Chemistry 9701/43 — May/June 2022

Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme

questions

100

marks

120

minutes

Topics Electrochemistry · Introduction to A Level Organic Chemistry · Equilibria · Transition Elements · Hydrocarbons · Carboxylic Acids and Derivatives · +7 more

Q1Group 2EquilibriaChemical EnergeticsFree sample

(a)

The solubility of the Group 2 sulfates decreases down the group.

Explain this trend.

(b)

Describe what is observed when magnesium and barium are reacted separately with an excess of dilute sulfuric acid.

magnesium

barium

(c)

The solubility product, $K_{\text{sp}}$ , of $\text{BaSO}_4$ is $1.08 \times 10^{-10}\text{ mol}^2\text{dm}^{-6}$ at $298\text{ K}$ .

Calculate the solubility of $\text{BaSO}_4$ in $\text{g per } 100\text{cm}^3$ of solution.

solubility of $\text{BaSO}_4 =$ .............................. $\text{g per } 100\text{cm}^3\text{ of solution}$

(d)

(i)

The equation for the formation of a gaseous sulfate ion is shown.

\text{S(s)} + 2\text{O}_2(\text{g}) + 2\text{e}^- \rightarrow \text{SO}_4^{2-}(\text{g}) \quad \Delta H = \Delta H_{\text{f}}^\ominus \text{ of } \text{SO}_4^{2-}(\text{g})

Calculate the standard enthalpy change of formation, $\Delta H_{\text{f}}^\ominus$ , of $\text{SO}_4^{2-}(\text{g})$ . It may be helpful to draw a labelled energy cycle. Use relevant data from Table 1.1 in your calculations.

Table 1.1

energy change	value / $\text{kJ}\text{ mol}^{-1}$
lattice energy of barium sulfate, $\text{BaSO}_4(\text{s})$	$-2469$
standard enthalpy change of formation of barium sulfate	$-1473$
standard enthalpy change of atomisation of barium	$+180$
first ionisation energy of barium	$+503$
second ionisation energy of barium	$+965$
standard enthalpy change of atomisation of sulfur	$+279$
standard enthalpy change for $\text{S(g)} \rightarrow \text{S}^{2-}(\text{g})$	$+440$
standard enthalpy change for $\text{O(g)} \rightarrow \text{O}^{2-}(\text{g})$	$+657$
$\text{O=O}$ bond energy	$+496$

$\Delta H_{\text{f}}^\ominus \text{ of } \text{SO}_4^{2-}(\text{g}) =$ .............................. $\text{kJ}\text{ mol}^{-1}$

(ii)

Suggest how the lattice energy of $\text{BaSO}_4(\text{s})$ differs from the lattice energy of $\text{Cs}_2\text{SO}_4(\text{s})$ . Explain your answer.

(e)

The reaction of solid hydrated barium hydroxide, $\text{Ba(OH)}_2\cdot 8\text{H}_2\text{O}$ , with ammonium salts is endothermic.

(i)

Calculate the minimum temperature at which the reaction of $\text{Ba(OH)}_2\cdot 8\text{H}_2\text{O}$ with $\text{NH}_4\text{NO}_3$ becomes feasible. Show all your working.

\begin{aligned} \text{Ba(OH)}_2\cdot 8\text{H}_2\text{O(s)} + 2\text{NH}_4\text{NO}_3(\text{s}) &\rightarrow 2\text{NH}_3(\text{g}) + \text{Ba(NO}_3)_2(\text{s}) + 10\text{H}_2\text{O(l)} & \Delta H_r^\ominus &= +132\text{ kJ}\text{ mol}^{-1} \\ & & \Delta S^\ominus &= +616\text{ J}\text{ K}^{-1}\text{mol}^{-1} \end{aligned}

temperature = .............................. $^\circ\text{C}$

(ii)

Barium hydroxide reacts readily with ammonium chloride on mixing at room temperature.

\text{Ba(OH)}_2\cdot 8\text{H}_2\text{O(s)} + 2\text{NH}_4\text{Cl(s)} \rightarrow 2\text{NH}_3(\text{g}) + \text{BaCl}_2\cdot 2\text{H}_2\text{O(s)} + 8\text{H}_2\text{O(l)} \quad \Delta H_r^\ominus = +133\text{ kJ}\text{ mol}^{-1}

Some relevant standard entropies are given in Table 1.2.

Table 1.2

substance	$\text{Ba(OH)}_2\cdot 8\text{H}_2\text{O(s)}$	$\text{NH}_4\text{Cl(s)}$	$\text{NH}_3(\text{g})$	$\text{BaCl}_2\cdot 2\text{H}_2\text{O(s)}$	$\text{H}_2\text{O(l)}$
$S^\ominus / \text{J}\text{ K}^{-1}\text{mol}^{-1}$	$427$	$95$	$192$	$203$	$70$

Calculate the standard Gibbs free energy change, $\Delta G^\ominus$ , for this reaction at $25^\circ\text{C}$ .

$\Delta G^\ominus =$ .............................. $\text{kJ}\text{ mol}^{-1}$

The rest of this paper

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