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

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.
(a) KI forms an ionic lattice that is soluble in water.
(i) Define enthalpy change of solution, $\Delta H_{\text{sol}}.$
.............................................................................................................................................. [1]
(ii) KI(s) has a high solubility in water although its enthalpy change of solution is endothermic.
Explain how this high solubility is possible.
.............................................................................................................................................. [2]
(b) Table 1.1 gives some data about the halide ions, $\text{Cl}^-$, $\text{Br}^-$ and $\text{I}^-$, and their potassium salts.
[Table_1]
(i) Explain the trend in the enthalpy change of hydration of the halide ions.
.............................................................................................................................................. [2]
(ii) The $\Delta H_{\text{sol}}$ values of these potassium halides are almost constant.
Use the $\Delta H_{\text{hyd}}$ and $\Delta H_{\text{latt}}$ data in Table 1.1 to suggest why.
.............................................................................................................................................. [1]
(iii) The enthalpy change of solution of KI(s) is +21.0 kJ mol$^{-1}$.
Use this information and the data in Table 1.1 to calculate the enthalpy change of hydration of the potassium ion, K$^+$(g).
$\Delta H_{\text{hyd}}$ of K$^+$(g) = ................................................. kJ mol$^{-1}$ [1]
(iv) Solid PbI$_2$ forms when KI(aq) is mixed with Pb$^{2+}$(aq) ions.
The solubility product, $K_{sp}$, of PbI$_2$ is $7.1 \times 10^{-9}$ mol$^3$ dm$^{-9}$ at 25$^\circ$C.
Calculate the solubility, in mol dm$^{-3}$, of PbI$_2$(s).
solubility of PbI$_2$(s) = ......................................... mol dm$^{-3}$ [2]
(v) The ionic radius of Pb$^{2+}$ is 0.120 nm compared to 0.133 nm for K$^+$.
Suggest how the $\Delta H^\circ_{\text{latt}}$ of PbI$_2$(s) differs from $\Delta H^\circ_{\text{latt}}$ of KI(s).
Explain your answer.
.............................................................................................................................................. [2]
(c) KI slowly oxidises in air, forming I$_2$.
reaction 1 $4$KI(s) $+$ $2$CO$_2$(g) $+$ O$_2$(g) $\rightarrow$ $2$K$_2$CO$_3$(s) $+$ $2$I$_2$(s) $\Delta H^\circ$ = $-203.4$ kJ mol$^{-1}$
Table 1.2 shows some data relevant to this question.
[Table_2]
(i) Calculate the standard entropy change, $\Delta S^\circ$, of reaction 1.
$\Delta S^\circ$ = ......................................... J K$^{-1}$ mol$^{-1}$ [2]
(ii) Use your answer to (c)(i) to show that reaction 1 is spontaneous at 298K.
[2]
(iii) The Group 1 carbonates are much more thermally stable than the Group 2 carbonates.
State and explain the trend in the thermal stability of the Group 2 carbonates.
.............................................................................................................................................. [2]
(d) A student electrolyses a solution of KI(aq) for 8 minutes using a direct current.
The half-equation for the reaction that occurs at the anode is given.
$2\text{I}^-$(aq) $\rightarrow$ I$_2$(aq) $+$ $2\text{e}^-$
(i) Write a half-equation for the reaction that occurs at the cathode.
Include state symbols.
............................................................................................................................... [1]
(ii) After the electrolysis, the I$_2$(aq) produced requires $21.35$ cm$^3$ of $0.100$ mol dm$^{-3}$ Na$_2$S$_2$O$_3$(aq) to react completely.
I$_2$(aq) $+$ 2Na$_2$S$_2$O$_3$(aq) $\rightarrow$ $2$NaI(aq) $+$ Na$_2$S$_4$O$_6$(aq)
Calculate the average current used in 8 minutes during the electrolysis.
current = ......................................................A [3]
(e) KI is used as a source of $\text{I}^-$ ions in organic synthesis.
One example of this is shown in the synthetic route in Fig. 1.1.

(i) Identify the reagents required for steps 1 and 2.
step 1 .......................................................................................................................................
step 2 ....................................................................................................................................... [2]
(ii) Step 3 occurs in two stages.
stage I NaNO$_2$ and HCl undergo an acid-base reaction to produce HNO$_2$.
stage II HNO$_2$ reacts with \textbf{C}, C$_6$H$_5$NH$_2$, to produce \textbf{D}, C$_6$H$_5$N$_2^+$.
Complete the equations for stage I and for stage II.
stage I NaNO$_2$ $+$ HCl $\rightarrow$ ..........................................................................
stage II ....................................................................................................................................... [2]
(iii) The $\text{I}^-$ from KI reacts with \textbf{D} in step 4. The mechanism is shown in Fig. 1.1.
Suggest the name for this mechanism.
............................................................................................................................................. [1]
Total: 26

(a) Equation 1 shows water acting as a Brønsted–Lowry acid.
equation 1 \[ \text{H}_2\text{O} + \text{NO}_2^- \rightleftharpoons \text{HNO}_2 + \text{OH}^- \]
(i) Identify the \textbf{two} conjugate acid–base pairs in equation 1.
acid I \[ \text{H}_2\text{O} \] conjugate base of acid I \[\text{............}\]
acid II \[\text{.........}\] conjugate base of acid II \[\text{............}\]
[1]

(ii) Water also behaves as a Brønsted–Lowry acid when it dissolves \text{CH}_3\text{NH}_2.
Explain the ability of \text{CH}_3\text{NH}_2 to act as a base.
...........................................................................................................................
.......................................................................................................................... [1]

(iii) Write an equation to show water acting as a base with \text{CH}_3\text{COOH}.
...........................................................................................................................
.......................................................................................................................... [1]

(b) The ionic product of water, \( \text{K}_w \) measures the extent to which water dissociates.
\[ \text{H}_2\text{O}(l) \rightleftharpoons \text{H}^+(aq) + \text{OH}^-(aq) \]
Fig. 2.1 shows how \text{K}_w varies with temperature.

!

(i) Write an expression for \( \text{K}_w \).
............................................................................................................................. [1]

(ii) Use information from Fig. 2.1 to deduce whether the dissociation of water is an exothermic or an endothermic process.
Explain your answer.
............................................................................................................................. [1]

(iii) An aqueous solution has \text{pH} = 7.00 at 30 ℃.
Use information from Fig. 2.1 to explain why this solution can be considered to be alkaline at 30 ℃.
.............................................................................................................................
.......................................................................................................................... [2]

(c) The three physical states of \( \text{H}_2\text{O} \) have different standard entropies, \( \text{S}^⦵ \), associated with them.
Table 2.1 shows these \text{S}^⦵ values.

![Table_1]

(i) Explain the difference in the \text{S}^⦵ values of \text{H}_2\text{O}(s) and \text{H}_2\text{O}(l).
.............................................................................................................................
............................................................................................................................. [1]

(ii) Explain why the increase in \text{S}^⦵ is \textbf{much} greater when \text{H}_2\text{O} boils than when it melts.
.............................................................................................................................
............................................................................................................................. [1]

(iii) The energy changes for \( \text{H}_2\text{O}(s) \rightarrow \text{H}_2\text{O}(l) \) are shown.
\[ \Delta G = 0.00 \text{ kJ mol}^{-1} \]
\[ \Delta H = +6.03 \text{ kJ mol}^{-1} \]
Use these data to show that the melting point of \( \text{H}_2\text{O}(s) \) is 0 ℃.
............................................................................................................................. [1]

(d) Metal–air batteries are electrochemical cells that generate electrical energy from the reaction of metal anodes with air.
The standard electrode potentials for the zinc–air battery are shown.
\[ [\text{Zn(OH)}_4]^{2-} + 2e^- \rightleftharpoons \text{Zn} + 4\text{OH}^- \quad \text{E}^⦵ = -1.22 \text{ V} \]
\[ \frac{1}{2}\text{O}_2 + \text{H}_2\text{O} + 2e^- \rightleftharpoons 2\text{OH}^- \quad \text{E}^⦵ = +0.40 \text{ V} \]

(i) Calculate the standard cell potential, \( \text{E}_{\text{cell}}^⦵ \), of the zinc–air battery.
\( \text{E}_{\text{cell}}^⦵ = \) ...............................................................V [1]

