Consider the gas-phase reaction: H₂(g) + I₂(g) → 2 HI(g) The reaction was experimentally determined to be first order in H₂ and first order in I₂. Consider the proposed mechanisms. Proposed mechanism I: H₂(g) + I₂(g) → 2 HI(g) Single step Proposed mechanism II: I₂(g) Δk1k-12 I(g) Fast H₂( g) + 2 I( g) → k22 HI( g) Slow b. What kind of experimental evidence might lead you to favor mechanism II over mechanism I?

Key Concepts

Reaction Order

Reaction order refers to the power to which the concentration of a reactant is raised in the rate law of a chemical reaction. In this case, the reaction is first order in both H2 and I2, indicating that the rate of reaction is directly proportional to the concentration of each reactant. Understanding reaction order is crucial for analyzing how changes in concentration affect the reaction rate and for evaluating proposed mechanisms.

Mechanism of Reaction

A reaction mechanism is a step-by-step description of the pathway taken by reactants to form products. It includes elementary steps, which can be either fast or slow, and helps explain the overall reaction rate. In the context of the question, comparing the single-step mechanism I with the multi-step mechanism II allows for an understanding of how intermediates and rate-determining steps influence the observed kinetics.

Rate-Determining Step

The rate-determining step (RDS) is the slowest step in a reaction mechanism that dictates the overall reaction rate. In proposed mechanism II, the slow step involves the formation of intermediates, which can significantly affect the kinetics of the reaction. Experimental evidence, such as changes in reaction rate with varying concentrations of reactants, can indicate whether the RDS aligns with the proposed mechanism, thus favoring one over the other.