AS & A Level Chemistry - 9701 Paper 4 2023 Summer Zone 1

(a) Group 2 nitrates decompose when heated. Describe how the thermal stability of Group 2 nitrates changes with increasing proton number. Explain your answer.
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(b) Copper(II) nitrate decomposes in a similar manner to Group 2 nitrates. Write an equation for the decomposition of Cu(NO_3)_2.
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(c) Cu(NO_3)_2 is added to water to form solution A.
Fig. 1.1 shows some reactions of solution A.

Complete Table 1.1 to show the formula and colour of each of the copper-containing species present in A, B, C and D.
[Table_1]

(d) EDTA^{4–} is a polydentate ligand.
(i) Explain what is meant by a polydentate ligand.
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[2]
(ii) Group 2 metal ions can form complexes similar to those of transition elements.
A solution of EDTA^{4–} is added to water containing [Ca(H_2O)_6]^{2+} to form a new complex, [CaEDTA]^2–, as shown.
equilibrium 1 \[ [Ca(H_2O)_6]^{2+} + EDTA^{4–} \rightleftharpoons [CaEDTA]^{2–} + 6H_2O \]
Circle on the structure of EDTA^{4–} in Fig. 1.2 the six atoms that form bonds with the metal ion.

[1]
(iii) The calcium ions in [Ca(H_2O)_6]^{2+} and [CaEDTA]^{2–} have a coordination number of 6.
Explain what is meant by coordination number.
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[1]
(iv) The complex [CaEDTA]^{2–} can be used to remove toxic metals from the body.
[Table_2]
An aqueous solution containing [CaEDTA]^{2–} is added to a solution containing equal concentrations of Cr^{3+}(aq), Fe^{3+}(aq) and Pb^{2+}(aq). The resulting mixture is left to reach a state of equilibrium.
State the type of reaction when [CaEDTA]^{2–} reacts with Cr^{3+}(aq), Fe^{3+}(aq) and Pb^{2+}(aq).
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[1]
(v) Deduce the relative concentrations of [CrEDTA]^–, [FeEDTA]^– and [PbEDTA]^{2–} present in the resulting mixture.
Explain your answer.
..............................................  > .......................................... > ...........................................
highest concentration                                           lowest concentration
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[1]

(e) The number of moles of water of crystallisation in a hydrated ionic salt can be determined by titration using aqueous EDTA^{4-} ions with a suitable indicator.
• 0.255 g of hydrated chromium(III) sulfate, Cr_2(SO_4)_3·nH_2O, is dissolved in water and made up to 100cm^3 in a volumetric flask.
• 25.0cm^3 of this solution requires 26.2cm^3 of 0.00800 mol dm^{–3} aqueous EDTA^{4–} ions to reach the end-point.
The reaction occurs as shown.
\[ [Cr(H_2O)_6]^{3+} +  EDTA^{4–} ightarrow [CrEDTA]^– + 6H_2O \]
Use the data to calculate the value of n in the formula of Cr_2(SO_4)_3·nH_2O.
Show your working.

n = .......................................................

(f) A solution of Cr^{3+}(aq) and a solution of Fe^{3+}(aq) have different colours.
Explain why the two complexes have different colours.
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(a) Some transition element complexes can show stereoisomerism.
State two types of stereoisomerism shown by transition element complexes.
1 ..........................................................................................................................
2 ..........................................................................................................................

(b) The complexes $[Pt(NH_3)_2Cl_2]$ and $[Pt(en)_2]^{2+}$ have the same geometry (shape) around the metal ion.
$[Pt(NH_3)_2Cl_2]$ exists as two stereoisomers whereas $[Pt(en)_2]^{2+}$ only has one possible structure.
State the geometry around the metal ion.
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(c) The complex $[Cr(en)_3]^{2+}$ exists as two stereoisomers whereas the complex $[Cr(OCH_2CH_2NH_2)_3]^-$ exists as four stereoisomers.
Complete the three-dimensional diagrams in Fig. 2.1 to show the four stereoisomers of $[Cr(OCH_2CH_2NH_2)_3]^-$.
Represent the ligand $-OCH_2CH_2NH_2$ by using


(d) The complex $[Cr(OCH_2CH_2NH_2)_3]^-$ is formed by reacting $Cr^{2+}(aq)$ with the conjugate base of 2-aminoethanol.
A synthesis of 2-aminoethanol is shown in Fig. 2.2.