(ii) The zinc–air battery usually operates at \text{pH 11} and \text{298 K}. The overall cell potential is dependent on \([\text{OH}^-]\).
The Nernst equation shows how the electrode potential at the cathode changes with \([\text{OH}^-]\).
\[ \text{E} = 0.40 - \frac{(0.059)}{z} \log{([\text{OH}^-]^2)} \]
Calculate the electrode potential, \text{E}, at \text{pH 11}.
\text{E} = ...............................................................................V [2]

Iron is a transition metal in Group 8 of the Periodic Table.
(a) (i) Explain why iron has variable oxidation states.
............................................................................................................................. [1]
(ii) Complete the shorthand electronic configurations of Fe and $\text{Fe}^{3+}$.
Fe [Ar] .........................................................................................................
$\text{Fe}^{3+}$ [Ar] ......................................................................................................... [1]

(b) An aqueous solution of $\text{Fe(NO}_3)_3$ contains the complex $\text{[Fe(H}_2\text{O)}_6]^{3+}$.
When solutions of KSCN(aq) and $\text{[Fe(H}_2\text{O)}_6]^{3+}$(aq) are mixed, a colour change is observed.
The red complex $\text{[Fe(H}_2\text{O)}_5\text{SCN]}^{2+}$ forms.
(i) Define complex.
............................................................................................................................. [1]
(ii) State the coordination number of Fe in $\text{[Fe(H}_2\text{O)}_6]^{3+}$.
............................................................................................................................. [1]
(iii) The H—O—H bond angle in water is 104.5°. Suggest the H—O—H bond angle in $\text{[Fe(H}_2\text{O)}_6]^{3+}$.
Explain your answer.
............................................................................................................................. [1]
(iv) Explain why iron complexes are coloured.
............................................................................................................................. [3]
(v) Aqueous solutions of complexes $\text{[Fe(H}_2\text{O)}_6]^{3+}$ and $\text{[Fe(H}_2\text{O)}_5\text{SCN]}^{2+}$ are different colours.
Explain why these complexes are different colours.
............................................................................................................................. [2]

(c) Table 3.1 gives values for the stability constants, $K_{\text{stab}}$, of different complexes of iron.
$$\begin{array}{|c|c|} \hline \text{complex} & \text{stability constant, } K_{\text{stab}} \\ \hline \text{[Fe(H}_2\text{O)}_5\text{(H}_2\text{PO}_4)]^{2+} & 5.90 \times 10^1 \\ \text{[Fe(H}_2\text{O)}_5\text{SCN]}^{2+} & 1.30 \times 10^2 \\ \hline \end{array}$$
(i) $\text{[Fe(H}_2\text{O)}_5\text{(H}_2\text{PO}_4)]^{2+}$ can form when $\text{H}_3\text{PO}_4$ reacts with $\text{[Fe(H}_2\text{O)}_6]^{3+}$.
Write an equation for this reaction.
............................................................................................................................. [1]
(ii) Write an expression for $K_{\text{stab}}$ of $\text{[Fe(H}_2\text{O)}_5\text{SCN]}^{2+}$ and give its units.
$$K_{\text{stab}} = \text{.......................................................}$$
units = ............................................................. [2]
(iii) Use the stability constant data in Table 3.1 to calculate the value of the equilibrium constant, $K_c$, for the following equilibrium.
$$\text{[Fe(H}_2\text{O)}_5\text{(H}_2\text{PO}_4)]^{2+} + \text{SCN}^{-} \rightleftharpoons \text{[Fe(H}_2\text{O)}_5\text{SCN]}^{2+} + \text{H}_2\text{PO}_4^{-}$$
value of $K_c = \text{.......................................................}$ [1]
[Total: 14]

