AS & A Level Chemistry - 9701 Paper 4 2022 Spring Zone 2

Iodine is found naturally in compounds in many different oxidation states.
(a) Iodide ions, $\text{I}^-$, react with acidified $\text{H}_2\text{O}_2$(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 $\text{H}_2\text{O}_2$(aq) in its reaction with $\text{I}^-$ ions in acidic conditions.

Write an ionic equation for the reaction.

role ...............................................................
ionic equation ....................................................
.....................................................................
[2]

(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 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, $K_{pc}$, is 93.8.

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

mass of $\text{I}_2$ in aqueous layer = .............................. g [2]

(iii) Suggest how the value of $K_{pc}$ 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.

.....................................................................
.....................................................................
[2]

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

(i) Define enthalpy change of solution.
.....................................................................
[1]

(ii) Use the data in Table 1.1 to calculate the enthalpy change of solution of potassium iodide, KI.

Table 1.1

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

enthalpy change of solution = .......................... kJ mol$^{-1}$ [1]

(iii) Suggest the trend in the magnitude of the lattice energies of the Group 1 iodides, LiI, NaI, KI.
Explain your answer.
.....................................................................
.....................................................................
[2]

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

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

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.

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

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

Calculate the concentration of $\text{Cu}^{2+}$(aq) in the original solution.
concentration of $\text{Cu}^{2+}$(aq) = ............................ mol dm$^{-3}$ [2]

(ii) Identify a suitable indicator for the titration.
.....................................................................
[1]

(iii) Copper(I) and copper(II) both contain electrons in all five 3d orbitals.
Sketch the shape of a 3d$_{xy}$ orbital on the axes provided.

[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.
.....................................................................
[2]

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

Calculate the solubility product, $K_{sp}$, of $\text{HgI}_2$. Include units in your answer.
[ $M_r$: $\text{HgI}_2$, 454.4 ]

value of $K_{sp}$ = ............................
units = ............................
[3]

Silicon is the second most abundant element by mass in the Earth's crust.
(a) In industry, silicon is extracted from SiO$_2$ by reaction with carbon at over 2000 °C.
reaction 1 SiO$_2$(s) + 2C(s) → Si(l) + 2CO(g)
(i) Explain why the entropy change, ΔS, of reaction 1 is positive.
........................................................................................................................................... [1]
(ii) Reaction 1 is highly endothermic. Suggest the effect of an increase in temperature on the feasibility of this reaction. Explain your answer.
........................................................................................................................................... [2]

(b) Silicon is purified by first heating it in a stream of HCl(g) to form SiHCl$_3$. The SiHCl$_3$ formed is then distilled to remove other impurities.
reaction 2 Si(s) + 3HCl(g) → SiHCl$_3$(g) + H$_2$(g)
(i) Table 2.1 shows some standard entropy data.
[Table_1]
Use the data in Table 2.1 to calculate ΔS° for reaction 2.
ΔS° = ............................... JK⁻¹mol⁻¹ [2]
(ii) Reaction 3 is the reverse of reaction 2 and is used to obtain pure silicon.
reaction 3 SiHCl$_3$(g) + H$_2$(g) → Si(s) + 3HCl(g) ΔH = +219.3kJmol⁻¹
Use this information and your answer to (b)(i) to calculate the temperature, in K, at which reaction 3 becomes feasible. Show your working.
[If you were unable to answer (b)(i), you should use ΔS° = -150 JK⁻¹mol⁻¹ for reaction 2. This is not the correct answer to (b)(i).]
temperature = ............................... K [2]

(c) Silicon can also be produced by electrolysis of SiO$_2$ dissolved in molten CaCl$_2$. The relevant half-equation for the cathode is shown.
SiO$_2$ + 4e⁻ → Si + 2O²⁻
Calculate the time, in seconds, required to produce 1.00 g of Si by this electrolysis, using a current of 6.00A. Assume no other substances are produced at the cathode.
time = ............................... s [2]

[Total: 9]

