Secondary 4 Pure Chemistry Acids Bases Salts Quiz

Secondary 4 Pure Chemistry Quiz - Acids Bases Salts

ANSWER KEY AND MARKING SCHEME

Total Marks: 40

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Section A: Short Answer (10 marks)

1. (a) SO₂(g) + H₂O(l) → H₂SO₃(aq) [1 mark]

• Accept: SO₂ + H₂O → H₂SO₃ (state symbols not penalised if omitted in this part, but award mark for correct equation)
• Do NOT accept: SO₃ (sulfur trioxide) or H₂SO₄ (sulfuric acid)

(b) Sulfurous acid [1 mark]

• Accept: H₂SO₃
• Do NOT accept: Sulfuric acid

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2. (a) Any TWO of: [2 marks, 1 mark each]

• Effervescence / fizzing / bubbles of gas produced
• Solid zinc carbonate dissolves / disappears
• Colourless solution forms
• Heat is released (exothermic reaction)
• Accept: Carbon dioxide gas is produced

(b) ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + CO₂(g) + H₂O(l) [2 marks]

• 1 mark for correct formulae
• 1 mark for correct balancing and state symbols
• Deduct 1 mark if state symbols omitted or incorrect

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3. [2 marks]

• Hydrochloric acid is a strong acid that fully/completely ionises/dissociates in water (1 mark)
• Ethanoic acid is a weak acid that only partially ionises/dissociates in water (1 mark)
• Therefore, at the same concentration, HCl produces a higher concentration of H⁺ ions, resulting in a lower pH / ethanoic acid produces fewer H⁺ ions, resulting in a higher pH
• Accept: Reference to degree of ionisation/dissociation

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4. (a) Neutralisation [1 mark]

• Accept: Acid-base reaction

(b) To ensure all the acid is completely reacted / to ensure complete neutralisation of the acid [1 mark]

• Accept: So that the resulting solution contains only the salt and water (no excess acid)
• Do NOT accept: "To speed up the reaction" or "To make more product"

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5. A base is a proton (H⁺) acceptor. [1 mark]

• Accept: A substance that accepts hydrogen ions.

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Section B: Structured Questions (18 marks)

6. (a) Sodium sulfate [1 mark]

• Accept: Na₂SO₄

(b) Pink/red/magenta to colourless [1 mark]

• Must state both initial and final colour
• Accept: Pink to colourless (phenolphthalein in base to neutral)

(c) [2 marks]

• Sodium sulfate is soluble (1 mark)
• Lead(II) sulfate is insoluble / forms a precipitate (1 mark)
• Therefore, titration is suitable for preparing soluble salts but not for insoluble salts / lead(II) sulfate would precipitate during the reaction, making the end-point difficult to determine
• Accept: Reference to solubility rules

(d) [2 marks]

• Heating to saturation produces a hot, concentrated solution (1 mark)
• On cooling, crystals form as solubility decreases with temperature (1 mark)
• If all water is removed by heating to dryness, the salt would be obtained as a powder rather than crystals / crystals would not form properly
• Accept: Reference to crystallisation process

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7. (a) Cu²⁺ / copper(II) ion [1 mark for identification]

• With NaOH: Blue precipitate forms, insoluble in excess — characteristic of Cu²⁺ (½ mark)
• With NH₃: Blue precipitate forms, soluble in excess to form deep blue solution — confirms Cu²⁺ (½ mark)
• [Total: 2 marks]

(b) [3 marks]

• Add aqueous potassium iodide (or any soluble iodide salt) (1 mark)
• If Al³⁺ is present: No precipitate / no visible reaction / solution remains colourless (1 mark)
• If Pb²⁺ is present: Yellow precipitate of lead(II) iodide forms (1 mark)
• Accept: Use of sulfate ions (white precipitate with Pb²⁺, no precipitate with Al³⁺) or chloride ions (white precipitate with Pb²⁺, no precipitate with Al³⁺)
• Must state reagent AND observations for BOTH ions

(c) Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) [2 marks]

• 1 mark for correct formulae
• 1 mark for correct state symbols
• Accept: Fe²⁺ + 2OH⁻ → Fe(OH)₂ (state symbols not penalised if omitted)

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8. (a) NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq) [2 marks]

• 1 mark for correct formulae
• 1 mark for correct state symbols
• Accept: NH₃(g) + HNO₃(aq) → NH₄NO₃(aq)

