 Solution

The reverse elementary rate constant is extraordinarily small at 298 K. Its reaction would require hydrogen chloride and a chlorine free radical to produce a hydrogen free radical and chlorine, reversing a highly exothermic propagation step. The value found above makes this channel negligible for the intended reacting mixture compared with forward propagation.

A large equilibrium constant alone does not permit a reverse reaction to be ignored under every conceivable composition: if forward reactants were exhausted, concentration ratios would matter. The simplification applies during the forward chain-reaction regime specified, rather than constituting a universal statement about all late-time or imposed reverse conditions.