= Solution
\b[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.
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