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2024 O Level Chem P1 Q31 — Energetics / Activation Energy Solution

Energetics / Activation Energy 2024 O Level Pure Chemistry · Paper 1 · Q31

Question

The energy profile diagram for a reversible reaction is shown. Which statement for this reversible reaction is correct? Question diagram

Question diagram

AThe activation energy of the backward reaction has the same value as the enthalpy change of reaction of the forward reaction but has the opposite sign.
BThe activation energy of the forward reaction has the same value as the enthalpy change of reaction of the backward reaction.
CThe activation energy of the forward reaction is four times the activation energy of the backward reaction.
DThe overall energy change of the forward and backward reactions have the same value and sign.

Correct answer: C

Video Solution

Explanation

Answer: C. Read the gridded profile carefully. Reactants sit 1 unit above the baseline; the transition state peak sits at 5 units; products sit at 4 units.

Forward activation energy = peak − reactants = \(5 - 1 = 4\) units.
Backward activation energy = peak − products = \(5 - 4 = 1\) unit.
Forward \(E_a\) ÷ backward \(E_a\) = 4.

So forward \(E_a\) is four times the backward \(E_a\) → option C.

A and B confuse \(E_a\) with \(\Delta H\). D claims forward and backward \(\Delta H\) have the same sign — but they are equal in magnitude and opposite in sign (a fundamental property of reversible reactions).

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