Final answer:
The overall reaction order is second order, with first-order dependence on ICl and H₂. A consistent two-step mechanism involves HI as an intermediate, with a fast first step and a slow, rate-determining second step involving bimolecular reactions.
Step-by-step explanation:
To determine the overall order of the reaction for H₂(g) + 2ICl(g) → I₂(g) + 2HCl(g) using the given data, observe the changes in the reaction rate as the pressures of reactive species change across different experiments. Since the rate law is experimentally determined to be rate = k[ICl][H₂], we conclude that the order is 1 with respect to both ICl and H₂, making the overall reaction second order. To provide a plausible two-step mechanism consistent with the rate law, consider the following:
- Step 1: H₂(g) + ICl(g) → HI(g) + HCl(g) (fast and reversible)
- Step 2: HI(g) + ICl(g) → I₂(g) + HCl(g) (slow, rate-determining step)
This mechanism shows that HI is an intermediate in the reaction and explains why the rate law includes only the concentrations of ICl and H₂. The first step rapidly establishes equilibrium, producing HI, while the second step, which consumes HI and ICl, is slower and therefore determines the rate of the reaction. The rate law is consistent with the second step because the rate-determining step only involves the bimolecular reactions of HI and ICl.