2020 O Level Chem P1 Q16 — Rate of Reaction / Catalysts and Energy Profiles Solution

Answer: C. A catalyst works by providing an alternative reaction pathway with a lower activation energy, so the energy hump in the profile becomes smaller. Crucially, a catalyst does not change the energy levels of the reactants or the products, and it does not change \(\Delta H\).

The original profile here is endothermic, with the products at a higher energy level than the reactants. So the correct catalysed profile must keep the reactants and the products at exactly the same heights as before, keep the overall endothermic shape, and simply show a lower hump.

Option C does exactly this: a reduced activation energy with the reactant and product levels unchanged. Option D keeps the original large hump, so it shows no catalytic effect; option B wrongly turns the reaction exothermic; and the remaining option alters the product level. The answer is C.