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Why is hydrogen iodide considered an intermediate in the reaction between hydrogen and iodine chloride?

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dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

In the reaction between hydrogen ( $H_{2}$ ) and iodine chloride ( $I C l$ ), hydrogen iodide ( $H I$ ) is considered a chemical intermediate because it is produced in the first step of the reaction and consumed in the subsequent step.12

The reaction proceeds through a two-step composite reaction mechanism:

Step 1 (Slow): $H_{2} + I C l \to H I + H C l$
Step 2 (Fast): $H I + I C l \to H C l + I_{2}$

In this process, $H I$ is generated as a product of the initial step and immediately acts as a reactant for the second step. Because the second reaction is rapid, $H I$ is removed as quickly as it is formed, meaning it does not accumulate as a final product but instead serves as a transient species that bridges the starting materials and the final products. This mechanism explains why the observed reaction rate is proportional to the first power of both $H_{2}$ and $I C l$ concentrations, rather than the second power of $I C l$ as the balanced equation might suggest.13