Chemistry 9701/42 — February/March 2022

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

questions

100

marks

120

minutes

Topics Introduction to A Level Organic Chemistry · Electrochemistry · Nitrogen Compounds · Equilibria · Chemical Energetics · Transition Elements · +6 more

Q1ElectrochemistryEquilibriaIntroduction to A Level Organic ChemistryChemical EnergeticsTransition ElementsAnalytical TechniquesReaction KineticsFree sample

Iodine is found naturally in compounds in many different oxidation states.

(a)

Iodide ions, $\text{I}^-$ , react with acidified $\text{H}_2\text{O}_2(\text{aq})$ to form iodine, $\text{I}_2$ , and water.
This reaction mixture is shaken with cyclohexane, $\text{C}_6\text{H}_{12}$ , to extract the $\text{I}_2$ .
Cyclohexane is immiscible with water.

(i)

Identify the role of $\text{H}_2\text{O}_2(\text{aq})$ in its reaction with $\text{I}^-$ ions in acidic conditions.

Write an ionic equation for the reaction.

(ii)

$15.0\text{ cm}^3$ of $\text{C}_6\text{H}_{12}$ is shaken with $20.0\text{ cm}^3$ of an aqueous solution containing $\text{I}_2$ until no further change is seen.
It is found that $0.390\text{ g}$ of $\text{I}_2$ is extracted into the $\text{C}_6\text{H}_{12}$ .
The partition coefficient of $\text{I}_2$ between $\text{C}_6\text{H}_{12}$ and water, $K_{\text{pc}}$ , is $93.8$ .

Calculate the mass of $\text{I}_2$ that remains in the aqueous layer.
Show your working.

(iii)

Suggest how the value of $K_{\text{pc}}$ of $\text{I}_2$ between hexan-2-one, $\text{CH}_3(\text{CH}_2)_3\text{COCH}_3$ , and water compares to the value given in (a)(ii).
Explain your answer.

(b)

The Group 1 iodides all form stable ionic lattices and are soluble in water.

(i)

Define enthalpy change of solution.

(ii)

Use the data in Table 1.1 to calculate the enthalpy change of solution of potassium iodide, $\text{KI}$ .

Table 1.1

process	enthalpy change, $\Delta H / \text{kJ mol}^{-1}$
$\text{K}^+(\text{g}) + \text{I}^-(\text{g}) \rightarrow \text{KI}(\text{s})$	$-629$
$\text{K}^+(\text{g}) \rightarrow \text{K}^+(\text{aq})$	$-322$
$\text{I}^-(\text{g}) \rightarrow \text{I}^-(\text{aq})$	$-293$

(iii)

Suggest the trend in the magnitude of the lattice energies of the Group 1 iodides, $\text{LiI}$ , $\text{NaI}$ , $\text{KI}$ .
Explain your answer.

(c)

The concentration of $\text{Cu}^{2+}(\text{aq})$ in a solution can be determined by the reaction of $\text{Cu}^{2+}$ ions with $\text{I}^-$ ions.

$\text{reaction 1} \quad 2\text{Cu}^{2+} + 4\text{I}^- \rightarrow 2\text{CuI} + \text{I}_2$

The $\text{I}_2$ produced in reaction 1 is titrated against a solution containing thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ , using a suitable indicator.

$\text{reaction 2} \quad 2\text{S}_2\text{O}_3^{2-} + \text{I}_2 \rightarrow \text{S}_4\text{O}_6^{2-} + 2\text{I}^-$

(i)

A $25.0\text{ cm}^3$ portion of a $\text{Cu}^{2+}(\text{aq})$ solution reacts with an excess of $\text{I}^-(\text{aq})$ .
The end-point of the titration occurs when $22.30\text{ cm}^3$ of $0.150\text{ mol dm}^{-3}$ $\text{S}_2\text{O}_3^{2-}(\text{aq})$ is added.

Calculate the concentration of $\text{Cu}^{2+}(\text{aq})$ in the original solution.

(ii)

Identify a suitable indicator for the titration.

(iii)

Copper(I) and copper(II) both contain electrons in all five 3d orbitals.

Sketch the shape of a $3\text{d}_{xy}$ orbital on the axes provided.

Fig. 1.1

(d)

The reaction of $\text{I}^-$ ions with persulfate ions, $\text{S}_2\text{O}_8^{2-}$ , can be catalysed by $\text{Fe}^{3+}$ ions.

$2\text{I}^- + \text{S}_2\text{O}_8^{2-} \rightarrow \text{I}_2 + 2\text{SO}_4^{2-}$

Write equations to show how $\text{Fe}^{3+}$ catalyses this reaction.

(e)

An orange precipitate of $\text{HgI}_2$ forms when $\text{Hg}^{2+}$ ions are added to $\text{KI}(\text{aq})$ .
The solubility of $\text{HgI}_2$ at $25^\circ\text{C}$ is $1.00 \times 10^{-7}\text{ g dm}^{-3}$ .

Calculate the solubility product, $K_{\text{sp}}$ , of $\text{HgI}_2$ .
Include units in your answer.

[ $M_{\text{r}}$ : $\text{HgI}_2$ , $454.4$ ]

The rest of this paper

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