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2021 O Level Chem P1 Q19 — Energetics / Activation Energy Solution

Energetics / Activation Energy 2021 O Level Pure Chemistry · Paper 1 · Q19

Question

The energy profile diagram for the formation of ammonia is shown. The reaction ½N₂ + 1½H₂ → NH₃ is exothermic, releasing 46 kJ/mol, and the activation energy of the forward reaction is 119 kJ/mol. What is the value of the activation energy for the decomposition of ammonia into its elements? Question diagram

Question diagram

A46 kJ/mol
B73 kJ/mol
C119 kJ/mol
D165 kJ/mol

Correct answer: D

Video Solution

Explanation

Answer: D (165 kJ/mol). On the energy profile the peak rises 119 kJ/mol above the reactants — that is the activation energy of the forward reaction (forming ammonia).

The forward reaction is exothermic by 46 kJ/mol, so the products (ammonia) lie 46 kJ/mol below the reactants.

The reverse reaction — the decomposition of ammonia into its elements — starts from the products and must climb all the way back to the same peak. Its activation energy is therefore the peak height measured from the products: \(119 + 46 = 165\,\text{kJ/mol}\).

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