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Question #44746

Difficulty: 8/10 | 🟢 Free

The Question

The specific acid-catalyzed hydrolysis of ethyl acetate ( $\ce{CH3COOC2H5}$ ) is carried out in an aqueous solution at $298 \text{ K}$ . The reaction is catalyzed by a weak acid $\ce{HA}$ ( $K_{a} = 1.0 \times 10^{-5}$ ) with an initial concentration of $0.1 \text{ M}$ . The initial concentration of ethyl acetate is $0.2 \text{ M}$ .

Given that the rate law for the hydrolysis is $\text{Rate} = k [\ce{Ester}][\ce{H+}]$ , and the acid dissociation constant of the product acetic acid ( $\ce{CH3COOH}$ ) at $298 \text{ K}$ is $K_{a} = 2.0 \times 10^{-5}$ .

Assuming the volume of the solution remains constant and neglecting the autoionization of water, what is the ratio of the rate of reaction at $t = 0$ to the rate of reaction when the hydrolysis is $75\%$ complete?

Options

A.
2
B.
4
C.
8
D.
1

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Degree of Dissociation (Alpha)|Pseudo-First Order Kinetics|Rate Law Derivation from Mechanism

Why This Question Is Tricky

Common Mistakes

The problem tests the intersection of chemical kinetics and ionic equilibrium.

Students often memorize that acid-catalyzed ester hydrolysis follows pseudo-first-order kinetics, assuming $[\ce{H+}]$ is constant.

However, when the catalyst is a weak acid, the production of another weak acid (acetic acid) during the reaction increases the total $[\ce{H+}]$ , which simultaneously suppresses the degree of dissociation ( $\alpha$ ) of the original catalyst via the common ion effect.

Failing to account for the dynamic $[\ce{H+}]$ leads to the incorrect assumption that the rate simply scales with the ester concentration (yielding a distractor ratio of 4).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Electrochemistry - Class 12
Concept	Degree of Dissociation (Alpha)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free