Edexcel IAL Chemistry 13.2 Kp expressions

Use p(species), not concentration brackets, raise every gaseous partial pressure to its coefficient and omit solids and liquids from the expression.

Exam points

Write Kp from gaseous partial pressures raised to stoichiometric powers.
Omit solid and liquid species while retaining every gas in a heterogeneous Kp expression.
Derive Kp units from the net power of atm after simplifying numerator and denominator.

[Maximum number: 1]

This question is about sulfuric acid and its salts.

The manufacture of sulfuric acid involves the equilibrium

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-197 \mathrm{~kJ} \mathrm{~mol}^{-1}

Write the expression for the equilibrium constant, $K_{p}$ , for this equilibrium. State symbols are not required.

expression for $K_{\mathrm{p}}$

Example of expression for $K_{\mathrm{p}}$ :
$\begin{aligned} K_{\mathrm{p}} & =\frac{p\left(\mathrm{SO}_{3}(\mathrm{~g})\right)^{2}}{p\left(\mathrm{SO}_{2}(\mathrm{~g})\right)^{2}(\mathrm{x}) p\left(\mathrm{O}_{2}(\mathrm{~g})\right)} \end{aligned}$

Allow P / P P / p p etc for partial pressure and this can be inside the brackets

Allow e.g. $p^{2} \mathrm{SO}_{3}$

Ignore missing (g) / brackets around formulae

Do not award square brackets

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13.2—Deduce an expression for Kp for homogeneous and heterogeneous systems, in terms of equilibrium partial pressures in atm question 1