Secondary 4 Pure Chemistry Preliminary Examination Paper 2

TuitionGoWhere Practice Paper - Pure Chemistry Secondary 4

Preliminary Examination — Version 2 — Answer Key and Marking Scheme

Subject: Pure Chemistry (6092) Level: Secondary 4 Express / G3 Paper: Paper 2 — Structured and Free-Response Total Marks: 80

Section A: Structured Questions [50 marks]

Question 1 [9 marks]

(a) Magnesium reacts more vigorously because magnesium is higher in the reactivity series than zinc / magnesium is more reactive than zinc [1]. Magnesium atoms lose electrons more readily than zinc atoms, so the reaction with H⁺ ions is faster [1].

(b) Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) [2]

• Award [1] for correct formulae and balancing; [1] for correct state symbols.
• Accept Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g) for [2].

(c)(i) The burning splint extinguishes with a 'pop' sound. [1]

(c)(ii) Hydrogen [1]

(d) Copper is below hydrogen in the reactivity series / copper is less reactive than hydrogen [1], so it cannot displace hydrogen ions from the acid.

(e) Lead reacts with dilute hydrochloric acid initially, but the reaction stops quickly [1] because an insoluble layer of lead(II) chloride forms on the surface of the lead, preventing further contact between the acid and the metal [1].

Question 2 [9 marks]

(a) Neutralisation [1]

(b) NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq) [2]

• Award [1] for correct formulae; [1] for correct state symbols.

(c)(i) Both ammonia solution and nitric acid are soluble / the reaction produces a soluble salt (ammonium nitrate) [1]. Titration allows exact neutralisation so that a pure solution of ammonium nitrate is obtained without excess acid or alkali [1].

(c)(ii) Heat the solution to evaporate some of the water / concentrate the solution until a saturated solution is obtained [1]. Allow the solution to cool so that crystals of ammonium nitrate form [1]. Filter the crystals, wash with a little cold distilled water, and dry between sheets of filter paper [1].

(d) Ammonium nitrate is used in the manufacture of explosives [1]. (Accept: as an oxidiser in rocket propellants / in cold packs.)

Question 3 [9 marks]

(a) The cation is Zn²⁺ (zinc ion) [1]. A white precipitate forms with both NaOH and NH₃, which is characteristic of Zn²⁺, Al³⁺, or Pb²⁺ [1]. The precipitate dissolves in excess NaOH but is insoluble in excess NH₃, which confirms Zn²⁺ (Al³⁺ would dissolve in both; Pb²⁺ would be insoluble in both) [1].

(b) The anion is SO₄²⁻ (sulfate ion) [1]. A white precipitate forms with barium nitrate in the presence of dilute nitric acid, which is the test for sulfate ions [1].

(c) Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) [2]

• Award [1] for correct formulae; [1] for correct state symbols.

(d) No change to damp red litmus paper / litmus paper remains red [1]. Zn²⁺ does not produce ammonia gas when heated with NaOH; only ammonium ions (NH₄⁺) produce ammonia under these conditions [1].

Question 4 [10 marks]

(a) Natural source: volcanic eruptions [1]. Human-made source: burning of fossil fuels (coal) in power stations / smelting of metal sulfide ores [1].

(b)(i) SO₂(g) + H₂O(l) → H₂SO₃(aq) [1]

(b)(ii) Sulfurous acid [1]

(c) H₂SO₃(aq) + CaCO₃(s) → CaSO₃(s) + H₂O(l) + CO₂(g) [2]

• Award [1] for correct formulae; [1] for correct state symbols.
• Accept H₂SO₄(aq) + CaCO₃(s) → CaSO₄(s) + H₂O(l) + CO₂(g) if candidate refers to sulfuric acid from oxidation of SO₂.

(d)(i) 2SO₂(g) + 2CaCO₃(s) + O₂(g) → 2CaSO₄(s) + 2CO₂(g) [2]

• Award [1] for correct reactants and products; [1] for correct balancing.

(d)(ii) Flue gas desulfurisation removes sulfur dioxide from waste gases before they are released into the atmosphere [1]. This prevents sulfur dioxide from dissolving in rainwater to form acid rain, which damages buildings, harms aquatic life, and acidifies soil [1].

Question 5 [9 marks]

(a) Solution A (pH 1.0) has the highest concentration of H⁺ ions [1]. pH is a measure of H⁺ ion concentration; the lower the pH, the higher the concentration of H⁺ ions [1].

(b) Solution A may be a strong acid while solution B may be a weak acid [1]. A strong acid ionises completely in water, producing a higher concentration of H⁺ ions [1]. A weak acid ionises partially, producing a lower concentration of H⁺ ions even at the same acid concentration [1].

(c) The hydroxide ion, OH⁻ [1]. H⁺(aq) + OH⁻(aq) → H₂O(l) [1].

(d) Calcium hydroxide (slaked lime) or calcium oxide (quicklime) or calcium carbonate (limestone) [1]. The substance is a base that neutralises the excess H⁺ ions in the acidic soil, raising the pH [1].

Question 6 [9 marks]

(a) CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) [2]

• Award [1] for correct formulae; [1] for correct state symbols.

