Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2025 / 5 / c / Solution

This labelled part introduces the reversible reaction pair without a separate calculation to perform. For the following derivations, define its forward and reverse elementary reaction rates asand the initiating rate as $r_1=k_1[\mathrm{N}{\mathrm{O}}_2][{\mathrm{O}}_3]$. The law of mass action gives $k_2$ bimolecular units and $k_{-2}$ first-order units. Reversibility does not require $r_2=r_{-2}$ while the complete network is reacting: that equality would impose chemical equilibrium on the pair, an extra assumption not needed here.