AS & A Level Chemistry - 9701 Paper 4 2019 Summer Zone 3

(a) Aqueous solutions of copper(II) salts contain the blue-coloured $\text{[Cu(H}_2\text{O)}_6]^{2+}$ complex ion. Separate portions of this blue solution react with aqueous sodium hydroxide and with concentrated hydrochloric acid.

Give the following information for each of these reactions.

● reaction with aqueous sodium hydroxide

ionic equation ................................................................................................................................................................
type of reaction .................................................................................................................................................................
colour and state of the copper-containing product .................................................................................................

● reaction with concentrated hydrochloric acid

ionic equation ................................................................................................................................................................
type of reaction .................................................................................................................................................................
colour and state of the copper-containing product .................................................................................................
[6]

(b) Chloride ions can be identified using aqueous silver nitrate, AgNO$_3$(aq).

\[\text{Ag}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} \rightarrow \text{AgCl(s)}\]

0.303 g of a chloride of sulfur is completely hydrolysed with water. All the chlorine atoms present in the chloride of sulfur are converted into chloride ions. The solution is diluted to 100.0 cm$^3$. A 25.00 cm$^3$ sample of this solution is titrated with 0.0500 mol dm$^{-3}$ AgNO$_3$(aq). The titration requires 22.40 cm$^3$ of 0.0500 mol dm$^{-3}$ AgNO$_3$(aq).

Calculate the empirical formula of the chloride of sulfur. Show all your working.

empirical formula of chloride of sulfur = .................................. [3]

(a) Group 2 nitrates decompose when heated.
Write an equation for the decomposition of strontium nitrate.
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(b) Describe and explain how the thermal stability of Group 2 nitrates changes with increasing atomic number.
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(c) The variation in the thermal stability of Group 2 amides is similar to that of Group 2 nitrates.
(i) Suggest whether calcium amide, Ca(NH$_2$)$_2$, will decompose more or less readily than barium amide, Ba(NH$_2$)$_2$. Explain your answer.
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(ii) Ba(NH$_2$)$_2$ decomposes when heated to form barium nitride, Ba$_3$N$_2$, and ammonia as the only products.
Write an equation for this reaction.
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(d) Ba(NH$_2$)$_2$ contains the NH$_2^-$ ion.
Predict the bond angle of NH$_2^-$. Explain your answer using the qualitative model of electron-pair repulsion.
bond angle ...........................................
explanation ...............................................................................................................................
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Chlorate(V) ions are powerful oxidising agents.
(a) The reduction of chlorate(V) ions, $\text{ClO}_3^-$, with $\text{SO}_2$ forms chlorine dioxide, $\text{ClO}_2$, and sulfate ions, $\text{SO}_4^{2-}$, as the only products.
Construct an equation for this reaction.
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(b) (i) Chlorine dioxide, $\text{ClO}_2$, disproportionates with hydroxide ions, $\text{OH}^-$ (aq), to form a mixture of $\text{ClO}_2^-$ and $\text{ClO}_3^-$ ions.
$2\text{ClO}_2 + 2\text{OH}^- \rightarrow \text{ClO}_2^- + \text{ClO}_3^- + \text{H}_2\text{O}$
Explain, using this reaction as an example, what is meant by disproportionation.
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(ii) Deduce the ionic half-equations for the reaction in (b)(i).
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(c) A lithium-iodine electrochemical cell can be used to generate electricity for a heart pacemaker. The cell consists of a lithium electrode and an inert electrode immersed in body fluids. When current flows lithium is oxidised and iodine is reduced.
(i) Use the Data Booklet to write half-equations for the reactions taking place at the two electrodes. Hence write the overall equation for when a current flows.
• ...........................................................................................................
• ...........................................................................................................
overall equation ...........................................................................................................
(ii) Use the Data Booklet to calculate the $E^\text{o}_\text{cell}$ for this cell.
$E^\text{o}_\text{cell}$ = ........................... V
(iii) A current of $2.5 \times 10^{-5}$A is drawn from this cell.
Calculate the time taken for $0.10g$ of lithium electrode to be used up. Assume the current remains constant throughout this period.
time = .......................... s

