Chemistry 9701 · AS & A Level · Nitrogen and sulfur

Nitrogen and sulfur — practice question

At $450\,\text{K}$ phosphorus(V) chloride, PCl$_5$(g), breaks down to give phosphorus(III) chloride, PCl$_3$(g), and chlorine, Cl$_2$(g). A dynamic equilibrium is set up, as shown. $\text{PCl}_5$(g) $\rightleftharpoons$ $\text{PCl}_3$(g) + \text{Cl}_2$(g)$ $\Delta H = +124\,\text{kJ mol}^{-1}$
(a)[1]

The enthalpy change of formation of PCl$_3$(g) under these conditions is provided. $\Delta H_f$ PCl$_3$(g) $= -320\,\text{kJ mol}^{-1}$. Calculate the enthalpy change of formation of PCl$_5$(g) under these conditions. Include a sign with your answer.

(b(i))[2]

State and explain how increasing temperature affects the rate of decomposition of PCl$_5$(g).

(b(ii))[2]

State and explain how increasing temperature affects the percentage of PCl$_5$(g) that decomposes.

(c)[2]

Explain the meaning of the term dynamic equilibrium and the conditions needed for it to be established.

(d(i))[2]

At $450\,\text{K}$ and $1.00 \times 10^5\,\text{Pa}$, when $2.00\,\text{mol}$ of $\text{PCl}_5(g)$ decompose, the equilibrium mixture contains $0.800\,\text{mol}$ of $\text{Cl}_2(g)$. Calculate the partial pressure of phosphorus(V) chloride, $p\text{PCl}_5$, in this equilibrium mixture.

(d(ii))[1]

Write the equilibrium constant expression, $K_p$, for the decomposition of $\text{PCl}_5(g)$.

(d(iii))[2]

The partial pressures of $\text{PCl}_3(g)$ and $\text{Cl}_2(g)$ in this equilibrium mixture are each $2.86 \times 10^4\,\text{Pa}$. Calculate the value of $K_p$ and state its units.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correct answer is $-444$

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