For A2 + B2 2AB Ea for forward and backward reaction are 180 and 200 kJ mol-1 respectively. If catalyst lowers Ea for both reactions by 100 kJ mol-1. Which of the following statement is correct?
- Catalyst does not alter the Gibbs energy change of a reaction.
- The enthalpy change for the reaction is +20 kJ mol-1.
- Catalyst can cause non-spontaneous reactions to occur.
- The enthalpy change for the catalysed reaction is different from that of uncatalysed reaction.
Answer
(1) Catalyst does not alter the Gibbs energy change of a reaction.
Solution
r H = Ea(f) - Ea(b)
r H = 180 - 200 = -20 kJ mol-1 (not +20), so (B) is wrong.
With the catalyst: Ea(f) = 80 and Ea(b) = 100 kJ mol-1
r H = 80 - 100 = -20 kJ mol-1, the same as before, so (D) is wrong.
A catalyst only provides an alternate path of lower activation energy; the initial and final states are unchanged.
As per NCERT, a catalyst does not alter the Gibbs energy, G, of a reaction. It catalyses only spontaneous reactions and cannot make a non-spontaneous reaction occur, so (C) is wrong and (A) is correct.
Answer: (1)