Chemistry 9701 · AS & A Level · Equilibria

Equilibria — practice question

Hydrogen can be obtained from natural gas by means of the reaction between hydrogen sulfide and methane: $2\text{H}_2\text{S(g)} + \text{CH}_4\text{(g)} \rightleftharpoons \text{CS}_2\text{(g)} + 4\text{H}_2\text{(g)}$.
(a)[2]

Write the expression for $K_p$ for this reaction, and state its units.

(b(i))[2]

Calculate the equilibrium partial pressures of $\text{H}_2\text{S(g)}$ and $\text{H}_2\text{(g)}$.

(b(ii))[1]

Calculate $K_p$ at this temperature.

(b)

At $1000\,\text{K}$, the starting partial pressures of the two gases in a mixture are given below. $\text{H}_2\text{S}(g)\;200\,\text{atm}$ $\text{CH}_4(g)\;100\,\text{atm}$ The mixture is then allowed to reach equilibrium. It is observed that the equilibrium partial pressure of $\text{CS}_2(g)$ is $2\,\text{atm}$ and that of the unreacted $\text{CH}_4(g)$ is $98\,\text{atm}$.

(c(i))[1]

Predict the sign of $\Delta S^\circ$ for this reaction. Explain your answer. $\text{2H}_2\text{S}(g) + \text{CH}_4(g) \rightleftharpoons \text{CS}_2(g) + 4\text{H}_2(g)$ $\Delta H^\circ = +241\,\text{kJ mol}^{-1}$

(c(ii))[2]

For this reaction at $1000\,\text{K}$, the free energy change, $\Delta G^\circ$, is $+51\,\text{kJ mol}^{-1}$. Calculate the value of $\Delta S^\circ$ for this reaction, stating its units.

(d)[2]

How would $\Delta G^\circ$, and therefore the spontaneity (feasibility) of this reaction, alter as the temperature increases? Explain your answer.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: Equilibrium constant expression: $K_p = \dfrac{p(\text{CS}_2)\, p(\text{H}_2)^4}{p(\text{H}_2\text{S})^2 \, p(\text{CH}_4)}$

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