AS & A Level Chemistry - 9701 Paper 4 2017 Winter Zone 2

(a) Using oxidation numbers, explain why this reaction is a redox reaction.
NaClO_2 + \frac{1}{2}Cl_2 \rightarrow ClO_2 + NaCl
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(b) The central atom in the molecule of ClO_2 is chlorine.
Draw the 'dot-and-cross' diagram for ClO_2. Show outer electrons only.


(c) The reaction between ClO_2 and F_2 is shown.
2ClO_2 + F_2 \rightarrow 2ClO_2F
The rate of the reaction was measured at various concentrations of the two reactants and the following results were obtained.

(a) When water is added to magnesium nitride, Mg$_3$N$_2$, the products are a white suspension of Mg(OH)$_2$ and an alkaline gas.
(i) Write an equation for this reaction.
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(ii) A 2.52 g sample of Mg$_3$N$_2$ is added to an excess of water. Calculate the mass of Mg(OH)$_2$ formed.

mass of Mg(OH)$_2$ = ......................... g [2]

(b) State and explain how the solubility of the Group 2 hydroxides varies down the group.
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(c) Magnesium hydroxide is sparingly soluble in water. The concentration of its saturated solution at 298 K is 1.7 x 10$^{-4}$ mol dm$^{-3}$.

(i) Write an expression for the solubility product, $K_{sp}$, of Mg(OH)$_2$.

$K_{sp}$ = .............................................................. [1]

(ii) Calculate the value of $K_{sp}$ for Mg(OH)$_2$ at 298 K and state its units.

$K_{sp}$ = .............................. units .............................. [2]

(d) The temperature at which the Group 2 hydroxides and carbonates start to decompose increases down the group.

Suggest an explanation for this trend in the decomposition temperature of the Group 2 hydroxides.
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(a) (i) In a buffer solution at pH 5.9, a sample of tryptophan does not move during electrophoresis.
Draw the structures of the ions formed by tryptophan at pH 1.0 and pH 5.9.
[pH 1.0 structure here] [pH 5.9 structure here]
(ii) Tryptophan can combine with valine to form a dipeptide.
Use the Data Booklet to draw the structure of this dipeptide.

(b) Complete the following table to show the structures of the products formed and the type of reaction occurring when serotonin reacts with the four reagents in separate reactions.
[Table: reagent, structure of product, type of reaction]
[Image_1: Table Here]

(c) Serotonin is converted by enzymes in the liver to compound M.
[Image: Diagram showing serotonin and compound M]
(i) By reference to the Data Booklet, suggest how the infra-red spectrum of M would differ from that of serotonin.
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[1]
(ii) The proton NMR spectrum of M dissolved in $\text{CDCl}_3$ shows eight peaks due to the eight different types of proton present in the molecule.
The proton NMR spectrum of M dissolved in $\text{D}_2\text{O}$ was recorded.
Predict the number of peaks that would be seen in the proton NMR spectrum of M in $\text{D}_2\text{O}$.
number of peaks .....................................................................................................
explanation .....................................................................................................................
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[2]

(d) Compound M can be polymerised under certain conditions to form polymer N, shown.
[Image: Structure of polymer N]
Polymer N is biodegradable, unlike polyethene which is not.
Explain why N is biodegradable.
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[1]

An electrochemical cell consists of a half-cell containing V^{3+}(aq) and V^{2+}(aq) ions and another half-cell containing VO_2^{+}(aq) and VO^{2+}(aq) ions.
(a) (i) Use data from the Data Booklet to calculate a value for the $E^⦵_{cell}$.

$E^⦵_{cell} = \text{.........................}$ V [1]

(ii) Write the ionic equation for the cell reaction.

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(iii) Draw a fully labelled diagram of the apparatus you could use to measure the potential of this cell. Include the necessary chemicals.


[2]

(b) Use data from the Data Booklet to predict whether a reaction might take place when the following pairs of aqueous solutions are mixed. If a reaction occurs, write an equation for it and calculate the $E^⦵_{cell}$.

• V^{2+}(aq) and Sn^{4+}(aq)

Does a reaction occur? ..........................
equation ............................................................................................................
$E^⦵_{cell} .................................................................$

• VO_2^{2+}(aq) and Fe^{3+}(aq)

Does a reaction occur? ..........................
equation ...............................................................................................................
$E^⦵_{cell} .................................................................$

[3]

(a) The arrangement of the anions around a cation is called the geometry of the cation; e.g. in $[CuCl_4]^{2-}$ the geometry of copper is tetrahedral and the co-ordination number of copper is 4.