Titanium is a transition element in Period 4. It is commonly found as TiO₂ in minerals.
(a) (i) Define transition element.
........................................................................................................................ [1]
(ii) Identify two typical properties of transition elements.
........................................................................................................................ [1]
(b) The TiO₂²⁺ ion forms when TiO₂ reacts with an excess of sulfuric acid.
TiO₂²⁺ can be reduced by zinc metal in acidic conditions to form a purple solution containing Ti³⁺(aq).
(i) TiO₂²⁺(aq) is a colourless ion.
Suggest why.
........................................................................................................................ [2]
(ii) Give the electronic configuration of an isolated Ti³⁺ ion.
1s² ........................................................................................................................ [1]
(iii) Write an ionic equation for the reduction of TiO₂²⁺ by zinc metal in acidic conditions.
........................................................................................................................ [1]
(c) Acidified Ti³⁺(aq) reacts with oxygen dissolved in water as shown.
4Ti³⁺ + O₂ + 2H₂O → 4TiO₂²⁺ + 4H⁺   ΔG^o = -436.1 kJ mol^-1
The standard reduction potential, E^o, of O₂ + 4H⁺ + 4e^- ↔ 2H₂O is +1.23 V.
(i) Calculate the standard reduction potential, E^o, in V, of the TiO₂²⁺/Ti³⁺(aq) half-cell. Show your working.
E^o = ............................. V [3]
(ii) When aqueous citrate ions, C₆H₅O₇^3-, are added to Ti³⁺(aq), the [Ti(C₆H₅O₇)₂]^3-(aq) complex forms.
Explain, in terms of d-orbitals, why Ti³⁺ is able to form complex ions.
........................................................................................................................ [1]
(iii) Acidified [Ti(C₆H₅O₇)₂]^3-(aq) does not react with oxygen dissolved in water, unlike acidified Ti³⁺(aq).
Suggest what this means for the value of the standard reduction potential, E^o, of the following half-cell.
[Ti(C₆H₅O₇)₂]^2-(aq) + e^- ↔ [Ti(C₆H₅O₇)₂]^3-(aq)
Explain your answer.
........................................................................................................................ [1]
(d) Some reactions of TiO₂ are shown in Fig. 3.1.
The anion, acac^-, is a bidentate ligand.



(i) The titanium ions in TiF₆^2- and Ti(acac)₂Cl₂ have a coordination number of 6.
State what is meant by coordination number.
........................................................................................................................ [1]
(ii) Write an equation for the formation of TiF₆^2- from TiO₂.
........................................................................................................................ [1]
(iii) State what is meant by bidentate ligand.
........................................................................................................................ [2]
(iv) Ti(acac)₂Cl₂ shows both optical and geometrical (cis/trans) isomerism.
Ti(acac)₂Cl₂ exists as three stereoisomers.
The structure of one stereoisomer of Ti(acac)₂Cl₂ is shown in Fig. 3.2.



Complete the structures of the other two stereoisomers of Ti(acac)₂Cl₂.





[2]
(v) The acac^- anion is symmetrical.
Deduce which, if any, of stereoisomers 1, 2 and 3 in (d)(iv) are polar. Explain your answer.
........................................................................................................................ [2]
[Total: 19]

Compounds F and J are shown in Fig. 4.1.

Fig. 4.1

(a) F and J both contain the arene functional group.

(i) Identify the other functional groups in F and J.
F: ................................................................................................................
J: ................................................................................................................
[2]

(ii) State the number of chiral centres in a molecule of F and in a molecule of J.
number of chiral centres in: F = .......................................... J = ..........................................
[1]

(b) A student proposes a multi-step synthesis of F from benzene, as shown in Table 4.1.

(i) Complete Table 4.1 by providing relevant details of the reagents and conditions for steps 1 and 4, and the structure of product D.

[3]

(ii) In a second multi-step synthesis, the student changes the order in which the reagents and conditions are used. The reaction scheme is shown in Fig. 4.2. G is the major product of this synthesis.

Fig. 4.2

Draw the structure of G.
Explain why G is the major product of the synthesis rather than E.
.......................................................... .......................................................... .......................................................... ..........................................................
[2]

(c) J reacts under suitable conditions with NaOH(aq). After acidification of the reaction mixture, compounds K and L form.
Fig. 4.3
(i) Give the molecular formula of L.
..............................................................................................................
[1]

(ii) State the two types of reaction that occur when J reacts with NaOH(aq).
............................................................................................................
[2]

(d) K can also be synthesised from phenol, C$_6$H$_5$OH.
Fig. 4.4 shows several reactions of phenol.
Fig. 4.4
(i) Write an equation for the formation of M in reaction 1.
..............................................................................................................
[1]

(ii) Draw N, the product of reaction 2.
..............................................................................................................
[1]

(iii) Explain why phenol is a weaker acid than K.
..............................................................................................................
[2]

(e) Phenol and benzene both react with nitric acid, as shown in Fig. 4.5.
Fig. 4.5
Explain why the reagents and conditions for these two reactions are different.
..............................................................................................................
[3]
[Total: 18]

