2018 O Level Chem P1 Q18 — Speed of Reaction / Catalysts Solution

Answer: B. A catalyst provides an alternative pathway with a lower activation energy, but it does not change the energies of the reactants or products, so \(\Delta H\) is unchanged.

Compared with the original profile, the catalysed profile must keep the same reactant (start) level and the same product (end) level, but have a lower peak. Diagram B shows exactly this.

Diagram A keeps the same levels but does not lower the peak, so it shows no catalyst effect. Diagram C raises the products above the reactants, changing \(\Delta H\). Diagram D lowers the product level, also changing \(\Delta H\). Answer B.