AS & A Level Chemistry - 9701 Paper 4 2018 Winter Zone 2

(a) An alkene, a carboxylic acid and a ketone, all of similar volatility, are mixed together. The mixture is then analysed by gas chromatography.

The gas chromatogram produced is shown.



The separation of the three compounds depends on their relative solubilities in the liquid stationary phase. The liquid stationary phase is an alkane.

(i) Complete the table to suggest which compound in the mixture is responsible for each peak J, K and L. Explain your answer by reference to the intermolecular forces of each compound.

[Table_1]
peak - organic compound - explanation
J
K
L


(ii) The area underneath each peak is proportional to the mass of the respective compound.

Calculate the percentage by mass in the original mixture of the compound responsible for peak K.

% of mixture responsible for peak K = .........................

(b) Chlorobenzene can be prepared from benzene as shown.



Aluminium chloride, $AlCl_3$, catalyses this reaction.

(i) Write an equation to show how $AlCl_3$ generates the electrophile needed in this reaction.
.........................................................................................................................
(ii) Draw the mechanism of the reaction between this electrophile and benzene to form chlorobenzene. Include all relevant curly arrows and charges.


(iii) Write an equation to show how the catalyst is regenerated.
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(c)(i) Catalysts can be heterogeneous or homogeneous. Explain what is meant by a homogeneous catalyst.
.........................................................................................................................
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(ii) Complete the table by placing one tick (✓) in each row to indicate the mode of action of the catalyst in each reaction.

[Table_2]
Rh in the removal of $NO_2$ from exhaust gases of cars - heterogeneous - homogeneous
$Fe^{3+}$ in the $I^-/S_2O_8^{2-}$ reaction

(a) Complete the electronic configuration for Cu and Cu$^{2+}$.



(b) (i) The 3d orbitals in an isolated Cu$^{2+}$ ion are degenerate.
Explain what is meant by the term “degenerate” in this context.
.....................................................................................................................
..................................................................................................................... [1]

(ii) Complete the diagram to describe the splitting of the 3d orbital energy levels in an octahedral complex.


[1]

(c) (i) 1,2-diaminoethane, H$_2$NCH$_2$CH$_2$NH$_2$, en, is a bidentate ligand.
Explain what is meant by the term bidentate.
.....................................................................................................................
.....................................................................................................................
..................................................................................................................... [1]

(ii) Cu$^{2+}$ ions and en form the complex ion [Cu(en)$_3$]$^{2+}$.
Draw the two optical isomers of this complex ion.
You may use



to represent en.


[2]

(a) The reaction scheme shows some reactions of $[Co(H_2O)_6]^{2+}$.


$$[Co(H_2O)_6]^{2+} \xrightarrow{\text{reaction 1}} [Co(H_2O)_4(OH)_2] \xrightarrow{\text{reaction 2, excess } NH_3(aq)} \text{solution of A}$$
$$\text{reaction 3} \downarrow \text{$CO_3^{2-}(aq)$}$$
$$\text{precipitate B}$$

(i) Write the formulae of the following species.
A $\text{......................}$
B $\text{......................}$ [2]

(ii) State a suitable reagent for reaction 1.
$\text{.......................................................................................................................}$ [1]

(iii) Write an equation for reaction 2.
$\text{.......................................................................................................................}$ [1]

(iv) Write an ionic equation for reaction 3.
$\text{.......................................................................................................................}$ [1]

(b) $\text{Co}^{2+}$ ions catalyse the oxidation of 2,3-dihydroxybutanedioate ions, $C_4H_4O_6^{2-}$, to methanoate ions, $HCO_2^{-}$, carbon dioxide and water.

(i) What property of transition elements allows $\text{Co}^{2+}$ ions to act as a catalyst?
$\text{.......................................................................................................................}$ [1]

(ii) Draw the structure of the 2,3-dihydroxybutanedioate ion. [1]

(iii) Complete the equation for the oxidation of 2,3-dihydroxybutanedioate. Use [O] for the oxidising agent in this reaction.
$$C_4H_4O_6^{2-} + \text{......................} \rightarrow \text{......................}$$ [1]

(c) Platin, $\text{Pt}(NH_3)_2Cl_2$, displays extit{cis-trans} isomerism.

(i) Draw the structures of these isomers.

(a) Calcium nitride, Ca$_3$N$_2$, reacts readily with water to form a white precipitate suspended in an alkaline solution. The oxidation number of nitrogen does not change during the reaction.

Construct an equation for the reaction of Ca$_3$N$_2$ with water.
..............................................................................................................................

(b) The enthalpy changes of solution, $\Delta H^{\circ}_{\text{sol}}$, of the hydroxides of the Group 2 elements become less endothermic down the group.

