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Question #44063

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The Question

Consider the gas-phase exothermic reaction $X(g) + Y(g) \rightleftharpoons Z(g)$ catalyzed by a solid metal surface at $400\text{ K}$ . The reaction proceeds via a Langmuir-Hinshelwood mechanism where $X$ and $Y$ compete for the same active sites. The adsorption of $X$ is highly favored such that its surface coverage $\theta_X \approx 1$ , while $Y$ adsorbs weakly.

If the volume of the reaction vessel is suddenly halved at constant temperature before any $Z$ is formed, which of the following correctly describes the changes to the initial forward reaction rate ( $r_f$ ), the thermodynamic equilibrium constant ( $K_p$ ), and the final equilibrium moles of $Z$ ( $n_Z$ )?

Options

A.
$r_f$ remains unchanged; $K_p$ remains unchanged; $n_Z$ increases.
B.
$r_f$ quadruples; $K_p$ remains unchanged; $n_Z$ increases.
C.
$r_f$ doubles; $K_p$ increases; $n_Z$ remains unchanged.
D.
$r_f$ remains unchanged; $K_p$ decreases; $n_Z$ decreases.

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

Homogeneous vs. Heterogeneous Equilibrium|Temperature Coefficient (Q10)|Collision Frequency vs. Effective Collisions

Why This Question Is Tricky

Common Mistakes

This problem tests the critical distinction between thermodynamic equilibrium and heterogeneous reaction kinetics.

Students often erroneously apply gas-phase collision theory to surface-catalyzed reactions, assuming that an increase in partial pressures will universally increase the reaction rate (predicting a quadrupling of $r_f$ ).

The 'Hidden Variable' is the kinetic saturation of the catalyst surface by the strongly adsorbing species ( $X$ ), which acts as a limiting reagent for active sites.

This makes the rate inversely proportional to $P_X$ and directly proportional to $P_Y$ , rendering the overall rate independent of total pressure changes at constant composition.

Simultaneously, it ensures students do not confuse the shift in equilibrium position (yield of $Z$ ) with a change in the equilibrium constant ( $K_p$ ), which is strictly temperature-dependent.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Equilibrium - Class 11 Part 1
Concept	Homogeneous vs. Heterogeneous Equilibrium
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free