AS & A Level Chemistry - 9701 Paper 2 2021 Winter Zone 1

Sulfides are compounds that contain sulfur but not oxygen.

(a) Carbon disulfide, CS₂, is a volatile liquid at room temperature and pressure.

(i) State the meaning of volatile.
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(ii) Draw a 'dot-and-cross' diagram of the CS₂ molecule.
[2]

(iii) Suggest the bond angle in a molecule of CS₂.
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(iv) CS₂ is a liquid under room conditions, while CO₂ is a gas.
Explain what causes the difference in the physical properties between CS₂ and CO₂.
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(b) The enthalpy change of combustion of CS₂(l) is represented by the following equation.
$CS_2(l) + 3O_2(g) \xrightarrow{\Delta H_c} CO_2(g) + 2SO_2(g)$

(i) Define enthalpy change of combustion.
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(ii) The table shows the enthalpy changes of formation of CS₂(l), CO₂(g) and SO₂(g).

[Table_1]
Use the data in the table to calculate the enthalpy change of combustion, $\Delta H_c$, of CS₂(l), in kJ mol⁻¹.
Show your working.
$\Delta H_c$ of CS₂(l) = ............................................. kJ mol⁻¹ [2]

(c) Hydrogen sulfide gas, H₂S(g), is slightly soluble in water. It acts as a weak acid in aqueous solution.

(i) State the meaning of weak acid.
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(ii) Give the formula of the conjugate base of H₂S.
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(iii) H₂S(aq) reacts slowly with oxygen dissolved in water. The reaction is represented by the following equation.
$H_2S(aq) + \frac{1}{2}O_2(aq) \rightarrow H_2O(l) + S(s)$
Explain, with reference to oxidation numbers, why this reaction is a redox reaction.
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(d) The compound As₂S₃ is a common mineral.
When As₂S₃ is heated strongly in air, it forms a mixture of products, as shown.
$2As_2S_3(s) + 9O_2(g) \rightarrow As_4O_6(s) + 6SO_2(g)$

(i) A sample containing 0.198 g As₂S₃ is placed in 0.100 dm³ of pure oxygen, an excess, in a reaction chamber connected to a gas syringe at room temperature.
The reactants are heated until no further change is observed. The products are then allowed to cool to room temperature.
Calculate the volume, in dm³, of gas present at the end of the experiment.
The molar volume of gas is 24.0 dm³mol⁻¹ under these conditions. Assume that the pressure is constant throughout the experiment.
Show your working.
volume of gas remaining = ................................ dm³ [4]

(ii) State the environmental consequences of releasing SO₂(g) into the atmosphere.
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(iii) SO₂(g) can be removed from the air by reacting it with NaOH(aq).
Construct an equation for the reaction of SO₂(g) with NaOH(aq). Include state symbols.
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[Total: 21]

The reaction scheme shows some reactions of calcium.


(a) (i) Reaction 1 produces Ca(NO$_3$)$_2$ and one other product.
Identify the other product.
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(ii) Construct an equation for the thermal decomposition of Ca(NO$_3$)$_2$(s).
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(iii) State the trend in the thermal stability of the Group 2 nitrates down the group.
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(iv) In reaction 3, excess CO$_2$ is bubbled through water containing CaCO$_3$. A solution of Ca(HCO$_3$)$_2$(aq) forms.
Construct an equation for reaction 3.
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(b) Describe how Ca(OH)$_2$ is used in agriculture.
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(c) In reaction 4, calcium carbide, CaC$_2$, is formed from CaO.
CaC$_2$ contains the C$_2^{2-}$ anion. Each carbon in C$_2^{2-}$ is sp hybridised.

(i) Describe how sp hybridised orbitals are formed.
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(ii) Sketch a diagram to show how two sp hybrid orbitals can form a sigma (σ) bond.
[2]

(d) The flowchart shows some reactions of CaC$_2$.


(i) Reaction 5 can be used to prepare NH$_3$.
CaCN$_2$ + 3H$_2$O → CaCO$_3$ + 2NH$_3$
Calculate the minimum mass, in tonnes, of calcium cyanamide, CaCN$_2$, that is required to produce 1.50 × 10$^6$ tonnes of NH$_3$.
Show your working.
1 tonne = 1.00 × 10$^6$ g
minimum mass of CaCN$_2$ = ............................... tonnes [2]

(ii) Draw the structure of the organic products formed in the following reactions.
[3]

Phosphorus is a reactive Period 3 element.
(a) Phosphorus has several allotropes. Details of two allotropes are given.