The geometry of a cation in an ionic compound can be predicted from the ratio of the ionic radii of the cation and anion involved.

[Table_1]

Use data from the Data Booklet to predict the geometry of, and hence the co-ordination number of, the cation for
• sodium chloride, NaCl,

geometry of Na$^+$ = .......................... co-ordination number of Na$^+$ = ..........................

• magnesium chloride, MgCl$_2$.

geometry of Mg$^{2+}$ = .......................... co-ordination number of Mg$^{2+}$ = ..........................

(b) Magnesium(I) chloride, MgCl, is an unstable compound and readily decomposes as shown.

$$2MgCl(s) \rightarrow Mg(s) + MgCl_2(s)$$

Use the following data to calculate the enthalpy change of this reaction.

$\Delta H_f^{\circ} MgCl(s) = -106 \text{kJ mol}^{-1}$
$\Delta H_f^{\circ} MgCl_2(s) = -642 \text{kJ mol}^{-1}$

enthalpy change = .......................... kJ mol$^{-1}$

(c) (i) The equation for which $\Delta H$ is the lattice energy for MgCl is shown.

$$Mg^{+}(g) + Cl^{-}(g) \rightarrow MgCl(s)$$

Use the equation, the following data, and relevant data from the Data Booklet to calculate a value for the lattice energy of MgCl. You might find it helpful to construct an energy cycle.

electron affinity of Cl(g) = -349 kJ mol$^{-1}$
enthalpy change of atomisation of Mg(s) = +147 kJ mol$^{-1}$
enthalpy change of formation of MgCl(s) = -106 kJ mol$^{-1}$

lattice energy MgCl = .......................... kJ mol$^{-1}$

(ii) Suggest how the lattice energies of MgCl$_2$ and NaCl will compare to that of MgCl. Explain your answers.

MgCl$_2$ and MgCl ......................................................................................................................................
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NaCl and MgCl .......................................................................................................................................
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(d) Define the term electron affinity.

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(a) Define the term transition metal complex.
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(b) Platinum can form the compound $[\text{Pt} (\text{NH}_3)_4\text{Cl}_2] [\text{PtCl}_4]$.
State the co-ordination numbers and the oxidation numbers of the platinum in the two ions of this compound.

| co-ordination number | oxidation number |
|-----------------------|--------------------|
| $[\text{Pt} (\text{NH}_3)_4\text{Cl}_2]^{2+}$ | |
| $[\text{PtCl}_4]^{2-}$ | | [2]

(c) Draw three-dimensional diagrams to show the structures of the two isomers of $[\text{Pt} (\text{NH}_3)_4\text{Cl}_2]^{2+}$.
[2]

(d) Solutions of the compounds $[\text{Pt} (\text{NH}_3)_4\text{Cl}_2]\text{Br}_2$ and $[\text{Pt} (\text{NH}_3)_4 \text{Br}_2]\text{Cl}_2$ can be distinguished from each other by a simple chemical test. Assume that any species bonded to the platinum ion does not react in this test.
Complete the table with a test that could be used to positively identify each compound. Give details of expected observations with each compound.

| test | observation with $[\text{Pt} (\text{NH}_3)_4\text{Cl}_2]\text{Br}_2$ | observation with $[\text{Pt} (\text{NH}_3)_4\text{Br}_2]\text{Cl}_2$ |
[2]

(e) In this question you should consider geometrical and optical isomerism.
What type of isomerism is shown by the following complexes? You should answer geometrical, optical, both or neither.

octahedral $[\text{Co} (\text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2)_2\text{Cl}_2]^+$ ..................................................................................................................
square planar $[\text{Ni} (\text{CN})_2\text{Cl}_2]^{2-}$ ........................................................................................................................
tetrahedral $[\text{CuBr}_2\text{Cl}_2]^{2-}$ ................................................................................................................................ [3]

(f) Many enzymes contain transition metal complexes.
Describe, with the aid of a suitably labelled diagram, how an enzyme catalyses the breakdown of a substrate molecule.
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(a) Calcium carbide, $\text{CaC}_2$, reacts readily with water, forming ethyne, $\text{C}_2\text{H}_2$, and a sparingly soluble white ionic compound.
(i) Write an equation for the reaction of $\text{CaC}_2$ with water.
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(ii) Draw a 'dot-and-cross' diagram for the carbide ion, $\text{C}_2^{2-}$. Show outer electrons only.

