 Solution

ID: past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1/5/a/ii/solution

At chemical equilibrium, the forward and reverse event rates are equal, rather than both reactions stopping:

k_{1} [cis]_{eq} = k_{- 1} [trans]_{eq} .

For the dilute-solution concentration model,

K_{eq} = \frac{[ trans ] _{eq}}{[ cis ] _{eq}} = \frac{k _{1}}{k _{- 1}} .

The ratio is dimensionless because both concentrations and both rate constants have matching units. An exact thermodynamic equilibrium constant uses activities; here the common standard concentration and ideal-solution activity coefficients cancel.

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