[Table_1]
$$\begin{array}{|c|c|c|}\hline \text{allotrope of phosphorus} & \text{formula} & \text{melting point/°C} \\ \hline \text{white} & P_4 & 44 \\ \text{red} & P & 590 \\ \hline \end{array}$$

(i) White phosphorus and red phosphorus both have covalent bonding.
Suggest the types of structure shown by white phosphorus \((P_4)\) and red phosphorus \((P)\).
Explain why red phosphorus \((P)\) has a higher melting point than white phosphorus \((P_4)\).
structure of \(P_4\) \text{.....................}
structure of \(P\) \text{.....................}
explanation \text{.....................}
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(ii) Red phosphorus \((P)\) forms when white phosphorus \((P_4)\) is exposed to sunlight.
$$\frac{1}{4}P_4(s) \rightarrow P(s) \quad \Delta H = -17.6\, \text{kJ mol}^{-1}$$
Use this information to draw a reaction pathway diagram to show the formation of red phosphorus \((P)\) from white phosphorus \((P_4)\).



(b) Some reactions of \(P_4(s)\) are shown in the reaction scheme.



(i) State the oxidation number of phosphorus in \(P_4O_{10}\).
\text{.....................} [1]

(ii) Deduce the identity of \(Q\) and hence construct chemical equations for reactions 1 and 2.
reaction 1 \(PCl_5 + \ldots\ldots H_2O \rightarrow \ldots\ldots\ldots\ldots\) ...
reaction 2 \(P_4O_{10} + \ldots\ldots H_2O \rightarrow \ldots\ldots\) ... ... [2]

(c) Triphenylphosphine is used in a type of reaction known as a Wittig reaction.

triphenylphosphine

where \(=\, C_6H_5\)

(i) Give the empirical formula of triphenylphosphine.
\text{.....................} [1]

In a Wittig reaction, an aldehyde reacts with a halogenoalkane to form an alkene. The conversion is shown in the following unbalanced equation.

Compound \(H\) can be made from propanal, \(C_2H_5CHO\). Stage 3 in the reaction scheme is a Wittig reaction.
$$\begin{align*} \text{stage 1} \quad & C_2H_5CHO \xrightarrow{\,\text{NaBH}_4\,} G \\ \text{stage 2} \quad & G \xrightarrow{\,\text{red phosphorus and } I_2\,} C_2H_5CH_2I \\ \text{stage 3} \quad & \text{(Wittig reaction)} \quad C_2H_5CH_2I + C_2H_5CHO \xrightarrow{\,\text{triphenylphosphine}\,} H\\ \end{align*}$$

(ii) State the types of reaction that occur in stages 1 and 2.
stage 1 \text{.....................}
stage 2 \text{.....................} [2]

(iii) Draw the structures of \(G\) and \(H\) in the boxes provided.

[2]

(d) Identify the organic products formed when compound \(J\), shown below, is heated with hot concentrated acidified manganate(VII) ions.


\(J\) \xrightarrow{\text{hot concentrated } \text{MnO}_4^- \text{ and } \text{H}_2\text{SO}_4} \quad \text{.....................} [2]

(a) (i) Reaction 1 is an oxidation reaction.
Give the reagent(s) and conditions required for reaction 1.
reagent(s) ........................................................
conditions ........................................................
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(ii) Construct an equation to represent reaction 1.
Use [O] to represent an oxygen atom from the oxidising agent in this reaction.
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(iii) Suggest the type of reaction that occurs in reaction 2.
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(iv) $\text{H}_2\text{SO}_4$ acts as a homogeneous catalyst in reaction 3.
Explain why $\text{H}_2\text{SO}_4$ is described as homogeneous.
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(b) Reaction 2 needs to take place in the absence of water to prevent formation of compound C.

If C is present in the reaction mixture of reaction 3, a different compound, compound D, will also form. Compound D has two identical functional groups.
The infrared spectrum of D shows strong absorptions at 1100 cm$^{-1}$ and 1720 cm$^{-1}$, but no absorption due to O–H bonds.
Use the \textit{Data Booklet} to identify the functional group present in D.
Explain your answer as fully as you can.
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(c) Some other reactions of C are shown.

(i) Draw the structure of E. [1]

(ii) Suggest why NaBH$_4$ is not a suitable reagent to make F, (CH$_2$OH)$_2$, from C.
Explain your answer.
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(iii) Construct an equation for the reaction of (CH$_2$OH)$_2$ with SOCl$_2$ to form G, (CH$_2$Cl)$_2$.
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(d) Explain why C is very soluble in water.
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