Questions 1
(a) Topic -1.1 Particles in the atom and atomic radius
(b)Topic -1.4 Ionisation energy
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.
(ii) The melting point of silicon is 1410°C. The melting point of sulfur is 113°C. Explain this difference.
(b) Table 1.1 shows some properties of the elements Si to S. The first ionisation energy of P is not shown.
(iii) Three possible values for the first ionisation energy of P are given.
619 \(kJmol^{–1}\) 893 \(kJmol^{–1}\) 1060\(kJmol^{–1}\)
Circle the correct value. Explain your choice, including a comparison of your chosen value to those of Si and S.
(iv) SiCl₄ and PCl₅ each react with water, forming misty fumes. Identify the chemical responsible for the misty fumes.
(v) Predict the shape of the SCl₂ molecule.
▶️Answer/Explanation
Ans:
(a)(i) decreases (from Si to S)
(ii) M1 Si giant covalent AND S is simple molecular
OR
Si giant covalent AND S has IMFs
M2 Si involves breaking covalent bonds AND S involves breaking IMFs
M3 more energy needed to break covalent bonds than IMFs
OR
covalent bonds are strong AND IMFs are weak
(b)(i) total e⁻ in s subshell 6 6 6
total e– in p subshell 8 9 10
(ii) Si(g) → \(Si^+\)(g) + e⁻
(iii) 1060 (is circled)
P / it has greater attraction of nucleus for outer electrons compared to Si
ORA
P / it has greater nuclear charge than Si
ORA
S has two electrons in a (3)p orbital
AND
resulting in spin-pair repulsion
(b)(i)(v) hydrogen chloride / HCl
(v) non-linear
Questions 2
(a) Topic -7.1 Chemical equilibria: reversible reactions, dynamic equilibrium
(b)Topic -7.1 Chemical equilibria: reversible reactions, dynamic equilibrium
(c)Topic -7.1 Chemical equilibria: reversible reactions, dynamic equilibrium
(d)Topic -12.1 Nitrogen and sulfur
(e)Topic -13.2 Characteristic organic reactions
(f)Topic -10.1 Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds
NO and NO₂ react at 25°C to give N₂O₃ as shown in the equation.
\(NO(g) + NO_2(g)\rightleftharpoons N_2O_3(g)\) ΔH =\( –7.2kJmol^{–1}\)
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.
(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.
(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.
Calculate \(K_p\), the equilibrium constant with respect to partial pressures. Deduce the units of \(K_p\).
(d) Identify one natural process and one man-made process that cause the formation of atmospheric NO and NO₂.
natural process …………………………………
man-made process …………………………….
(e) NO₂ is a brown gas that can be used to form nitric acid.
(i)NO₂ is a free radical. Define free radical.
(ii) NO₂ has a catalytic role in the oxidation of atmospheric sulfur dioxide. Write equations to show the catalytic role of NO₂ in this oxidation.
(iii) State one environmental consequence of the oxidation of atmospheric sulfur dioxide.
(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.
(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.
(iii) State how the thermal stability of Group 2 nitrates changes down the group.
▶️Answer/Explanation
Ans:
(a) curved line from (0,0) to reach same horizontal line
(b) equilibrium position would / it moves to left / reactants
forward reaction is exothermic (so disfavoured by increase in T)
(d) natural process: lightning
man-made process: internal combustion engines
(e)(i) species with 1 or more unpaired electrons
(ii) NO₂ + SO₂ → SO₃ + NO
NO + ½ O₂ → NO₂
(iii) (formation of) acid rain
(f)(i) magnesium hydroxide / Mg(OH)₂
(ii) Mg(NO₃)₂ → MgO + 2NO₂ + ½O₂
(iii) increases
Questions 3
(a) Topic -7.2 Brønsted–Lowry theory of acids and bases
(b)Topic -5.1 Enthalpy change, ΔH
(c)Topic -27.1 Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds
(d)Topic -18.2 Esters
Phosphoric(V) acid, H₃PO₄, is used in both inorganic and organic reactions.
(a) H₃PO₄ 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 H₃PO₄.
(i) Write an equation for each step.
step 1 ………………………………………………………………………………………………………………..
step 2 ………………………………………………………………………………………………………………..
(ii) H₃PO₄ is a weak Brønsted–Lowry acid. Define weak Brønsted–Lowry acid.
(b) H₃PO₄ is also formed in the process shown in reaction 1.
reaction 1 \(4H_3PO_3 \to 3H_3PO_4 + PH_3\)
Table 3.1 shows some relevant thermodynamic data.
(i) Define enthalpy change of formation.
(ii) Use the data in Table 3.1 to calculate the enthalpy change, \(\Delta H_r\), of reaction 1.
(iii) Explain why reaction 1 is a disproportionation reaction. Explain your reasoning with reference to relevant oxidation numbers.
(c) Fig. 3.1 shows a reaction scheme that involves H₃PO₄ in several reactions.
(i) Identify A, which reacts with propene in the presence of H₃PO₄ in reaction 2.
(ii) Draw the structure of B.
(iii) Name the type of reaction that occurs in reaction 3.
(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 ……………………..
(d) H₃PO₄ 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 H₃PO₄ with methanol, CH₃OH.
(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.
▶️Answer/Explanation
Ans:
(a)(i)\( 4P / P_4 + 5O_2 \to P_4O_{10}\)
P₄O₁₀ + 6H₂O → 4H₃PO₄
(ii) proton / H⁺ donor
partially dissociates (in solution / in water / when aqueous)
(b)(i) enthalpy change when one mole of a compound / substance is formed from its constituent elements in their standard states
(ii) \(\Delta H_f\) = +9 + 3(–1281) – 4(–972) = (+)54 (kJ mol⁻¹) (1)
Any two from (1):
• use correct stoichiometry for all three values
• use correct expression regardless of stoichiometry for all three values
• calculated correctly
(iii) P (in H₃PO₃) is (both) oxidised and reduced (simultaneously)
P (is oxidised) from (+)3 / (+)III → (+)5 / (+)V AND (reduced to) –3 / –III
(c)(i) H₂O/ steam
(iii) condensation
(iv) C=O
absorption changes to frequency / wavenumber / from 1670–1740 to 1710–1750 cm⁻¹.
(d)(i) H₃PO₄ + 3CH₃OH → (CH₃O)₃PO + 3H₂O
(ii) \(\frac{0.84}{12.7} \times \frac{100}{1.1} (=6.013)\)
Questions 4
(a) Topic -13.4 Isomerism: structural isomerism and stereoisomerism
(b)Topic -21.1 Organic synthesis
Lactic acid, CH₃CH(OH)COOH, and pyruvic acid, CH₃COCOOH, both contain two functional groups.
(a) (i) Explain why lactic acid exists as optical isomers.
(ii) Give the systematic name of lactic acid.
(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.
(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 Br₂. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.
(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.
CH₃CH(OH)COOH + ………………….
(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.
▶️Answer/Explanation
Ans:
(a)(i) (it has molecules that) contain a chiral carbon / centre OR are non-superimposable mirror images
(ii) 2-hydroxypropanoic acid
(iii) dashed (or distinctly labelled) bond between O and H in different molecules of H-O-H and propanoic acid
correct sequence of three partial charges over the H-bond
lone pair on O in line with H-bond
(ii) CH₃CH(OH)COOH + [O] → CH₃COCOOH + H₂O
(iii) HCN & KCN OR NaCN with H₂SO₄
NaOH(aq) (1)
dilute / aq(ueous)HCl / H2SO4 (1)
acidified K₂Cr₂O₇