(a) Sketches of the shapes of some atomic orbitals are shown.
Identify the type of orbital, s, p, or d.

shape of orbital x y z
type of orbital x

[1]

(b) Cadmium forms the two ions, Cd2 2+ and Cd2+. The electronic configuration of cadmium in these ions is shown.
• [Kr] 4d10 5s1
• [Kr] 4d10
Use this information to explain why cadmium is not a transition element.
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(c) Methylamine, CH3NH2, is a monodentate ligand.
(i) State what is meant by the term monodentate in this context.
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[1]
In the presence of aqueous methylamine, [Cd(H2O)6]2+ reacts to form a mixture of two isomeric octahedral complexes.
equilibrium 1 [Cd(H2O)6]2+ + 4CH3NH2 ⇌ [Cd(CH3NH2)4(H2O)2]2+ + 4H2O ΔH˚r = −57 kJ mol⁻¹
(ii) Complete the three-dimensional diagrams to show the isomers of [Cd(CH3NH2)4(H2O)2]2+.
Use L to represent CH3NH2 in your diagrams.

[2]

(d) (i) State what is meant by the term stability constant.
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(ii) Complete the table by placing one tick (✓) in each row to suggest how increasing temperature will affect Kstab and the equilibrium concentration of the cadmium complex, [[Cd(CH3NH2)4(H2O)2]2+], for equilibrium 1. Explain your answer.
[Table_1]
explanation ............................................................................................................................................
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[2]
EDTA4− is a polydentate ligand. When a solution of EDTA4− is added to [Cd(H2O)6]2+ a new complex [CdEDTA]2− is formed.
The values for the stability constants for two Cd2+ complexes are shown.
[Table_2]
(iii) A solution containing equal numbers of moles of CH3NH2 and EDTA is added to [Cd(H2O)6]2+.
Predict which complex is formed in the larger amount. Explain your answer.
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(e) Methylamine is a Brønsted–Lowry base.
Write an equation showing how methylamine dissolves in water to give an alkaline solution.
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[1]

(f) Methylamine is a useful reagent in organic chemistry.
(i) Write an equation for the reaction of ethanoyl chloride with methylamine.
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[2]
(ii) Methylamine also reacts with propanone to form compound P as shown.

Deduce the type of reaction shown here.
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[1]

(a) Chlorate(I) ions undergo the following reaction under aqueous conditions.

$2NH_3 + ClO^- \rightarrow N_2H_4 + Cl^- + H_2O$

A series of experiments was carried out at different concentrations of $ClO^-$ and $NH_3$.

The table shows the results obtained.

experiment	[ClO-] / mol dm-3	[NH3] / mol dm-3	initial rate / mol dm-3 s-1
1	0.200	0.100	0.256
2	0.400	0.200	2.05
3	0.400	0.400	8.20

(i) Use the data in the table to determine the order with respect to each reactant, $ClO^-$ and $NH_3$.
Show your reasoning.
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(ii) Write the rate equation for this reaction.

rate = ..................................................................................................................... [1]

(iii) Use the results of experiment 1 to calculate the rate constant, $k$, for this reaction. Include the units of $k$.

$k = ext{...........................}$
units = ext{...........................} [2]

(iv) On the axes sketch a graph to show how the value of $k$ changes as temperature is increased.

[Graph Image]
[1]

(b) In another experiment, the reaction between chlorate(I) ions and iodide ions in aqueous alkali was investigated.

A solution of iodide ions in aqueous alkali was added to a large excess of chlorate(I) ions and $[I^-]$ was measured at regular intervals.

