 Solution

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.