Choose the correct statements from the following regarding the reaction kinetics
(A) A catalyst does not affect the Gibbs energy of a reaction.
(B) A catalyst alters the equilibrium constant.
(C) For a zero order reaction, half life is dependent on the initial concentration of the reactant.
(D) Orientation of reactant molecules affects the reaction rate.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) only
Each statement rests on a basic idea of chemical kinetics: what a catalyst changes, how the half-life depends on concentration for each order, and what makes a collision effective.
(A) A catalyst does not affect the Gibbs energy of a reaction. — Correct. A catalyst only provides a path of lower activation energy; \( \Delta G \) depends on the reactants and products, which stay the same. So a catalyst can speed up only a spontaneous reaction.
(B) A catalyst alters the equilibrium constant. — Incorrect. The forward and backward reactions are speeded up equally, so the equilibrium constant is unchanged; equilibrium is only reached faster.
(C) For a zero order reaction, half life is dependent on the initial concentration of the reactant. — Correct. For a zero order reaction \( [\mathrm{R}] = [\mathrm{R}]_0 - kt \). At \( t_{1/2} \), \( [\mathrm{R}] = [\mathrm{R}]_0/2 \), which gives \( t_{1/2} = \frac{[\mathrm{R}]_0}{2k} \): the half-life is directly proportional to the initial concentration. (Only for a first order reaction is \( t_{1/2} = 0.693/k \) independent of it.)
(D) Orientation of reactant molecules affects the reaction rate. — Correct. By collision theory, only collisions with enough energy and proper orientation form products. For example, \( \ce{OH-} \) must strike \( \ce{CH3Br} \) on the side away from Br to give methanol; other collisions are ineffective, which is why the rate is \( P Z_{AB} e^{-E_a/RT} \) with an orientation (probability) factor P.