(i) Describe how the results of this experiment can be used to confirm that the reaction is first-order with respect to $[I^-]$.
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[2]

A three-step mechanism for this reaction is shown.

step 1 $ClO^- + H_2O \rightarrow HClO + OH^-$
step 2 $I^- + HClO \rightarrow HIO + Cl^-$
step 3 $HIO + OH^- \rightarrow H_2O + IO^-$

(ii) Use this mechanism to deduce the overall equation for this reaction.
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[1]

(iii) Identify a step that involves a redox reaction. Explain your answer.
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[1]

(a) Complete the table by placing one tick (✓) in each row to indicate the sign of each type of energy change under standard conditions.

[Table]
energy change | always positive | always negative | either negative or positive
bond energy
enthalpy change of formation
[/Table]
[1]

(b) Explain what is meant by the term enthalpy change of atomisation.
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(c) The overall reaction for the atomisation of liquid bromine molecules, $\text{Br}_2(l)$, is shown.

$$\text{Br}_2(l) \rightarrow 2\text{Br}(g)$$
This happens via a two-step process.

- Construct a labelled energy cycle to represent this atomisation process, including state symbols.
- Use your cycle and relevant data from the Data Booklet to calculate the enthalpy change of vaporisation of $\text{Br}_2(l)$, $\Delta H^\circ_{\text{vap}}$;

The enthalpy change of atomisation of bromine, $\Delta H_{\text{at}}$, = +112 kJ mol$^{-1}$.

$$\Delta H^\circ_{\text{vap}} = \text{..................... kJ mol}^{-1}$$ [3]

(d) Suggest how the $\Delta H^\circ_{\text{vap}}$ of iodine, $\text{I}_2(l)$, would compare to that of bromine, $\text{Br}_2(l)$. Explain your answer.
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(e) (i) Explain what is meant by the term enthalpy change of hydration.
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(ii) Suggest why the enthalpy change of hydration of $\text{Br}^{-}(g)$ is more exothermic than that of $\text{I}^{-}(g)$.
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(a) Benzene can be converted into cyclohexane.

(i) For this reaction name the type of reaction and identify the reagent and conditions needed.
type of reaction ........................................................
reagent and conditions ...........................................
[2]
(ii) State the bond angles in benzene and cyclohexane.
bond angle in benzene ................................................... bond angle in cyclohexane .................................................
Explain your answers.
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[2]

(b) When benzene reacts with $SO_3$, benzenesulfonic acid is produced.

The mechanism of this reaction is similar to that of the nitration of benzene. Concentrated $H_2SO_4$ is used in an initial step to generate the $SO_3H^+$ electrophile as shown.
$$SO_3 + H_2SO_4 ightarrow SO_3H^+ + HSO_4^-$$
(i) Draw a mechanism for the reaction of benzene with $SO_3H^+$ ions. Include all necessary curly arrows and charges.

[3]
(ii) Write an equation to show how the $H_2SO_4$ catalyst is reformed.
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[1]

(c) 3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid.

Suggest the reagents and conditions and name the mechanism for this reaction.
reagents and conditions ..........................................................
mechanism ..........................................................
[2]

(d) When concentrated sulfuric acid is added to water, dissociation takes place in two stages.
stage 1 $H_2SO_4 ightleftharpoons H^+ + HSO_4^-$
stage 2 $HSO_4^- ightleftharpoons H^+ + SO_4^{2-}$ $K_{a2} = 1.0 imes 10^{-2} mol dm^{-3}$
$K_{a2}$ is the acid dissociation constant for stage 2.
(i) Write the expression for the acid dissociation constant $K_{a2}$.
$K_{a2} =$ ..........................................................
[1]
(ii) $H_2SO_4$ is considered a strong acid whereas $HSO_4^-$ is considered a weak acid.
Suggest how the magnitude of the acid dissociation constant for stage 1 compares to $K_{a2}$.
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[1]

