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NEET CHEMISTRYChemical KineticsMedium

Question

The reaction of hydrogen and iodine monochloride is given as: H2(g)+2ICl(g)2HCl(g)+I2(g)H_2(g) + 2ICl(g) \rightarrow 2HCl(g) + I_2(g). This reaction is of first order with respect to H2(g)H_2(g) and ICl(g)ICl(g), for which of the following proposed mechanisms: Mechanism A: H2(g)+2ICl(g)2HCl(g)+I2(g)H_2(g) + 2ICl(g) \rightarrow 2HCl(g) + I_2(g) Mechanism B: H2(g)+ICl(g)HCl(g)+HI(g)H_2(g) + ICl(g) \rightarrow HCl(g) + HI(g); slow HI(g)+ICl(g)HCl(g)+I2(g)HI(g) + ICl(g) \rightarrow HCl(g) + I_2(g); fast

A

B Only

B

A and B both

C

Neither A nor B

D

A only

Step-by-Step Solution

The order of a reaction and the rate law are determined experimentally and cannot be predicted simply from the balanced chemical equation unless it is an elementary reaction. For complex reactions, the overall rate is controlled by the slowest step, called the rate-determining step.

Given: The reaction is first order w.r.t H2H_2 and first order w.r.t IClICl. This implies the Rate Law is Rate=k[H2][ICl]Rate = k[H_2][ICl].

Mechanism A: This suggests a single-step (elementary) trimolecular collision involving one H2H_2 and two IClICl molecules. The rate law would be Rate=k[H2][ICl]2Rate = k[H_2][ICl]^2, which is third order. This contradicts the given data. Furthermore, trimolecular collisions are very rare .

Mechanism B: The reaction occurs in two steps. The first step is slow and involves one H2H_2 molecule colliding with one IClICl molecule. Since the slow step is rate-determining, the predicted rate law is Rate=k[H2][ICl]Rate = k[H_2][ICl]. This matches the experimentally observed first-order dependence on both reactants. The second step is fast and does not affect the rate law.

Therefore, Mechanism B is the correct mechanism .

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Chemical Kinetics. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYChemical Kineticsreactionhydrogeniodinemonochloriderightarrow

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