State and explain the trend in the solubilities of the Group 2 hydroxides.
..............................................................................................................................
..............................................................................................................................
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(c) Complete the energy cycle to show the enthalpy changes that occur in the transformations between aqueous ions, gaseous ions and an ionic solid.

On your diagram label each enthalpy change with its appropriate symbol; lattice energy, $\Delta H^{\circ}_{\text{lat}}$, enthalpy change of hydration, $\Delta H^{\circ}_{\text{hyd}}$, or enthalpy change of solution, $\Delta H^{\circ}_{\text{sol}}$.

Complete the three arrows showing the correct direction of each enthalpy change.



(d) The numerical value of the solubility product, $K_{sp}$, of CaF$_2$ is $3.45 \times 10^{-11}$ at 298 K.

(i) Write an expression for the solubility product of CaF$_2$. Include its units.
$K_{sp}$ = .....................................................
units = ........................................... [2]

(ii) Calculate the solubility of CaF$_2$ at 298 K.
solubility = ......................... mol dm$^{-3}$ [1]

(a) Explain why the thermal stability of the Group 2 nitrates increases down the group.
..........................................................................................................................................................
..........................................................................................................................................................
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(b) Sodium nitrite, NaNO$_2$, is a decomposition product from heating sodium nitrate, NaNO$_3$.
A student analysed a sample of sodium nitrite by titration with aqueous cerium(IV) ions, Ce$^{4+}$(aq). The equation for the titration reaction is shown.
NO$_2^-$(aq) + 2Ce$^{4+}$(aq) + H$_2$O(l) → 2Ce$^{3+}$(aq) + NO$_3^-$(aq) + 2H$^+$(aq)
• 0.138 g of impure sodium nitrite was dissolved in water and made up to 100 cm$^3$ in a volumetric flask.
• 25.0 cm$^3$ of this solution required 21.80 cm$^3$ of 0.0400 mol dm$^{-3}$ Ce$^{4+}$(aq) to reach the end-point.
You should assume the impurity does not react with Ce$^{4+}$(aq).
Calculate the percentage purity of the sample of sodium nitrite.
............................ %

(c) Acidified manganate(VII) ions, MnO$_4^-$, can also be used to analyse solutions containing nitrite ions, NO$_2^-$, by titration. In acidic solution, NO$_2^-$ ions exist as HNO$_2$.
(i) Use the Data Booklet to construct an ionic equation for this reaction.
..........................................................................................................................................................
..........................................................................................................................................................
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(ii) Use E° values to calculate the E$_{cell}^\ominus$ for this reaction.
E$_{cell}^\ominus$ = ............................... V

(d) Nitrous acid, HNO$_2$, is a weak acid with a K$_a$ of 6.9 × 10$^{-4}$ mol dm$^{-3}$ at 298 K.
(i) Explain the difference between a strong acid and a weak acid.
..........................................................................................................................................................
..........................................................................................................................................................
(ii) Write the expression for the acid dissociation constant, K$_a$, for HNO$_2$.
K$_a$ = ..........................................................................................................................
(iii) Calculate the pH of 0.15 mol dm$^{-3}$ HNO$_2$.
pH = .................................
(iv) Calculate the percentage of HNO$_2$ molecules that are ionised in 0.15 mol dm$^{-3}$ HNO$_2$.
% ionisation = .................................

(e) Solutions containing a mixture of HNO$_2$ and NaNO$_2$ are buffer solutions.
(i) Define what is meant by the term buffer solution.
..........................................................................................................................................................
..........................................................................................................................................................
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(ii) Write two equations to show how a solution containing a mixture of HNO$_2$ and NaNO$_2$ acts as a buffer.
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(f) Nitrous acid is used in the preparation of diazonium salts. The –N$_2^+$ group in the diazonium ion can be replaced with Cl, Br or CN as shown.
The reagent used is a copper(I) salt, CuX.
[Image]
This reaction can be used in the synthesis of compound Z as shown.
[Image]
(i) Suggest the reagent used in reaction 1.
..........................................................................................................................................................
(ii) Suggest structures of compounds Y and Z and draw them in the boxes above.
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(g) Predict the number of peaks in the carbon-13 NMR spectra of W and Z.
[Table]
..........................................................................................................................................................

2-hydroxypropanoic acid can be synthesised in four steps from ethanoic acid.

(a) (i) Suggest a reagent for step 2.
............................................................................................................................... [1]
(ii) Suggest reagents and conditions for steps 1 and 4.
step 1 ..............................................................................................................................
step 4 .............................................................................................................................. [2]
(b) Compound R can be used in the synthesis of compound T as shown.

