= Solution
\b[Step 1 is initiation; steps 2 and 3 are propagation; step 4 is termination.] Initiation dissociates a <chlorine> <molecule> into two <chlorine> <free radicals>. Each propagation step consumes one <free radical> and creates another, carrying the chain forward without destroying its <free radical> carrier overall. Termination removes two <chlorine> <free radicals> by recombination.
The inert collision partner accepts or supplies energy as needed and is regenerated. It is not a <free radical> chain carrier or a net reagent. Adding the two propagation steps cancels the <hydrogen> and <chlorine> <free radicals> and gives one complete conversion of <hydrogen> plus <chlorine> to two <molecules> of <hydrogen chloride>.
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