(i) Suggest the mechanism for step 1 of the reaction of oxirane with ammonia in Fig. 2.3.
Include all relevant curly arrows, lone pairs of electrons, charges and partial charges.
Draw the structure of the organic intermediate.

(ii) A small amount of by-product E, shown in Fig. 2.4, is produced during the reaction shown in Fig. 2.2.
Suggest how the formation of by-product E can be minimised.
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(iii) Compound F, $C_4H_9NO$, can be formed from the reaction of by-product $E, C_4H_{11}NO_2$, with concentrated $H_2SO_4$.
Compound F is a saturated and basic organic compound.
Suggest a structure for compound F. State the type of reaction undergone by E to form F.

type of reaction ..................................................................

(a) Aqueous acidified iodate(V) ions, $IO_3^-$, react with iodide ions, as shown.

$$IO_3^- + 6H^+ + 5I^- \rightarrow 3I_2 + 3H_2O$$

The initial rate of this reaction is investigated. Table 3.1 shows the results obtained.

[Table_1]

Table 3.1
experiment | $[IO_3^-]/$mol dm$^{-3}$ | $[H^+]/$mol dm$^{-3}$ | $[I^-]/$mol dm$^{-3}$ | initial rate / mol dm$^{-3}$ min$^{-1}$
1 | 0.0400 | 0.0150 | 0.0250 | $4.20 \times 10^{-2}$
2 | 0.120 | to be calculated | 0.0125 | $7.09 \times 10^{-2}$

The rate equation for this reaction is rate = $k[IO_3^-][H^+]^2[I^-]^2$.

(i) Explain what is meant by order of reaction.
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(ii) Complete Table 3.2.

[Table_2]

Table 3.2

the order of reaction with respect to $[IO_3^-]$
the order of reaction with respect to $[H^+]$
the order of reaction with respect to $[I^-]$
the overall order of reaction

[1]

(iii) Use your answer to (a)(ii) to sketch lines in Fig. 3.1 to show the relationship between the initial rates and the concentrations of $[IO_3^-]$ and $[I^-]$.



Fig. 3.1

[1]

(iv) Use data from Table 3.1 to calculate the rate constant, $k$, for this reaction.
Include the units of $k$.

$k =$............................................. units............................................. [2]

(v) Use data from Table 3.1 to calculate the concentration of hydrogen ions, $[H^+]$, in experiment 2.

$[H^+] =$............................................. mol dm$^{-3}$ [1]

(vi) This reaction is repeated in two separate experiments.
The experiments are carried out at the same temperature and with the same concentrations of $I^-$ and $IO_3^-$.
One experiment takes place at pH 1.0 and the other experiment takes place at pH 2.0.
Calculate the value of $$\frac{\text{rate at pH 1.0}}{\text{rate at pH 2.0}}$$
value of $$\frac{\text{rate at pH 1.0}}{\text{rate at pH 2.0}} =$$............................................. [1]

(b) In aqueous solution, iron(III) ions react with iodide ions, as shown.

$$2Fe^{3+} + 2I^- \rightarrow 2Fe^{2+} + I_2$$

The initial rate of reaction is first order with respect to $Fe^{3+}$ and second order with respect to $I^-$.
The mechanism for this reaction has three steps.
Each step involves only two ions reacting together.
Suggest equations for the three steps of this mechanism. Identify the rate-determining step.

step 1 ....................................................................................................................................................
step 2 ....................................................................................................................................................
step 3 ....................................................................................................................................................
rate-determining step = .............................................

[3]

(a) State the hybridisation of the carbon atoms and the $\text{C–C–H}$ bond angle in benzene, $\text{C}_6\text{H}_6$. Explain how orbital overlap leads to the formation of $\sigma$ and $\pi$ bonds in benzene.
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(b) Compound $Z$ can be synthesised from benzene in three steps by the route shown in Fig. 4.1.
[Image_1: Fig. 4.1]
(i) Draw structures for $X$ and $Y$ in Fig. 4.1.
(ii) Give the reagents and conditions for steps 1, 2 and 3.
step 1 .....................................................................................................................................
step 2 .....................................................................................................................................
step 3 ....................................................................................................................................

