AS & A Level Chemistry - 9701 Paper 2 2023 Winter Zone 2

The elements silicon, phosphorus and sulfur are in Period 3 of the Periodic Table.

(a) (i) Describe the variation in atomic radius from silicon to sulfur.
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(ii) The melting point of silicon is 1410°C. The melting point of sulfur is 113°C. Explain this difference.
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(b) Table 1.1 shows some properties of the elements Si to S. The first ionisation energy of P is not shown.

[Table_1]

Table 1.1

| property | Si | P | S |
|------------------------------|-----|-------|-------|
| total number of electrons in s subshells | | | |
| total number of electrons in p subshells | | | |
| first ionisation energy /kJ mol$^{-1}$ | 786 | | 1000 |
| formula of most common chloride | SiCl$_4$ | PCl$_5$ | SCl$_2$ |

(i) Complete Table 1.1 to show the total number of s and p electrons in an atom of Si, P and S.
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(ii) Construct an equation to represent the first ionisation energy of Si.
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(iii) Three possible values for the first ionisation energy of P are given.

619 kJ mol$^{-1}$   893 kJ mol$^{-1}$   1060 kJ mol$^{-1}$

Circle the correct value. Explain your choice, including a comparison of your chosen value to those of Si and S.
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(iv) SiCl$_4$ and PCl$_5$ each react with water, forming misty fumes. Identify the chemical responsible for the misty fumes.
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(v) Predict the shape of the SCl$_2$ molecule.
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NO and NO_2 react at 25 °C to give N_2O_3 as shown in the equation.
NO(g) + NO_2(g) \rightleftharpoons N_2O_3(g) \hspace{5mm} \Delta H = -7.2 \, \text{kJ mol}^{-1}
The reaction is reversible and reaches equilibrium in a closed system.
(a) Fig. 2.1 shows how the rate of the forward reaction changes with time.
Initially, the rate of the reverse reaction is zero.
Complete Fig. 2.1 to sketch how the rate of the reverse reaction changes with time.

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(b) State how the position of equilibrium changes, if at all, when the reaction takes place at 100 °C.
Explain your answer.
Assume the pressure remains constant.
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(c) Table 2.1 shows the composition of an equilibrium mixture of NO(g), NO_2(g) and N_2O_3(g) at 101 kPa.
[Table_1]
Calculate K_p, the equilibrium constant with respect to partial pressures.
Deduce the units of K_p.
K_p = .................................... units .......................................
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(d) Identify one natural process and one man-made process that cause the formation of atmospheric NO and NO_2.
natural process ..........................................................................................................................
man-made process........................................................................................................................[2]
(e) NO_2 is a brown gas that can be used to form nitric acid.
(i) NO_2 is a free radical.
Define free radical.
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(ii) NO_2 has a catalytic role in the oxidation of atmospheric sulfur dioxide.
Write equations to show the catalytic role of NO_2 in this oxidation.
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(iii) State one environmental consequence of the oxidation of atmospheric sulfur dioxide.
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(f) A student titrates nitric acid with a base to form a solution containing aqueous magnesium nitrate.
(i) Identify a base that the student could use.
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(ii) The student evaporates the water to obtain magnesium nitrate solid. When this solid is heated it decomposes.
Write an equation for the decomposition of magnesium nitrate.
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(iii) State how the thermal stability of Group 2 nitrates changes down the group.
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Phosphoric(V) acid, $H_3PO_4$, is used in both inorganic and organic reactions.
(a) $H_3PO_4$ is made in a two-step process from phosphorus.
step 1 Phosphorus reacts with an excess of oxygen to form a white solid.
step 2 The white solid then reacts with water to form $H_3PO_4$.
(i) Write an equation for each step.
step 1 .........................................................................................................................
step 2 ......................................................................................................................... [2]
(ii) $H_3PO_4$ is a weak Brønsted–Lowry acid.
Define weak Brønsted–Lowry acid.
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(b) $H_3PO_4$ is also formed in the process shown in reaction 1.
reaction 1 $4H_3PO_3 \rightarrow 3H_3PO_4 + PH_3$
Table 3.1 shows some relevant thermodynamic data.

Table 3.1
[Table_1]
(i) Define enthalpy change of formation.
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(ii) Use the data in Table 3.1 to calculate the enthalpy change, $\Delta H_r$, of reaction 1.
$\Delta H_r = ............................................................$ kJ mol$^{-1}$ [2]
(iii) Explain why reaction 1 is a disproportionation reaction.
Explain your reasoning with reference to relevant oxidation numbers.
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(c) Fig. 3.1 shows a reaction scheme that involves $H_3PO_4$ in several reactions.

(i) Identify $A$, which reacts with propene in the presence of $H_3PO_4$ in reaction 2.
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(ii) Draw the structure of $B$.
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(iii) Name the type of reaction that occurs in reaction 3.
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(iv) Reaction 3 is monitored using infrared spectroscopy. It is not possible to use the O—H absorption frequency to monitor the reaction.
Use Table 3.2 to identify a suitable bond whose absorption frequency can be used to monitor the progress of reaction 3.
State the change you would see in the infrared spectrum during reaction 3.
bond .........................................................................................................................
change in infrared spectrum ...................................................................................
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Table 3.2
[Table_2]
(d) $H_3PO_4$ also reacts with alcohols to form organophosphates.
Organophosphates are compounds similar to esters. They have the general structure shown in Fig. 3.2.

(i) Complete the equation to suggest the products of the reaction of $H_3PO_4$ with methanol, $CH_3OH$.
$H_3PO_4 + 3CH_3OH \rightarrow ............................................................$ [1]
(ii) Compound T is a simple organophosphate.
The mass spectrum of T shows a molecular ion peak at $m/e = 182$. This peak has a relative intensity of 12.7.
The relative intensity of the M+1 peak is 0.84.
Deduce the number of carbon atoms in T.
Hence suggest the molecular formula of T.
Assume that phosphorus and oxygen exist as single isotopes.
Show your working.
number of carbon atoms in T = ..........................................
molecular formula of T = .................................................... [3]

Lactic acid, $\text{CH}_3\text{CH(OH)COOH}$, and pyruvic acid, $\text{CH}_3\text{COCOOH}$, both contain two functional groups.


(a) (i) Explain why lactic acid exists as optical isomers.
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(ii) Give the systematic name of lactic acid.
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(iii) Lactic acid forms hydrogen bonds with water.
Complete Fig. 4.2 to show the formation of a hydrogen bond between one molecule of lactic acid and one molecule of water. Label the hydrogen bond. Show any relevant dipoles and lone pairs of electrons.
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(b) Two possible syntheses of pyruvic acid are shown in Fig. 4.3 and Fig. 4.4.
Each synthesis has a total of three steps.
(i) Complete the diagram in Fig. 4.5 to show the mechanism for the reaction of propene with $\text{Br}_2$.
Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.
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(ii) Write an equation for the oxidation of lactic acid to pyruvic acid, the third step of Fig. 4.4. Use [O] to represent one atom of oxygen from an oxidising agent.
$\text{CH}_3\text{CH(OH)COOH} + \text{...............................................}\rightarrow \text{CH}_3\text{COCOOH}$ [1]
(iii) Complete Table 4.1 to give details of the reagents and conditions used in each of the two syntheses shown in Fig. 4.3 and Fig. 4.4.
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