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Question #44809

Difficulty: 8/10 | 🟢 Free

The Question

The acid-catalyzed hydrolysis of ethyl acetate (\ceCH3COOCH2CH3\ce{CH3COOCH2CH3}) is studied at 298 K298 \text{ K}. In Experiment I, the reaction is carried out in pure water ([\ceH2O]=55.5 M[\ce{H2O}] = 55.5 \text{ M}) with 0.1 M \ceHCl0.1 \text{ M} \ \ce{HCl} as the catalyst and an initial ester concentration of 0.01 M0.01 \text{ M}. Under these conditions, the reaction exhibits a half-life of 40 min40 \text{ min}. In Experiment II, the reaction is performed in an inert water-dioxane solvent mixture where [\ceH2O]=11.1 M[\ce{H2O}] = 11.1 \text{ M}, keeping the initial ester and catalyst concentrations identical to Experiment I. Assuming the intrinsic third-order rate constant (kk) remains unaffected by the solvent medium, determine the half-life of the ester in Experiment II and the apparent overall order of the reaction under these new conditions.

Options

  • A.

    200 min200 \text{ min}, Apparent first-order

  • B.

    40 min40 \text{ min}, Apparent first-order

  • C.

    8 min8 \text{ min}, Apparent first-order

  • D.

    200 min200 \text{ min}, Apparent second-order

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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

First Order Half-Life Independence|Pseudo-First Order Kinetics

Why This Question Is Tricky

Common Mistakes

This question tests the misconception that the half-life of a pseudo-first-order reaction is an absolute constant.

Students often memorize that 'first-order half-life is independent of initial concentration' and erroneously apply this to the solvent/excess reactant in pseudo-first-order kinetics (Option B).

The trap exposes that the pseudo-rate constant kobsk_{obs} embeds the concentration of the excess reagent ([\ceH2O][\ce{H2O}]).

Changing the solvent composition alters kobsk_{obs} and thus the half-life, even though the reaction remains apparently first-order because water is still in large excess (11.1 M≫0.01 M11.1 \text{ M} \gg 0.01 \text{ M}), invalidating Option D.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterChemical Kinetics - Class 12
ConceptFirst Order Half-Life Independence
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree