AS & A Level Chemistry - 9701 Paper 4 2024 Summer Zone 2

(a) Describe the trend in the solubility of the sulfates of magnesium, calcium and strontium. Explain your answer.
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most soluble                 least soluble
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(b) Define lattice energy, $\Delta H_{latt}$.
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(c) State and explain the main factors that affect the magnitude of lattice energies.
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(d) Table 1.1 shows some energy changes.

[Table_1]

(i) Born–Haber cycles can be used to determine the lattice energies of ionic compounds. Complete the Born–Haber cycle in Fig. 1.1 for potassium sulfide, $K_2S(s)$. Include state symbols for all of the species.


(ii) Calculate the lattice energy, $\Delta H^\circ_{latt}$, of $K_2S(s)$ using relevant data from Table 1.1. Show your working.
$\Delta H^\circ_{latt}$ of $K_2S(s)$ = ...............................kJ mol$^{-1}$

(a) (i) Lithium nitrate, LiNO₃, decomposes on heating in a similar way to Group 2 nitrates to give the metal oxide, a brown gas and oxygen.
Write an equation for the decomposition of LiNO₃.
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(ii) The other Group 1 nitrates, MNO₃, decompose on heating to form the metal nitrite, MNO₂, and oxygen.
The thermal stability of these nitrates increases down the group.
Suggest why the thermal stability of MNO₃ increases down the group.
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(b) Acidified manganate(VII) ions, MnO₄⁻, can be used to analyse solutions containing nitrite ions, NO₂⁻, by titration.
X is a solution of NaNO₂.
250.0 cm³ of X is added to 50.0 cm³ of 0.125 mol dm⁻³ acidified MnO₄⁻(aq). The MnO₄⁻(aq) ions are in excess; all the NO₂⁻ ions are oxidised in the reaction.
The unreacted MnO₄⁻(aq) required 22.50 cm³ of 0.0400 mol dm⁻³ Fe2+(aq) to reach the end-point.
The relevant half-equations are shown.
$$\begin{align*} \text{NO}_2^- + \text{H}_2\text{O} &\rightleftharpoons \text{NO}_3^- + 2\text{H}^+ + 2\text{e}^- \\ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- &\rightleftharpoons \text{Mn}^{2+} + 4\text{H}_2\text{O} \\ \text{Fe}^{2+} &\rightleftharpoons \text{Fe}^{3+} + \text{e}^- \end{align*}$$
Calculate the concentration, in mol dm⁻³, of NaNO₂ in X.
concentration of NaNO₂ in X = ............................... mol dm⁻³ [3]

(c) Table 2.1 shows electrode potentials for some electrode reactions involving manganese compounds.

[Table_2.1]

(i) Aqueous manganate(VI) ions, MnO₄²⁻, are unstable in acidic conditions and undergo a disproportionation reaction.
The $E^\ominus_{\text{cell}}$ for this reaction is +1.14 V.
Construct an overall ionic equation for this disproportionation reaction.
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(ii) Suggest and explain how the $E_{\text{cell}}$ value of the disproportionation reaction changes with an increase in pH.
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(a) Carbon disulfide, $\text{CS}_2$, is flammable and reacts readily with oxygen, as shown in reaction 1.

reaction 1 $\text{CS}_2(g) + 3\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{SO}_2(g)$

Table 3.1 shows the standard enthalpy of formation, $\Delta H_f^\ominus$, and the standard entropy, $S^\ominus$, for some substances.

[Table_1]

Calculate the standard Gibbs free energy change, $\Delta G^\ominus$, in $\text{kJ mol}^{−1}$, for reaction 1 at 25 °C.

$\Delta G^\ominus = \text{............................} \text{kJ mol}^{−1}$ [3]

(b) Carbon disulfide reacts with chlorine to form tetrachloromethane, as shown in reaction 2.

reaction 2 $\text{CS}_2 + 3\text{Cl}_2 \rightarrow \text{CCl}_4 + \text{S}_2\text{Cl}_2 \quad \Delta H^\ominus = -261.6 \text{kJ mol}^{−1}$

$\Delta S^\ominus = -365.5 \text{J K}^{−1} \text{mol}^{−1}$

Calculate the maximum temperature, in K, for reaction 2 to be feasible.

temperature = ............................. K [2]

(a) (i) Explain why transition elements have variable oxidation states.
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(ii) Sketch the shape of a $3d_{z^2}$ orbital in Fig. 4.1.

[1]

(b) Samples of $[Cu(H_2O)_6]^{2+}$(aq) are reacted separately with an excess of solution A and with an excess of solution B.
The reaction of $[Cu(H_2O)_6]^{2+}$(aq) with solution A is a precipitation reaction.
The reaction of $[Cu(H_2O)_6]^{2+}$(aq) with solution B is a ligand substitution reaction.
Suggest a possible identity for solution A and for solution B. Give relevant observations and the formula of the copper-containing product for each reaction.
solution A ..........................................................................................................................................
observations .......................................................................................................................................
formula of the copper-containing product ................................................................................
solution B ..........................................................................................................................................
observations .......................................................................................................................................
formula of the copper-containing product ................................................................................
[3]

(c) Solutions containing the $[Ag(NH_3)_2]^+$ complex are colourless. Explain why this complex is colourless.
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(d) Two bidentate ligands are shown in Fig. 4.2.

Explain what is meant by a bidentate ligand.
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(e) Ruthenium(III) ions, $Ru^{3+}$, form an octahedral complex, $[Ru(dpys)_2Cl_2]^+$, with the ligands dpys and chloride ions.
This complex shows the same kind of stereoisomerism as $[Ru(NH_3)_4Cl_2]^+$ but also shows a different type of stereoisomerism.
(i) Complete the three-dimensional diagrams in Fig. 4.3 to show the three different stereoisomers of $[Ru(dpys)_2Cl_2]^+$.
The dpys ligand can be represented using N ---- N.

[3]
(ii) State the different types of stereoisomerism shown by $[Ru(dpys)_2Cl_2]^+$.
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(iii) Deduce which stereoisomers in (e)(i) are non-polar. Explain your answer.
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(a) Nitrosyl chloride, NOCl, can be formed by the reaction between nitrogen monoxide and chlorine, as shown.

$2NO + Cl_{2} \rightarrow 2NOCl$

The initial rate of this reaction is investigated, starting with different concentrations of $NO$ and $Cl_{2}$. The results obtained are shown in Table 5.1.

[Table_5.1]

$$\begin{array}{cccc} \text{experiment} & [NO]/\text{mol dm}^{-3} & [Cl_{2}]/\text{mol dm}^{-3} & \text{initial rate}/\text{mol dm}^{-3}\text{min}^{-1} \\ 1 & 0.0250 & 0.0150 & 3.68 \times 10^{-2} \\ 2 & 0.0750 & 0.0150 & 3.32 \times 10^{-1} \\ 3 & 0.0500 & 0.0600 & 5.89 \times 10^{-1} \\ \end{array}$$

(i) Use the data in Table 5.1 to deduce the rate equation for this reaction.

Explain your reasoning.

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(ii) Use your rate equation from (a)(i) and the data from experiment 1 to calculate the rate constant, $k$, for this reaction. Include the units of $k$.

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

(b) $NO_{2}Cl$ is another compound containing nitrogen, oxygen and chlorine.

In sunlight, $NO_{2}Cl$ can undergo homolytic fission to release chlorine radicals which can catalyse the conversion of ozone, $O_{3}$, into oxygen.

Complete the mechanism for this process.

initiation (homolytic fission) \hspace{30pt} $NO_{2}Cl \rightarrow ............ + ............$
propagation step 1 \hspace{50pt} ..... + $O_{3} \rightarrow ............ + ............$
propagation step 2 \hspace{50pt} .... + ...... \rightarrow .......... + ..........$
[2]

(c) Ozone reacts with nitrogen dioxide, as shown.

