A-LevelChemistryChemical bonding9701/21 · Oct/Nov 2021 · Q121 Marks

Sulfides are compounds that contain sulfur but not oxygen.

(a)(i)State the meaning of volatile.[1]
(a)(ii)Draw a 'dot-and-cross' diagram of the CS₂ molecule.[2]
(a)(iii)Suggest the bond angle in a molecule of CS2.[1]
(a)(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.[2]
(b)(i)Define enthalpy change of combustion.[2]
(b)(ii)The table shows the enthalpy changes of formation of CS2(I), CO2(g) and SO2(g). compound | enthalpy change of formation, ∆Ηf/kJ mol-1 ---|--- CS2(I) | +89.7 CO2(g) | -394 SO2(g) | -297 Use the data in the table to calculate the enthalpy change of combustion, ∆H, of CS2(I), in kJ mol-1. Show your working. ∆H of CS2(I) = ...................................... kJ mol-1[2]
(c)(i)State the meaning of weak acid.[1]
(c)(ii)Give the formula of the conjugate base of H2S.[1]
(c)(iii)H₂S(aq) reacts slowly with oxygen dissolved in water. The reaction is represented by the following equation. H₂S(aq) + ½O₂(aq) → H₂O(l) + S(s) Explain, with reference to oxidation numbers, why this reaction is a redox reaction.[2]
(d)(i)A sample containing 0.198 g As2S3 is placed in 0.100dm³ 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 dm³ mol-1 under these conditions. Assume that the pressure is constant throughout the experiment. Show your working. volume of gas remaining = ................................ dm³[4]
(d)(ii)State the environmental consequences of releasing SO2(g) into the atmosphere.[1]
(d)(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.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] easily vaporised / easily evaporates / turns to gas easily
[2] M1 bonding pairs S C S (with 2 lone pairs on each S) M2 Correct number of remaining outer electrons (2 lone pairs on each S)
[1] 180°
[2] M1 CS2 has more electrons M2 So stronger induced dipole (forces) (between molecules)
[2] M1 (enthalpy/energy change when) 1 mole of a compound M2 burns/combusts/reacts in excess oxygen/O2 OR completely burns/ completely combusts/completely reacts in oxygen/O2
[2] M1 (−394 +2 (−297) – (+89.7) M2 = -1080 (kJ mol-1)
[1] weak [acid] partially dissociates/partially ionises (into H+ ions/protons)
[1] HS-
[2] M1 S (increases) oxidation number -2 → 0 so oxidation / or is oxidised M2 O (decreases) O.N. 0 → −2 so reduction / is reduced
[4] M1 moles of As2S3 = 0.198/246.1 / 8.05 × 10-4 M2 moles SO2 (using moles of As2S3 as limiting factor) = 2.41(36) × 10–3 moles (6 / 2 × 8.05 × 10-4) Volume SO2 = 2.41(36) × 10-3 × 24 = 0.0579 dm³ M3 Moles O2 used in reaction = 8.05 × 10-4 × 9/2 = 3.62 × 10-3 Volume O2 used in reaction = 3.62 × 10-3 × 24 = 0.0869 dm³ M4 Final total volume gas = (0.1 – 0.0869) + 0.0579 = [0.0131 + 0.0579] = 0.071(0) dm³ M4 ONLY award 4th mark if the final answer rounds to 0.071 Answer to minimum of 2 sig figs MAX 3 for using ecf from M1 to M2 to M3 and M4 Award all 4 marks if final answer rounds to 0.071
[1] acid rain
[2] M1 SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(1) AND correct species and balancing M2 State symbols