(e) Benzoic acid, $C_6H_5CO_2H$, is a weak acid. A solution of $0.0250 mol dm^{-3}$ benzoic acid has a pH of 2.90.
Calculate the $K_a$ of benzoic acid.
$$K_a = ext{.....................} mol dm^{-3}$$
[2]

(a) The mass spectrum of compound X, C₅H₁₀O₂, is recorded.
The peak heights of the M and M+1 peaks are 22.65 and 1.25 respectively.
(i) Use these data to show that there are five carbon atoms present in one molecule of X.
Show your working. [1]
(ii) The mass spectrum has a peak at $m/e = 57$.
Complete the equation to show the fragmentation of X to produce this peak.
$[C_5H_{10}O_2]^+ \rightarrow ................................... + ...................................$ [2]

(b) State the use of TMS and CDCl₃ in NMR spectroscopy.
• TMS ............................................................
• CDCl₃ ............................................................ [1]

(c) The proton NMR spectrum of compound X, C₅H₁₀O₂, is shown.
(i) By considering both the relative peak areas and their $\delta$ values, use the Data Booklet to
• deduce the part of the molecule that produces the peak at $\delta 2.2$, ...........................................................................
• deduce the part of the molecule that produces the peaks at $\delta 1.2$ and $\delta 3.5$, ...........................................................................
• deduce the part of the molecule that produces the peak at $\delta 4.0$.
........................................................................... [3]
(ii) When reacted with aqueous alkaline iodine, X produces a yellow precipitate.
Use this information and your answers to (c)(i) to suggest a structure for X. [1]

(d) Compound W is an ester with the molecular formula C₅H₁₀O₂.
The proton NMR spectrum of W contains only two peaks.
The relative areas of these two peaks are in the ratio 9 : 1.
Suggest a structure for this ester, W. [1]

(e) Compound V is a carboxylic acid which contains a chiral centre. It also has the molecular formula C₅H₁₀O₂.
(i) Explain what is meant by the term chiral centre. .......................................................................................... .......................................................................................... [1]
(ii) Suggest a structure for V. [1]

(a) Organochlorine compounds can undergo hydrolysis.

$$\text{R–Cl + H}_2\text{O} \rightarrow \text{R–OH + HCl}$$

State and explain the relative rates of hydrolysis of the following compounds.
$$\text{CH}_3\text{CH}_2\text{Cl}\ \text{CH}_3\text{COCl}\ \text{C}_6\text{H}_5\text{Cl}$$
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(b) Epibatidine is a naturally occurring organochlorine compound.


(i) Epibatidine is a weak base.
State what is meant by the term weak base.
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A molecule of epibatidine contains two nitrogen atoms, both of which can act as a base.

(ii) Epibatidine reacts with $$\text{HCl(aq)}$$.
Complete the structure to suggest the product formed in this reaction.
[1]

(c) Polyamides, such as nylon-6, can be prepared from a monomer that contains both an amine and an acyl chloride functional group.


(i) When the nylon-6 monomer is hydrolysed, bonds are broken and formed.
By considering the two steps in the mechanism of the reaction, complete the table by placing one tick (✓) in each row to indicate the types of bonds broken and formed during the mechanism.

[Table]
bonds broken
bonds formed
[1]

(ii) Draw two repeat units of nylon-6. The amide bond should be shown fully displayed.
[2]

(d) An addition polymer made from two different alkene monomers is called a co-polymer. A section of a polyalkene co-polymer is shown.

Draw the structure of the two alkene monomers which produce this co-polymer. [2]

(e) Explain why polyamides normally biodegrade more readily than polyalkenes.
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................................................................................ [1]

(f) The alkene phenylethene can be prepared from benzene in three steps.

(i) Deduce the identity of compound H and draw its structure in the box.
[1]

(ii) Suggest reagents and conditions for each of the steps 1–3.
step 1 ...........................................................................
step 2 ...........................................................................
step 3 ...........................................................................
[3]