer: (D)

The key idea is that a catalyst speeds up a reaction by providing a different pathway with a lower activation energy, while leaving the overall enthalpy change (ΔH) of the reaction unchanged. The energy difference between reactants and products is determined by the reactants and products themselves, not by the catalyst. By offering an alternate mechanism with a lower energy barrier, the catalyst makes it easier for molecules to react, increasing the rate. But because ΔH depends on the final and initial states, and those states are the same with or without a catalyst, a catalyst does not alter ΔH. This is why the statement describing a lower activation energy via an alternative pathway and no change to ΔH best fits how catalysts work.

The key idea is that a catalyst speeds up a reaction by providing a different pathway with a lower activation energy, while leaving the overall enthalpy change (ΔH) of the reaction unchanged. The energy difference between reactants and products is determined by the reactants and products themselves, not by the catalyst. By offering an alternate mechanism with a lower energy barrier, the catalyst makes it easier for molecules to react, increasing the rate. But because ΔH depends on the final and initial states, and those states are the same with or without a catalyst, a catalyst does not alter ΔH. This is why the statement describing a lower activation energy via an alternative pathway and no change to ΔH best fits how catalysts work.