[1]

(b) Ethyne is the simplest member of the alkyne homologous series.
\[\text{H}---\text{C} \equiv \text{C}---\text{H}\]
ethyne
Propyne, $\text{C}_3\text{H}_4$, and butyne, $\text{C}_4\text{H}_6$, are the next two members of the series.
Deduce the general formula for the alkynes.
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(c) Ethyne can be polymerised into poly(acetylene), which is a conducting polymer.
$$\begin{array}{ccc} & \text{H} & \\ | & | \\ \text{--- C} \equiv \text{C} --- & \\ | & | \\ & \text{H} & \end{array} \text{poly(acetylene)}$$
(i) Suggest why this polymer conducts electricity.
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(ii) State the empirical formula of poly(acetylene).
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(iii) By reference to a physical or chemical property, suggest one advantage of a conducting polymer when compared with metals.
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(d) Alkynes can react with carbonyl compounds under basic conditions as shown in reaction 1.
$$\text{R}---\text{C} \equiv \text{C}---\text{H} + \small\text{amount of base} \rightarrow \text{R}---\text{C} \equiv \text{C}---\text{C}\text{R}^{\prime}\text{OH}$$
reaction 1
(i) The first step of the mechanism of reaction 1 involves the alkyne anion reacting with the carbonyl compound.
Complete the first step of the mechanism and draw the intermediate for this reaction.
Include all relevant dipoles, charges and curly arrows.
$$\begin{array}{c} \text{O} \\ || \\ \text{C} \\ / \text{R}^{\prime} \text{R}^{\prime\prime} \rightarrow \end{array} \rightarrow \; \text{intermediate} \; \rightarrow \text{R}^{\prime} \text{R}---\text{C} \equiv \text{C}---\text{R}^{\prime}\text{OH}$$
[3]
(ii) Suggest the name of the mechanism in reaction 1.
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(iii) An alkyne, $Q$, and a carbonyl compound, $R$, react together to form compound $P$ as shown.
$$\text{2Q} + \text{R} \rightarrow \begin{array}{ccc} \text{OH} & | \text{CH}_3 \\ \text{C}_2\text{H}_5\text{C} \equiv \text{C} \equiv \text{C} \text{OH} \text{CH}_3 \\ \text{C}_2\text{H}_5\text{C} \equiv \text{C} \equiv \text{C} \text{OH} \text{CH}_3 \end{array}$$
Use reaction 1 to suggest the structures of $Q$ and $R$.
[2]
(e) A series of twelve separate experiments is carried out as shown in the table.
Complete the table by writing in each box a tick (✓) if a reaction occurs, or a cross (×) if no reaction occurs.
$$\begin{array}{c|c|c|c|} \text{CH}_3\text{CHO} & \text{HCO}_2\text{H} & \text{CH}_3\text{COCH}_3 & \text{HO}_2\text{CCO}_2\text{H} \\ \hline \text{hot, acidified MnO}_4^-\text{(aq)} & & & \\ \hline \text{alkaline I}_2\text{(aq)} & & & \\ \hline \text{warm Tollens' reagent} & & & \\ \hline \end{array}$$
[4]

(a) Compound A can be produced from a plant hormone.

(i) Compound A shows optical and geometrical isomerism.

On the structure of A above,
• draw a line through the bond(s) that give rise to geometrical isomerism,
• circle all chiral carbon atoms. [2]

(ii) Give the names of four functional groups present in A.
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(iii) A molecule of A has 17 carbon atoms.

State the number of carbon atoms that are sp, sp^2 and sp^3 hybridised in A.

sp carbons = ..................... sp^2 carbons = ..................... sp^3 carbons = ..................... [1]

(iv) When A is reacted with an excess of hot, concentrated manganate(VII) ions, a mixture of three organic compounds is formed.
A → X ($C_{10}H_{14}O_7$) and Y ($C_3H_4O_3$) and Z ($C_4H_6O_3$)
Suggest the structures of Y and Z.

Y ($C_3H_4O_3$) Z ($C_4H_6O_3$) [2]

(b) A mixture of three different compounds, J, K and L, was analysed by thin layer chromatography using a polar stationary phase and a non-polar mobile phase. The three compounds all have similar molecular masses. The resulting chromatogram is shown.

(i) Identify which spot corresponds to each compound.
[Table_1]
compound spot
J $CH_3COCO_2H$
K $HO_2CCO_2H$
L $CH_3CH_2COCH_2CH_3$ [1]

(ii) Explain your answers to (b)(i).
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(iii) What is meant by the term $R_f$ value?
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