CAIE A-Level Chemistry 7.1.7 Kc and Kp Calculations

Practise finding equilibrium concentrations or partial pressures and substituting them into Kc and Kp expressions.

Exam points

use stoichiometric changes to find every equilibrium amount before calculating concentration or pressure
convert equilibrium moles to concentrations with the stated volume or to partial pressures with mole fractions
substitute into the correct Kc or Kp expression and report the value with derived units when required

[Maximum number: 2]

Hydrogen halides are compounds formed when halogens (Group 17 elements) react with hydrogen. The bond polarity of the hydrogen halides decreases from HF to HI.

Some relevant data are shown in the table.

Table for Question 7.1.7—The Kc and Kp expressions to carry out question 1 — CAIE A-Level Chemistry AS

Hydrogen chloride undergoes a reversible reaction with oxygen.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

The reaction is carried out at $400^{\circ} \mathrm{C}$ in the presence of a copper(II) chloride catalyst.

In a separate experiment, an equilibrium reaction mixture was found to contain the four gases at the partial pressures shown in the table.

K_{\mathrm{p}}=\frac{\left(p_{\mathrm{Cl}_{2}}\right)^{2} \times\left(p_{\mathrm{H}_{2} \mathrm{O}}\right)^{2}}{\left(p_{\mathrm{HCl}}\right)^{4} \times p_{\mathrm{O}_{2}}}

Use this information and the expression given for $K_{p}$ to calculate a value for $K_{p}$ . State the units of $K_{\mathrm{p}}$ .

\begin{array}{r} K_{\mathrm{p}}= \\ \text { units }= \end{array}

$K_p=\frac{(3.6\times10^4)^2\times(3.6\times10^4)^2}{(4.8\times10^4)^4\times3.0\times10^4}=1.05\times10^{-5}$
Units $=\mathrm{Pa^{-1}}$ .

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7.1.7—The Kc and Kp expressions to carry out question 1