Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2023 / Paper 1 / 5 / c / iii

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 $\mathrm{d}[\mathrm{H}]/\mathrm{d}t\simeq 0$ and $\mathrm{d}[\mathrm{Cl}]/\mathrm{d}t\simeq 0$.

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.