= Solution
\b[Apply the <steady-state approximation in chemical kinetics> to the <hydrogen> and <chlorine> <free radicals>.] After a short initiation transient their formation and consumption can balance while the stable reactants continue to be depleted and <hydrogen chloride> accumulates. Accordingly set $\mathrm d[\mathrm H]/\mathrm dt\simeq0$ and $\mathrm d[\mathrm{Cl}]/\mathrm dt\simeq0$.
This does not mean the <free radicals> are absent or that the whole mixture is at equilibrium. <hydrogen> and <chlorine> <molecules> and the <hydrogen chloride> product generally cannot all be assigned steady concentrations over appreciable conversion. The inert-body <concentration> can be treated as an imposed or slowly varying bath quantity; it is not an intermediate to which a <free radical> balance must be applied.
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