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How does the reaction between hydrogen and iodine chloride show that an overall reaction can have two steps?

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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).

The reaction between hydrogen and iodine chloride ( $H_{2} + 2 I C l \to I_{2} + 2 H C l$ ) demonstrates that a reaction can occur in multiple steps because its observed kinetic behavior does not match its balanced chemical equation.1

If the reaction occurred in a single elementary step, the rate would be proportional to the concentration of hydrogen and the square of the iodine chloride concentration. Instead, experiments show the reaction is second-order, with a rate law of $v = k [H_{2}] [I C l]$ . This indicates the reaction proceeds through a composite reaction mechanism consisting of two steps:1

Slow step: $H_{2} + I C l \to H I + H C l$
Fast step: $H I + I C l \to H C l + I_{2}$

Because the second step is rapid, the intermediate hydrogen iodide is consumed as quickly as it is formed. Consequently, the overall reaction rate is determined solely by the slow first step, which explains why the rate depends on the concentrations of hydrogen and iodine chloride to the first power.1