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How do consecutive reactions produce intermediate substances?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

In chemical kinetics, consecutive reactions produce intermediate substances through a series of elementary steps where the product of one step serves as a reactant for the next.12

A chemical intermediate is any substance generated during the conversion of initial reactants to a final product. In a consecutive mechanism, the reaction does not occur in a single step; instead, it proceeds through a sequence where an intermediate is formed and subsequently consumed. For example, in the reaction between hydrogen and iodine chloride, the process involves two steps:12
* Step 1: A slow reaction produces hydrogen iodide (HI) as an intermediate: $H_{2} + ICl \to HI + HCl$ .2
* Step 2: A rapid reaction consumes the HI intermediate to form the final products: $HI + ICl \to HCl + I_{2}$ .2

Intermediates can range from stable substances to highly reactive, short-lived species like free radicals or atoms. Because these species are often present at low concentrations, chemists frequently use the steady-state approximation to model their behavior and determine the overall reaction rate.12