$O_{3} + 2NO_{2} \rightarrow N_{2}O_{5} + O_{2}$

The rate of reaction is first order with respect to $O_{3}$ and first order with respect to $NO_{2}$.

Suggest equations for a two-step mechanism for this reaction.

step 1 ......................................................................................................................................................
step 2 .......................................................................................................................................................
[2]

(a) Aqueous solutions of methanoic acid, HCOOH, and propanoic acid, CH$_3$CH$_2$COOH, are mixed together.
An equilibrium is set up between two conjugate acid–base pairs.
(i) Define conjugate acid–base pair.
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(ii) The pK$_a$ of HCOOH is 3.75 and of CH$_3$CH$_2$COOH is 4.87.
Complete the equation for the Brønsted–Lowry equilibrium between the stronger of these two acids and water.
..................................... + H$_2$O $\rightleftharpoons$ ..................................... + ..................................... [1]

(b) (i) Write an expression for the acid dissociation constant, K$_a$, for butanoic acid, CH$_3$CH$_2$CH$_2$COOH.
K$_a$ = ..................................................................................................................... [1]
(ii) The pK$_a$ of CH$_3$CH$_2$CH$_2$COOH is 4.82.
A solution of CH$_3$CH$_2$CH$_2$COOH(aq) has a pH of 3.25.
Calculate the concentration, in mol dm$^{-3}$, of CH$_3$CH$_2$CH$_2$COOH in this solution.
concentration of CH$_3$CH$_2$CH$_2$COOH = ................................. mol dm$^{-3}$ [2]

(c) (i) Define buffer solution.
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(ii) A buffer solution containing a mixture of CH$_3$COOH and CH$_3$COONa is prepared as follows.
A solution of 600 cm$^3$ of CH$_3$COOH is mixed with 400 cm$^3$ of 0.125 mol dm$^{-3}$ CH$_3$COONa.
The buffer solution has pH 5.70. The K$_a$ of CH$_3$COOH is 1.78 $\times$ 10$^{-5}$ mol dm$^{-3}$.
Calculate the initial concentration, in mol dm$^{-3}$, of CH$_3$COOH used.
concentration of CH$_3$COOH = ................................. mol dm$^{-3}$ [3]

(d) A fuel cell is an electrochemical cell that can be used to generate electrical energy by using oxygen to oxidise a fuel.
Methanoic acid, HCOOH, is being investigated as a fuel in fuel cells.
When the cell operates, HCOOH is oxidised to carbon dioxide.
The half-equation for the reaction at the cathode is: O$_2$ + 4H$^+$ + 4e$^-$ $\rightarrow$ 2H$_2$O.
In this fuel cell, the overall cell reaction is the same as that for the complete combustion of HCOOH.
(i) Deduce the half-equation for the reaction at the anode.
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(ii) Calculate the volume, in cm$^3$, of oxygen used when a current of 3.75A is delivered by the cell for 40.0 minutes. Assume the cell operates at room conditions.
volume of oxygen = ................................. cm$^3$ [2]

(a) (i) Give the systematic name of P.
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(ii) P can be synthesised as shown in Fig. 7.2.

[Image: Fig. 7.2]

Suggest reagents and conditions for this reaction.
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(iii) A student attempts to synthesise P by an alternative route, as shown in Fig. 7.3.

Compound T is the major product in this reaction rather than P.

[Image: Fig. 7.3]

Explain why T is the major product in this reaction.
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(b) S reacts in a similar way to phenol in step 3.

(i) Draw the structures of Q, R and S in the boxes in Fig. 7.1.

[Image: Fig. 7.1]

[3]

(ii) Suggest reagents and conditions for steps 1 and 2 in Fig. 7.1.
step 1 ...............................................................................................................
step 2 ............................................................................................................... [3]

(a) State the relative basicities of phenylamine, $\text{C}_6\text{H}_5\text{NH}_2$, benzylamine, $\text{C}_6\text{H}_5\text{CH}_2\text{NH}_2$, and ammonia, $\text{NH}_3$, in aqueous solution. Explain your answer.
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most basic                                      least basic

(b) An excess of $\text{Br}_2$(aq) is added to separate samples of $\text{C}_6\text{H}_5\text{NH}_2$ and benzene, $\text{C}_6\text{H}_6$.

