2020 O Level Chem P1 Q14 — Energetics / Activation Energy Solution

Answer: D (271 kJ/mol). Picture the energy profile. The reactants climb an activation energy of 75 kJ/mol to reach the peak, then drop down to the products, which lie 196 kJ/mol below the reactants because the forward reaction is exothermic.

The activation energy of the reverse reaction is the height the products must climb to reach the same peak. That height is the drop from peak to products, which equals the forward activation energy plus the energy released: \(75 + 196 = 271\) kJ/mol.

So the reverse activation energy is 271 kJ/mol, giving answer D. A useful relationship is \(E_{a,\text{reverse}} = E_{a,\text{forward}} - \Delta H\), and with \(\Delta H = -196\) kJ/mol this gives \(75 - (-196) = 271\). Common error: subtracting instead of adding, which wrongly gives 121 kJ/mol (option B).