Edexcel IAL Chemistry 13.3 Calculating Kc and Kp

Apply stoichiometric changes to reach equilibrium, divide by volume for concentrations or multiply mole fraction by total pressure, then substitute with units.

Exam points

Complete initial-change-equilibrium tables from one measured equilibrium amount.
Calculate gas partial pressures from mole fractions and total pressure before finding Kp.
Calculate Kc or convert between Kc and Kp with appropriate units and precision.

[Maximum number: 4]

Ethyl propanoate, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}$ , smells of pineapple and is used as a flavouring. It may be hydrolysed using hydrochloric acid as a catalyst to produce propanoic acid and ethanol.

\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}(\mathrm{l})+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{l})

A mixture was prepared using 0.100 mol of ethyl propanoate and 0.200 mol of water containing the catalyst.
The mixture was left to reach equilibrium at $25^{\circ} \mathrm{C}$ .
The equilibrium mixture contained 0.0440 mol of propanoic acid.

Calculate the value for $K_{\mathrm{c}}$ for this equilibrium at $25^{\circ} \mathrm{C}$ .

Give your answer to an appropriate number of significant figures.

Example of calculation:

(4)

- expression for $K_{\mathrm{c}}$

(1)

$K_{\mathrm{c}}=\frac{\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right]}{\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\right]\left[\mathrm{H}_{2} \mathrm{O}\right]}$

Ignore state symbols Brackets must be square brackets

- calculation of equilibrium quantities of reactants

(1)

$\begin{aligned} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3} =(0.100-0.0440) =0.056(0)(\mathrm{mol}) \mathrm{H}_{2} \mathrm{O}=(0.2-0.0440) =0.156(\mathrm{~mol}) \end{aligned}$

- calculation of equilibrium concentrations of
products

(1)

$\begin{aligned} {\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=0.044(0) / \mathrm{V}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)} {\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right]=0.044(0) / \mathrm{V}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)} \end{aligned}$ $K_{\mathrm{c}}=\frac{0.044 \times 0.044}{0.056 \times 0.156}=0.22161=0.222 / 0.22$

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13.3—A value, with units where appropriate, for the equilibrium constants (Kc and Kp) for homogeneous and heterogeneous question 1