CAIE A-Level Chemistry 7.1.6 Equilibrium Constant Kp Expressions

Practise constructing Kp expressions from gaseous equilibria using partial pressures and stoichiometric powers.

Exam points

place gaseous product partial pressures over gaseous reactant partial pressures with coefficient powers
exclude solids, liquids and aqueous species because Kp contains only gaseous partial pressures
deduce Kp units from the overall pressure exponent, including negative or zero net powers

[Maximum number: 1]

Ammonia, $\mathrm{NH}_{3}$ , is manufactured from nitrogen and hydrogen by the Haber process.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

In another equilibrium mixture the partial pressures are as shown.

Write the expression for the equilibrium constant, $K_{\mathrm{p}}$ , for the production of ammonia from nitrogen and hydrogen.

K_{p}=

$\mathrm{K}_{\mathrm{p}}=\frac{\mathrm{pNH}_{3}{ }^{2}}{\mathrm{pN}_{2} \times \mathrm{pH}_{2}{ }^{3}}$

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