Question

For $\mathrm{A}_{2}+\mathrm{B}_{2} \rightleftharpoons 2 \mathrm{AB}$ $\mathrm{E}_{\mathrm{a}}$ for forward and backward reaction are 180 and $200 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. If catalyst lowers $\mathrm{E}_{\mathrm{a}}$ for both reaction by $100 \mathrm{~kJ} \mathrm{~mol}^{-1}$. 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 $+20 \mathrm{~kJ} \mathrm{~mol}^{-1}$.
• D. The enthalpy change for the catalysed reaction is different from that of uncatalysed reaction.

Hint:

Catalysts lower the activation energy but do not change the thermodynamic properties of the reaction.

Answer 1

The Correct Option is A

1. Given: - $\mathrm{A}_{2}+\mathrm{B}_{2} \rightleftharpoons 2 \mathrm{AB}$ - $\mathrm{E}_{\mathrm{f}} = 180 \mathrm{~kJ} \mathrm{~mol}^{-1}$ - $\mathrm{E}_{\mathrm{b}} = 200 \mathrm{~kJ} \mathrm{~mol}^{-1}$ 
2. Calculate the enthalpy change ($\Delta \mathrm{H}$): \[ \Delta \mathrm{H} = \mathrm{E}_{\mathrm{f}} - \mathrm{E}_{\mathrm{b}} = 180 \mathrm{~kJ} \mathrm{~mol}^{-1} - 200 \mathrm{~kJ} \mathrm{~mol}^{-1} = -20 \mathrm{~kJ} \mathrm{~mol}^{-1} \] 
3. Effect of catalyst: - Catalyst lowers the activation energy but does not change the Gibbs free energy change ($\Delta \mathrm{G}$) or the enthalpy change ($\Delta \mathrm{H}$) of the reaction. 
Therefore, the correct answer is (1) Catalyst does not alter the Gibbs energy change of a reaction.