(i) $\text{C}_6\text{H}_5\text{NH}_2$ reacts readily with $\text{Br}_2$(aq) to form organic product $\text{M}$.
State the expected observations for this reaction. Draw the structure of $\text{M}$.
observations .........................................................................................................................
structure of $\text{M}$

(ii) $\text{C}_6\text{H}_6$ does not react with $\text{Br}_2$(aq).
Suggest why $\text{Br}_2$(aq) reacts with $\text{C}_6\text{H}_5\text{NH}_2$ but not with $\text{C}_6\text{H}_6$.

(c) Explain why benzamide, $\text{C}_6\text{H}_5\text{CONH}_2$, is a much weaker base than ammonia, $\text{NH}_3$.

(d) $\text{C}_6\text{H}_5\text{CONH}_2$ is formed by reacting benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$, with $\text{NH}_3$.
Complete the mechanism in Fig. 8.1 for the reaction of $\text{C}_6\text{H}_5\text{COCl}$ with $\text{NH}_3$.
Include all relevant lone pairs of electrons, curly arrows, charges and dipoles. Draw the structure of the organic intermediate.


(e) Phenylalanine, $\text{C}_6\text{H}_5\text{CH}_2\text{CH(NH}_2\text{)}\text{COOH}$, is an amino acid with an isoelectric point of 5.5.

(i) State what is meant by isoelectric point.
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(ii) Draw the structure of $\text{C}_6\text{H}_5\text{CH}_2\text{CH(NH}_2\text{)}\text{COOH}$ at pH 10.

(f) $\text{C}_6\text{H}_5\text{CH}_2\text{CH(NH}_2\text{)}\text{COOH}$ and alanine, $\text{CH}_3\text{CH(NH}_2\text{)}\text{COOH}$, react to form a dipeptide containing both amino acid residues.
Draw the structure of this dipeptide.
The peptide functional group formed should be displayed.

(a) Explain why trichloroethanoic acid, CCl$_3$COOH, is more acidic than ethanoic acid, CH$_3$COOH.
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(b) Acyl chlorides are formed by reacting carboxylic acids with thionyl chloride, SOCl$_2$.

(i) Ethanedioyl chloride, (COCl)$_2$, can be prepared by reacting ethanedioic acid, (COOH)$_2$, with an excess of SOCl$_2$.
Write an equation for this reaction.
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(ii) Samples of (COCl)$_2$ are reacted separately with an excess of warm acidified KMnO$_4$(aq) and with H$_2$NCH$_2$CH$_2$NH$_2$.
The carbon-containing product from the reaction with H$_2$NCH$_2$CH$_2$NH$_2$ has the molecular formula C$_4$H$_6$N$_2$O$_2$.
Complete the boxes in Fig. 9.1 to suggest the structure of the carbon-containing product in each reaction.

[2]

(iii) A polyester can be synthesised from the reaction of (COCl)$_2$ with ethane-1,2-diol, HOCH$_2$CH$_2$OH.
Draw extit{two} repeat units of the polymer formed. Any functional groups should be displayed.
[2]

(c) Compound H, C$_6$H$_{10}$O$_3$, reacts with alkaline I$_2$(aq) to form yellow precipitate J but does extit{not} react with Na$_2$CO$_3$(aq).
The proton ($^1$H) NMR spectrum of H in CDCl$_3$ is shown in Fig. 9.2.

[Table_1]

(i) Identify yellow precipitate J.
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(ii) Complete Table 9.2 for the proton ($^1$H) NMR spectrum of H, C$_6$H$_{10}$O$_3$.
[Table_2]
[4]

(iii) Suggest a structure for H, C$_6$H$_{10}$O$_3$. [1]