A chemist compares the reaction pathway for the same exothermic reaction, once without any catalyst present and once with a catalyst present, at the same constant temperature. Which statement correctly compares the two pathways?

AS Chemistry 9701: Catalysed vs Uncatalysed Reaction Pathways (MCQ)

Step 1: What a catalyst does and does not change

A catalyst provides an alternative reaction pathway with a lower activation energy, $E_A$ , than the uncatalysed route. This lower energy barrier means that, at the same temperature, a greater fraction of particle collisions have enough energy to react, so the rate increases.

Step 2: What stays the same

The enthalpy change, $\Delta H$ , of the reaction depends only on the energies of the reactants and the products. It is unaffected by the pathway taken between them. Since the same reactants form the same products in both cases, $\Delta H$ is identical for the catalysed and uncatalysed routes; only the height of the energy barrier in between (the activation energy) is different.

Step 3: Evaluating the options

A: Correctly identifies a lower $E_A$ for the catalysed pathway with an unchanged $\Delta H$ . This is correct.
B: Incorrectly claims $\Delta H$ changes; the size of the overall energy change between reactants and products does not depend on the pathway or the presence of a catalyst.
C: Incorrectly claims $E_A$ is unchanged (a catalyst’s entire function is to lower $E_A$ ) and incorrectly claims $\Delta H$ changes.
D: Incorrectly reverses the effect of a catalyst. A lower, not higher, $E_A$ is what increases the fraction of successful collisions.

Final answer

$\boxed{\text{A}}$ . The catalysed pathway has a lower activation energy; the enthalpy change of reaction is unchanged.