(c) Compound $W$ is an isomer of $Z$.
[Image_2: Fig. 4.2]
Give the systematic name of $W$.
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(d) Complete Table 4.1 to show the number of peaks observed in the carbon-13 NMR spectrum for $W$ and $Z$.
[Table_1: Table 4.1]
compound   number of peaks observed
$W$                ...
$Z$                  ...

(a) The exhaust systems of most modern gasoline-fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts.
(i) Name three metal catalysts used in catalytic converters.
1 .............................. 2 .............................. 3 .............................. [1]
(ii) Explain what is meant by a heterogeneous catalyst.
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(b) The exhaust systems of many diesel-fuelled cars contain an additional system to reduce vehicle emissions. This uses a liquid that is added to the exhaust system. This liquid contains urea, $(NH_2)_2CO$, which decomposes on heating to isocyanic acid, $HNCO$, and ammonia.
reaction 1 \((NH_2)_2CO \rightarrow HNCO + NH_3\)
Isocyanic acid reacts with water vapour to form ammonia and carbon dioxide.
reaction 2 \(HNCO(g) + H_2O(g) \rightarrow NH_3(g) + CO_2(g)\)
Some values for standard enthalpy changes of formation, $\Delta H_f^\circ$, and standard entropies, $S^\circ$, are given in Table 5.1.
[Table_1]
(i) Explain what is meant by the term entropy of a system.
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(ii) Use the data in Table 5.1 to calculate $\Delta G^\circ$ for reaction 2 at 25°C. Show your working.
\(\Delta G^\circ = \text{.......................... kJ mol}^{-1}\) [4]

(c) The ammonia formed in reactions 1 and 2 can be used to remove nitrogen dioxide from exhaust emissions, as shown.
reaction 3 \(8NH_3 + 6NO_2 \rightarrow 7N_2 + 12H_2O\)
Use the equations for reactions 1, 2 and 3 to construct an overall equation for the reduction of $NO_2$ by $(NH_2)_2CO$.
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(d) Isocyanic acid, $HNCO$, can form cyanuric acid, $C_3H_3N_3O_3$, under certain conditions.
$C_3H_3N_3O_3$ has a cyclic structure containing alternating carbon and nitrogen atoms in the ring system.
Suggest a structure for cyanuric acid.
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(e) Isocyanic acid, $HNCO$, is a weak acid.
\(HNCO + H_2O \rightleftharpoons H_3O^+ + NCO^-\) \(pK_a = 3.70\text{ at 25°C}\)
(i) Write the mathematical expressions for $pK_a$ and pH.
$pK_a = ....................................................................................................$
$pH = ....................................................................................................$ [1]
(ii) Calculate the pH of $0.120\text{ mol dm}^{-3} HNCO(aq)$.
Give your answer to three significant figures.
$pH = \text{............................}$ [2]
(iii) Calculate the percentage of $HNCO$ molecules that are ionised in $0.120\text{ mol dm}^{-3} HNCO$.
percentage ionisation of $HNCO = \text{............................}$ [1]

(a) Compound H has the structural formula $CH_2=CHCH(NH_2)COOH$.
(i) Name all the functional groups in H.
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[2]
(ii) Compound H exhibits stereoisomerism.
Draw three-dimensional structures for the two stereoisomers of H. Name this type of stereoisomerism.

type of stereoisomerism ............................................................
[2]

(b) Compound H can be prepared from the reaction of J with an excess of hot aqueous acid.

(i) Complete Fig. 6.2 to show the equation for this reaction.

[1]
(ii) Name the type of reaction in (b)(i).
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[1]

(c) Polymers consist of monomers joined together by undergoing either addition or condensation polymerisation.
Compound H can react to form an addition polymer, K, or a condensation polymer, L, depending on the conditions.
(i) Draw one repeat unit of addition polymer K.
[1]
(ii) Draw two repeat units of condensation polymer L.
The new functional group formed should be displayed.
[2]
(iii) Explain why condensation polymers can normally biodegrade more readily than addition polymers.
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[1]