(b) Excess copper(II) oxide ensures that all the sulfuric acid is used up / completely reacted [1], so the resulting solution contains only copper(II) sulfate and water.

(c) Filter the mixture to remove the excess (unreacted) copper(II) oxide [1]. Heat the filtrate to evaporate some of the water until a saturated solution is obtained / until crystallisation point is reached [1]. Allow the solution to cool so that blue copper(II) sulfate crystals form [1]. Filter the crystals, wash with a little cold distilled water, and dry between sheets of filter paper [1].

(d) Copper(II) sulfate crystals contain water of crystallisation / are hydrated [1]. On heating, the water of crystallisation is driven off, leaving anhydrous copper(II) sulfate which is white [1].

Section B: Free-Response Questions [30 marks]

Candidates answer any two questions. Each question is worth 15 marks.

Question 7 [15 marks]

(a)(i) Magnesium hydroxide / aluminium hydroxide / calcium carbonate / sodium hydrogencarbonate [1]

(a)(ii) Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l) [2]

• Award [1] for correct formulae; [1] for correct balancing and state symbols.
• Accept any valid equation matching the antacid named in (a)(i).

(b)(i) Precipitation [1]

(b)(ii) Lead(II) chloride is insoluble in water [1]. Precipitation is suitable because mixing two soluble solutions containing Pb²⁺ ions and Cl⁻ ions produces an insoluble precipitate of lead(II) chloride, which can be filtered, washed, and dried [1].

(b)(iii) Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s) [2]

• Award [1] for correct formulae; [1] for correct state symbols.

(c)(i) N₂(g) + 3H₂(g) ⇌ 2NH₃(g) [1]

(c)(ii) Temperature: 450 °C [1]; Pressure: 200–250 atm [1]

(c)(iii) A higher pressure would increase the yield of ammonia because the forward reaction produces fewer gas molecules (4 → 2) [1]. However, higher pressures are expensive to maintain and require stronger, more costly equipment; the increased cost outweighs the benefit of the additional yield [1].

(d)(i) Carbonate ion, CO₃²⁻ [1]

(d)(ii) CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g) [1]

Question 8 [15 marks]

(a)(i) Cu²⁺ (copper(II) ion) [1]

(a)(ii) Cl⁻ (chloride ion) [1]

(a)(iii) Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) [2]

• Award [1] for correct formulae; [1] for correct state symbols.

(b) Add dilute nitric acid followed by aqueous silver nitrate to each solution [1]. Sodium chloride produces a white precipitate of silver chloride; sodium iodide produces a yellow precipitate of silver iodide [1]. Alternatively, add aqueous lead(II) nitrate: NaCl gives a white precipitate (PbCl₂); NaI gives a yellow precipitate (PbI₂) [1].

(c)(i) Ammonia, NH₃ [1]

(c)(ii) Hold a piece of damp red litmus paper at the mouth of the test tube [1]. The litmus paper turns blue, confirming the presence of ammonia gas [1]. (Accept: hold a glass rod dipped in concentrated HCl near the gas; dense white fumes of ammonium chloride form.)

(d)(i) SO₄²⁻ (sulfate ion) [1]

(d)(ii) Run-off from agricultural fertilisers containing ammonium sulfate or potassium sulfate / discharge from industrial processes / acid rain reacting with sulfate minerals [1].

(d)(iii) Dilute nitric acid is added to react with and remove any carbonate ions that may be present [1]. Carbonate ions would also produce a white precipitate with barium nitrate (barium carbonate), giving a false positive result for sulfate ions [1].

Question 9 [15 marks]

(a) An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. [1]

(b)(i) Hydrochloric acid has the lower pH [1]. Hydrochloric acid is a strong acid that ionises completely in water, producing a high concentration of H⁺ ions [1]. Ethanoic acid is a weak acid that ionises only partially, producing a lower concentration of H⁺ ions even at the same concentration [1].

(b)(ii) Add a piece of magnesium ribbon (or zinc granules) to equal volumes of both acids of the same concentration [1]. The hydrochloric acid produces more vigorous effervescence / faster bubbling than ethanoic acid [1], showing that HCl has a higher concentration of H⁺ ions available for reaction [1].

(c)(i) 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) [1]

(c)(ii)

• Moles of H₂SO₄ = (20.0 / 1000) × 0.100 = 0.00200 mol [1]
• From the equation, mole ratio NaOH : H₂SO₄ = 2 : 1
• Moles of NaOH = 2 × 0.00200 = 0.00400 mol [1]
• Concentration of NaOH = 0.00400 / (25.0 / 1000) = 0.160 mol/dm³ [1]

(c)(iii)

• Mᵣ of NaOH = 23 + 16 + 1 = 40 [1]
• Concentration in g/dm³ = 0.160 × 40 = 6.40 g/dm³ [1]

(d) Universal indicator gives a range of colours over the pH scale rather than a sharp colour change at a specific pH [1]. In a titration, an indicator with a sharp endpoint at the equivalence point is needed (e.g., methyl orange or phenolphthalein) to determine the exact volume of acid required for neutralisation [1].

END OF ANSWER KEY

© TuitionGoWhere Secondary School (AI) — Preliminary Examination Version 2 — Marking Scheme