2-Chloropropanoic acid, CH₃CHClCOOH, is used in many chemical syntheses.
(a) (i) An equilibrium is set up when CH₃CHClCOOH is added to water.
Write the equation for this equilibrium. ................................................................................................................................................. [1]
(ii) 0.150 mol of CH₃CHClCOOH dissolves in 250 cm³ of distilled water to produce a solution of pH 1.51. Calculate the $pK_a$ of CH₃CHClCOOH. $pK_a =$ ................................. [2]
(iii) An equal concentration of aqueous propanoic acid has pH 2.55.
Explain the difference in the pH of solutions of equal concentration of CH₃CHClCOOH and propanoic acid. ................................................................................................................................................. [2]
(b) When CH₃CHClCOOH reacts with aqueous NH₃, alanine forms.
Fig. 5.1
Alanine is an amino acid. Its isoelectric point is 6.1.
(i) State what is meant by isoelectric point. ................................................................................................................................................. [1]
(ii) Give the structural formula of alanine at pH 2. ................................................................................................................................................. [1]
(iii) Alanine exists as a pair of optical isomers. The structure of one optical isomer is shown in Fig. 5.2. Draw the three-dimensional structure of the other optical isomer of alanine.
Fig. 5.2
[1]
(iv) Polymer C forms from the reaction between alanine and 4-aminobutanoic acid, H₂N(CH₂)₃COOH. Draw a repeat unit of C. The functional group formed should be displayed. ......................................................................................................................... [2]
(v) State the type of polymerisation shown in (b)(iv). ................................................................................................................................................. [1]
(vi) Scientists are investigating C as a replacement for poly(propene) in packaging. Suggest an advantage of using C instead of poly(propene). ................................................................................ [1]
(c) A student studies the reaction of CH₃CHClCOOH with aqueous NH₃ to determine the reaction mechanism.
The student finds that when CH₃CHClCOOH and NH₃ are added in a 1:1 stoichiometric ratio, the conjugate acid and base of the reactants are quickly formed.
reaction 1 CH₃CHClCOOH + NH₃ → CH₃CHClCOO⁻ + NH₄⁺
(i) Identify the conjugate acid–base pairs in reaction 1.
conjugate acid–base pair I .......................................... and ..........................................
conjugate acid–base pair II ......................................... and ..........................................
[1]
In an excess of NH₃, CH₃CHClCOO⁻ undergoes a nucleophilic substitution reaction.
reaction 2 CH₃CHClCOO⁻ + NH₃ → CH₃CH(NH₂)COO⁻ + H⁺ + Cl⁻
A student investigates the rate of reaction 2. The student mixes CH₃CHClCOO⁻ with a large excess of NH₃. The graph in Fig. 5.3 shows the results obtained.
Fig. 5.3
(ii) Use the graph in Fig. 5.3 to show that reaction 2 is first order with respect to [CH₃CHClCOO⁻]. ............................................................................................................................... [2]
(iii) Explain why a large excess of NH₃ needs to be used in order to obtain the results in Fig. 5.3. ............................................................................................................................... [1]
(iv) The student measures the effect of changing the concentration of NH₃ on the rate of reaction 2. Table 5.1 shows the results obtained.
Use the information in Table 5.1 and in (c)(ii) to determine whether the nucleophilic substitution reaction proceeds via an $S_N1$ or an $S_N2$ mechanism. Explain your answer. ............................................................................................................................... [2]
(v) Describe the effect of an increase in temperature on the rate of reaction of CH₃CHClCOO⁻ and NH₃. Explain your answer. ............................................................................................................................... [2]
(vi) When an excess of CH₃CHClCOO⁻ is used, further substitution reactions occur. One product has the formula C₆H₉NO₄²⁻. Suggest the structure of C₆H₉NO₄²⁻. ............................................................................................................................... [1]
[Total: 21]

Lidocaine is used as an anaesthetic. A synthesis of lidocaine is shown in Fig. 6.1.

(a) W can be formed by reacting HOCH$_2$COOH with an excess of SOCl$_2$.
Write an equation for this reaction.
[1]
(b) After W and X have reacted together, an excess of CH$_3$COONa(aq) is added to the reaction mixture.
Suggest why.
[1]
(c) The reaction of W with X, reaction 1, follows an addition-elimination mechanism.
Complete the mechanism for the reaction of W with X.
Include all relevant curly arrows, lone pairs of electrons, charges and partial charges. Use Ar-NH$_2$ to represent X.

[4]
(d) (C$_2$H$_5$)$_2$NH reacts with Y in reaction 2.
Explain why (C$_2$H$_5$)$_2$NH can act as a nucleophile.
[1]
(e) The purity of lidocaine can be checked using thin-layer chromatography. Ethyl ethanoate is used as the solvent.
The R$_f$ values of X and lidocaine are given in Table 6.1.
[Table_1]
(i) Identify the substances used as the mobile and stationary phases in this thin-layer chromatography experiment.
mobile phase .................................................................
stationary phase .................................................................
[1]
(ii) Describe how an R$_f$ value can be calculated.
..............................................................................
[1]
(iii) Suggest why the R$_f$ value for X is less than that for lidocaine.
................................................................................
[1]
(f) The proton (1H) NMR spectrum of lidocaine is shown in Fig. 6.2.

Table 6.2
[Table_2]
(i) Name the splitting patterns at δ 2.6 and δ 1.1.
δ 2.6 ........................................ δ 1.1 ..........................................
[1]
(ii) The relative peak area of the peaks at δ 3.0 and δ 2.3 is 1:3 respectively.
Identify the protons in the 1H NMR spectrum of lidocaine that are responsible for the peaks at the following chemical shift values.
δ 7.1 ..............................................................
δ 3.0 ..............................................................
δ 2.3 ..............................................................
[2]
(iii) Predict the number of peaks in the carbon-13 (13C) NMR spectrum of lidocaine.
..............................................................................
[1]
[Total: 14]