2019 O Level Chem P1 Q15 — Energetics / Energy Profiles and Activation Energy Solution

Answer: A (the forward reaction is endothermic with different activation energies for the forward and reverse reactions). In the energy profile for \(2\text{NH}_3 \rightleftharpoons \text{N}_2 + 3\text{H}_2\), the products N₂ and 3H₂ sit at a higher energy than the reactant 2NH₃.

Because the products are higher in energy than the reactant, the forward reaction absorbs energy overall and is endothermic. That makes statement about an endothermic forward reaction correct.

The activation energy of the forward reaction is measured from the reactant level up to the peak, while the activation energy of the reverse reaction is measured from the higher product level up to the same peak. Since the reactant and product levels are at different heights, the forward and reverse activation energies are different. Combining 'endothermic forward reaction' with 'different activation energies' gives row A. The common error is assuming the activation energies must be equal because they share the same peak.