JEE Main 2025 · 4 April · morning · Single correct · +4 / −1
For A₂+B₂ ⇌ 2 AB
Eₐ for forward and backward reaction are 180 and 200 kJ mol⁻¹ respectively
If catalyst lowers Eₐ 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) The enthalpy change for the reaction is +20 kJ mol⁻¹.
- (c) Catalyst can cause non-spontaneous reactions to occur.
- (d) The enthalpy change for the catalysed reaction is different from that of uncatalysed reaction.
Answer and solution
Answer (a) Catalyst does not alter the Gibbs energy change of a reaction.
Solution
Chemistry ma’am · 3 steps
A catalyst lowers both hills by the same amount, so the start and end energies never move.
- Enthalpy from barriers. On an energy profile, Δ H is the gap between the forward and the backward activation energies.
Δ H = E_(a,f) - E_(a,b)
Reaction E_(a,f) E_(a,b) Δ H Uncatalysed 180 kJ mol⁻¹ 200 kJ mol⁻¹ -20 kJ mol⁻¹ Catalysed 80 kJ mol⁻¹ 100 kJ mol⁻¹ -20 kJ mol⁻¹ - What the catalyst changes. Only the path between reactants and products changes. The energies of A₂ + B₂ and of 2AB stay the same, so Δ H, Δ G and K are all unchanged. A catalyst therefore cannot make a non-spontaneous reaction happen.
- So. The correct statement is (a): a catalyst does not alter the Gibbs energy change of a reaction.
The idea to keep. A catalyst only lowers the barrier. It speeds up forward and backward reactions equally and leaves Δ H, Δ G and the equilibrium constant untouched.
Answer from the NTA final answer key. Solution by PrepWiser.