JEE Main 2025 Apr 4 Shift 1, Chemistry Q7: Gibbs Energy Change and Equilibrium

JEE Main2025Apr 4, Shift 1Chemistry

For $A_{2} + B_{2} ⇌ 2 A B$

$E_{a}$ for forward and backward reaction are 180 and 200 kJ mol $^{- 1}$ respectively.

If catalyst lowers $E_{a}$ for both reaction by 100 kJ mol $^{- 1}$ .

Which of the following statement is correct?

A
Catalyst does not alter the Gibbs energy change of a reaction.
B
Catalyst can cause non-spontaneous reactions to occur.
C
The enthalpy change for the reaction is $+ 20$ kJ mol $^{- 1}$ .
D
The enthalpy change for the catalysed reaction is different from that of uncatalysed reaction.

Solution

A catalyst lowers the activation energy of both the forward and reverse steps by the same amount. It changes the kinetics (rate), not the thermodynamics (state functions) of the reaction.

$Δ H = E_{a, f} - E_{a, b} = 180 - 200 = - 20$ kJ mol $^{- 1}$ (so option 3 is wrong; the sign and value differ).

With catalyst: $E_{a, f} = 80$ , $E_{a, b} = 100$ ; $Δ H$ is still $- 20$ kJ mol $^{- 1}$ , unchanged (so option 4 is wrong).

Catalysts cannot turn non-spontaneous reactions into spontaneous ones since they do not alter $Δ G$ (so option 2 is wrong, and this very fact makes option 1 correct).

$∴$ Correct statement: a catalyst does not alter $Δ G$ of a reaction.

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JEE Main 2025 Apr 4 Shift 1, Chemistry Q7: Gibbs Energy Change and Equilibrium

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