IB Chemistry SL 2.3.2 Equilibrium Law Questions

Use real SL exam questions to write and apply Kc expressions from balanced homogeneous equilibrium equations.

Exam points

Write a Kc expression using product concentrations over reactant concentrations with powers taken from the balanced equation.
Calculate or transform Kc by substituting equilibrium concentrations or reversing/scaling the balanced equation while keeping stoichiometric exponents consistent.

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The reaction between ethanoic acid and ethanol is homogeneous and reversible.

\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \quad \Delta H^{\theta}{ }_{\mathrm{r}}=-4 \mathrm{~kJ} \mathrm{~mol}^{-1}

Deduce the expression for the equilibrium constant, K, for this reaction.

$\langle\mathrm{K}=\rangle \frac{\left[\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\right]\left[\mathrm{H}_{2} \mathrm{O}\right]}{\left[\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}\right]\left[\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}\right]}$

Marking guidance:

Do not accept expression without
$\mathrm{H}_{2} \mathrm{O}$ .

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IB Chemistry SL 2.3.2 Equilibrium Law Questions question 1