Ruthenium and osmium are transition metals below iron in Group 8 of the Periodic Table.
(a) Two different complex ions, $X$ and $Y$, can form when anhydrous $\text{RuCl}_3$ reacts with water under certain conditions.
$X$ and $Y$ have octahedral geometry.
Aqueous samples of $X$ and $Y$ react separately with an excess of $\text{AgNO}_3\,(aq)$. Different amounts of $\text{AgCl}$ are precipitated:
• 1 mole of complex ion $X$ produces 2 moles of $\text{AgCl}$
• 1 mole of complex ion $Y$ produces 1 mole of $\text{AgCl}$
(i) Complete Table 4.1 to suggest formulae for $X$ and $Y$.
[Table_1]
(ii) Both complexes react with an excess of bipyridine, bipy, to form a mixture of two stereoisomers of $[\text{Ru(bipy)}_3]^{3+}$.
Bipyridine is a bidentate ligand.
Draw three-dimensional diagrams of the two stereoisomers of $[\text{Ru(bipy)}_3]^{3+}$.
Use $\text{N} \begin{array}{c}\leftrightarrow\end{array} \text{N}$ to represent the bipy ligand in your structures.


(b) Fig. 4.1 shows another ruthenium complex.

This complex contains the neutral ligand pyrazine.
(i) Suggest how pyrazine is able to bond to two separate ruthenium ions.
..............................................................................................................................
(ii) Pyrazine is an aromatic compound. The bonding and structure of pyrazine is similar to that of benzene.
Describe and explain the shape of pyrazine.
In your answer, include:
• the hybridisation of the nitrogen and carbon atoms
• how orbital overlap forms $\pi$ bonds between the atoms in the ring.
..............................................................................................................................
(iii) Predict the number of peaks seen in the carbon-13 NMR spectrum of pyrazine.
Explain your answer.
..............................................................................................................................
(iv) The overall charge of the ruthenium complex in Fig. 4.1 is $5+$.
Deduce the possible oxidation states of the two ruthenium ions in the complex.
..............................................................................................................................

(c) Osmium tetroxide, $\text{OsO}_4$, reacts with alkenes in a similar manner to cold dilute acidified $\text{MnO}_4^-$.
Fig. 4.2 shows a proposed synthesis of a condensation polymer $G$.

(i) Suggest a reagent for step 1.
..............................................................................................................................
(ii) Draw the structure of exactly one repeat unit of the condensation polymer $G$.
The ester linkage should be shown fully displayed.

(a) (i) Write an equation for the formation of the electrophile for step 1. ...................................................................................................................................... [1]
(ii) Complete the mechanism in Fig. 5.2 for step 1, the alkylation of chlorobenzene. Include all relevant curly arrows and charges. Draw the structure of the intermediate.
[Image_1: Fig. 5.2]
[3]
(iii) Step 2 is an oxidation reaction. Construct an equation for the reaction in step 2. Use [O] to represent an atom of oxygen from an oxidising agent. ...................................................................................................................................... [1]
(iv) Suggest reagents for the conversion of K to M in steps 3 and 4.
step 3 .............................................................................................................
step 4 ............................................................................................................. [2]
(v) Identify the type of reaction that occurs in step 5. ...................................................................................................................................... [1]
(vi) Step 7 takes place when P is heated with a weak base such as $K_2CO_3$(aq).
Suggest why a strong base such as NaOH(aq) is not used for this reaction. ...................................................................................................................................... ...................................................................................................................................... [1]
(vii) Q is optically active. Explain the meaning of optically active. ...................................................................................................................................... [1]
(viii) Give two reasons why it might be desirable to synthesise a single optical isomer of Q for use as a drug. ...................................................................................................................................... [2]
(b) Q is commonly used in conjunction with aspirin.

Aspirin is a weak Brønsted–Lowry acid.
(i) The $pK_a$ of aspirin is 3.49. 75 mg of aspirin dissolves in water to form 100 $cm^3$ of an aqueous solution. Calculate the pH of this solution.
$[M_r: aspirin, 180.0]$
pH = .............................................................. [3]
(ii) Aspirin undergoes acid hydrolysis in the stomach. Give the structures of the organic products of this acid hydrolysis.
[2]
[Total: 17]

Amino acids are molecules that contain —NH₂ and —COOH functional groups.
Glycine, $H_2NCH_2COOH$, is the simplest stable amino acid.
(a) The isoelectric point of glycine is 6.2.
(i) Define isoelectric point.
............................................................. [1]
(ii) Draw the structure of glycine at pH 4. [1]
(b) Fig. 6.1 shows two syntheses starting with glycine.