(i) Suggest the structures of S and T and draw them in the boxes. [2]
(ii) Name the \textit{type of reaction} for step I and step II.
step I ..............................................................................................................................
step II .............................................................................................................................. [2]
(c) Compound R can be polymerised.
Draw a section of this polymer showing two repeat units. [2]

(a) Polyurethanes are polymers made by the reaction of a diisocyanate with a diol as shown. $R^1$ and $R^2$ are hydrocarbon groups.

O=C=N-R^1-N=C=O + HO-R^2-OH →

a diisocyanate a diol a polyurethane

Lycra® is a polyurethane formed from the diisocyanate P and HOCH$_2$CH$_2$OH.



(i) Give the molecular formula for P.
...................................................................................................... [1]

(ii) Draw the repeat unit of Lycra®.
...................................................................................................... [2]

(iii) Fibres of Lycra® are strong due to the intermolecular forces between the polymer chains.
Complete the table to identify two intermolecular forces responsible for this property and the group(s) involved.

[Table_1]
...................................................................................................... [2]

(b) Name one example of each of the following types of polymer.

[Table_2]
...................................................................................................... [3]

(a) Chloramine, $\text{NH}_2\text{Cl}$, can be used in the treatment of drinking water to kill bacteria. Excess chloramine in water is destroyed using UV light. The mechanism for this involves free radicals.
The initiation step in this process is shown.
$\text{NH}_2\text{Cl} \xrightarrow{\text{UV}} \cdot \text{NH}_2 + \cdot \text{Cl}$
(i) What is meant by the term free radical?
.................................................................................................................................
................................................................................................................................. [1]
The equation for a possible propagation step in the process is shown.
$\text{NH}_2\text{Cl} + \cdot \text{Cl} \rightarrow \cdot \text{NHCl} + \text{HCl}$
(ii) Suggest an equation for a possible termination step in this process.
................................................................................................................................. [1]

(b) (i) Draw the ‘dot-and-cross’ diagram of $\text{NH}_2\text{Cl}$. Show outer electrons only.

[1]
(ii) State the hybridisation of the nitrogen atom and suggest the $\text{H–N–Cl}$ bond angle in the $\text{NH}_2\text{Cl}$ molecule.
hybridisation of N ...............................................................................................................................
$\text{H–N–Cl}$ bond angle ............................................................. [1]

(c) Some values for standard enthalpy changes of formation, $\Delta H_f^\circ$, and standard entropies, $S^\circ$, are given in the table.
[Table_1]
\begin{array}{|c|c|c|}
\hline
& \Delta H_f^\circ/\text{kJ mol}^{-1} & S^\circ/\text{J K}^{-1} \text{mol}^{-1} \\ \hline
\text{NH}_2\text{Cl}(g) & +80.1 & +241 \\ \hline
\text{NH}_3(g) & -45.9 & +198 \\ \hline
\text{N}_2\text{H}_4(g) & +95.4 & +237 \\ \hline
\text{HCl}(g) & -92.3 & +187 \\ \hline
\end{array}
(i) Define the meaning of the term entropy.
................................................................................................................................. [1]
Hydrazine, $\text{N}_2\text{H}_4$, can be produced from chloramine and ammonia as shown.
$\text{NH}_2\text{Cl}(g) + \text{NH}_3(g) \rightarrow \text{N}_2\text{H}_4(g) + \text{HCl}(g)$
(ii) Calculate the standard entropy change, $\Delta S^\circ$, for this reaction.
$\Delta S^\circ = \text{............................... J K}^{-1}\text{ mol}^{-1}$ [1]
(iii) Calculate the standard enthalpy change, $\Delta H^\circ$, for this reaction.
$\Delta H^\circ = \text{.............................. kJ mol}^{-1}$ [1]
(iv) Calculate the standard Gibbs free energy change, $\Delta G^\circ$, for this reaction at 298 K.
$\Delta G^\circ = \text{.............................. kJ mol}^{-1}$ [2]
(v) Explain, with reference to $\Delta G^\circ$, why this reaction becomes less feasible at higher temperatures.
.................................................................................................................................
................................................................................................................................. [1]

(d) Compare and explain the basicities of ammonia, ethylamine and phenylamine.
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................................................................................................................................. [4]

(a) Use information from the Data Booklet to draw the structure of the dipeptide glu-cys.

[2]
The isoelectric point is the pH at which an amino acid exists as a zwitterion. The isoelectric point of glutamic acid is 3.2 and of cysteine is 5.0.
A mixture of the dipeptide glu-cys and its two constituent amino acids, glutamic acid and cysteine, was analysed by electrophoresis using a buffer at pH 5.2. The results obtained are shown.



(b) Suggest identities for the species responsible for spots E, F and G. Explain your answers.

[Table_1]
explanation .......................................
....................................................
....................................................

[3]