(a) State the relative basicities of ethanamide, diethylamine and ethylamine in aqueous solution.
Explain your answer.
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   most basic                          least basic
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(b) The amino acid alanine, $\text{H}_2\text{NCH(CH}_3\text{)COOH}$, can act as a buffer.
(i) Define a buffer solution.
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(ii) Write two equations to show how an aqueous solution of alanine can act as a buffer solution.
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(c) Glutamic acid is another amino acid that acts as a buffer.
glutamic acid


Fig. 7.1
(i) Draw the skeletal formula for glutamic acid. [1]
(ii) Draw the structure for the dipeptide, ala-glu, formed from one molecule of alanine and one molecule of glutamic acid.
The peptide bond formed should be displayed. [2]

(d) The isoelectric point of alanine is 6.0 and of glutamic acid is 3.2.
A mixture of the dipeptide, ala-glu, and its two constituent amino acids, alanine and glutamic acid, is analysed by electrophoresis using a buffer at pH 6.0.

Fig. 7.2
Draw and label three spots on Fig. 7.2 to indicate the predicted position of each of these three species after electrophoresis.
Explain your answer.
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(e) Alanine, $\text{H}_2\text{NCH(CH}_3\text{)COOH}$, reacts with methanol to form the ester $G$ under certain conditions.
The proton ($\text{^1H}$) NMR spectrum of $G$ dissolved in $\text{D}_2\text{O}$ is shown in Fig. 7.3.

Fig. 7.3
Table 7.1

[Table_1]

(i) Complete Table 7.2 for the proton ($\text{^1H}$) NMR spectrum of $G$.
[Table_2]

chemical shift $(\delta)$

splitting pattern

number of $\text{^1H}$ atoms responsible for the peak

number of protons on adjacent carbon atoms

1.4
3.5
4.0 [3]

(ii) The proton ($\text{^1H}$) NMR spectrum of $G$ dissolved in $\text{CDCl}_3$ is obtained.
Describe the difference observed between this spectrum and the proton NMR spectrum in $\text{D}_2\text{O}$ shown in Fig 7.3.
Explain your answer.
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(a) Complete Table 8.1 by placing one tick (✓) in each row to indicate the sign of each type of energy change under standard conditions.

[Table 8.1]

| energy change | always positive | always negative | can be either negative or positive |
|-----------------------------|-----------------|-----------------|-----------------------------------|
| lattice energy | | | |
| enthalpy change of hydration| | | |
| enthalpy change of solution | | | |

[1]

(b) Define enthalpy change of hydration.

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(c) Table 8.2 shows various energy changes which can be used in the following questions.

[Table 8.2]

| energy change | value/kJ mol⁻¹ |
|------------------------------------------------------------|---------------|
| standard enthalpy change of atomisation of calcium | +178.2 |
| first ionisation energy of calcium | +590 |
| second ionisation energy of calcium | +1145 |
| standard enthalpy change of atomisation of bromine | +111.9 |
| Br–Br bond energy | +192.9 |
| standard enthalpy change of solution of calcium bromide, CaBr₂(s) | −103.1 |
| standard enthalpy change of formation of calcium bromide, CaBr₂(s)| −682.8 |
| standard enthalpy change of hydration of Ca²⁺ | −1579 |
| first electron affinity of bromine | −324.6 |
| first ionisation energy of bromine | +1140 |

(i) Select and use relevant data from Table 8.2 to calculate the lattice energy, $\Delta H^\circ_{latt}$ of CaBr₂(s).

It may be helpful to draw a labelled energy cycle.
Show your working.

$\Delta H^\circ_{latt}$ of CaBr₂(s) = ............................ kJ mol⁻¹ [3]

(ii) Select and use relevant data from Table 8.2 and your answer to (c)(i) to calculate the standard enthalpy change of hydration, $\Delta H^\circ_{hyd}$ of Br⁻.

It may be helpful to draw a labelled energy cycle.
If you were not able to answer (c)(i), use −2500 kJ mol⁻¹ as your value for $\Delta H^\circ_{latt}$ of CaBr₂(s). This is not the correct value.
Show your working.

$\Delta H^\circ_{hyd}$ of Br⁻ = ............................ kJ mol⁻¹ [2]

(iii) The enthalpy change of hydration of the Br⁻ ion is more negative than the enthalpy change of hydration of the I⁻ ion. Explain why.

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