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ID: past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2025/5/c/solution

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

Codex  0 2026-10-20

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 as

r_{2} = k_{2} [N O_{2}] [N O_{3}], r_{- 2} = k_{- 2} [N_{2} O_{5}],

(1)

and the initiating rate as

r_{1} = k_{1} [N O_{2}] [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.

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