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How does a chain reaction regenerate an intermediate and continue reacting?

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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 a chemical chain reaction, the process of regenerating an intermediate occurs during the propagation stage. This stage consists of a repetitive cycle of elementary steps where a reactive intermediate—such as an atom or a free radical—reacts with original reactants to produce stable products and a new intermediate.12

For example, in the reaction between hydrogen and bromine, the mechanism includes these propagation steps:2
* $Br + H_{2} \to HBr + H$
* $H + {Br}_{2} \to HBr + Br$

In this cycle, a bromine atom is consumed in the first step but is regenerated in the second, allowing the sequence to repeat without the net loss of the intermediate. This regeneration ensures the reaction remains self-sustaining until the chain is eventually broken by termination steps, such as the recombination of radicals. The average number of times this cycle repeats is referred to as the chain length.213