(b) [3 marks]

• Temperature: 450°C (1 mark)
• Pressure: 200–250 atm (1 mark)
• Explanation of compromise (1 mark):
  • The forward reaction is exothermic, so a lower temperature favours higher yield, BUT a higher temperature is used to increase the rate of reaction
  • Higher pressure favours the forward reaction (fewer moles of gas on product side) and increases rate, BUT very high pressure is expensive and requires stronger equipment
  • The conditions chosen represent a compromise between yield, rate, and economic factors
• Award marks for any valid combination addressing temperature, pressure, and the compromise concept

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9. pH approximately 10–11; blue/purple colour. [1 mark]

• Accept: pH 10–12, dark blue/violet.

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10. [2 marks]

• Solid citric acid does not conduct electricity because the ions are not free to move / ions are held in a fixed lattice (1 mark)
• In aqueous solution, citric acid ionises/dissociates to produce mobile ions (H⁺ and citrate ions) which carry the electric current (1 mark)

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Section C: Data-Based and Extended Response (12 marks)

11. (a) [2 marks]

• Curve A represents hydrochloric acid (1 mark)
• HCl is a strong acid that fully ionises in water, producing a higher concentration of H⁺ ions / ethanoic acid is a weak acid that partially ionises, producing a lower concentration of H⁺ ions (1 mark)
• Therefore, the reaction with HCl is faster (steeper initial gradient) / reaches completion sooner

(b) [2 marks]

• Both acids have the same concentration (1.0 mol/dm³) and the same volume was used (1 mark)
• Therefore, both contain the same number of moles of acid / the same amount of H⁺ ions available for reaction (1 mark)
• The same amount of magnesium was used, so the limiting reagent is the same in both cases, producing the same volume of H₂
• [Award 2 marks for clear explanation linking equal moles to equal product]

(c) [2 marks]

• Curve should show:
  • Steeper initial gradient than Curve A (1 mark)
  • Same final volume (60 cm³) (½ mark)
  • Reaches completion faster than Curve A (less than 40 seconds) (½ mark)
• Explanation: Powder has larger surface area than ribbon, increasing rate of reaction

(d) [3 marks]

• Moles of H₂ = 60/24000 = 0.0025 mol (1 mark)
• Mg + 2HCl → MgCl₂ + H₂ (or implied 1:1 ratio) (½ mark)
• Moles of Mg = moles of H₂ = 0.0025 mol (½ mark)
• Mass of Mg = 0.0025 × 24 = 0.060 g (1 mark)
• Award full marks for correct answer with working shown
• Accept: 0.06 g

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12. (a) [2 marks]

• Gas: Carbon dioxide / CO₂ (1 mark)
• Equation: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) (1 mark)
• Accept: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (state symbols not penalised)
• Do NOT accept: CO₂ + CaO → CaCO₃

(b) [2 marks]

• Anion: Carbonate / CO₃²⁻ (1 mark)
• Evidence:
  • Test 1: Effervescence with dilute acid and gas turns limewater milky indicates carbonate (½ mark)
  • Test 3: White precipitate with acidified barium nitrate confirms carbonate (or could indicate sulfate, but Test 1 already confirms carbonate) (½ mark)
• Note: If student identifies sulfate, award 0 marks as Test 1 evidence contradicts this

(c) [3 marks]

• Add aqueous ammonia dropwise, then in excess (½ mark)
• If Ca²⁺: White precipitate forms, insoluble in excess ammonia (1 mark)
• If Mg²⁺: White precipitate forms, insoluble in excess ammonia (½ mark)
• If Zn²⁺: White precipitate forms, soluble in excess ammonia to form colourless solution (1 mark)
• Note: Ca²⁺ and Mg²⁺ give the same result with ammonia; additional test needed to distinguish between them (e.g., flame test: Ca²⁺ = brick-red, Mg²⁺ = no colour)
• Award marks for correct observations for each ion

(d) [1 mark]

• CaCO₃ / MgCO₃ / ZnCO₃
• Accept any one of these formulae
• Must be consistent with anion identified in (b)

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13. (a) [2 marks]

• In water: HCl(g) dissolves and ionises/dissociates to form H⁺(aq) and Cl⁻(aq) ions, so the solution is acidic (1 mark)
• In methylbenzene: HCl(g) dissolves but does NOT ionise (methylbenzene is a non-polar/non-aqueous solvent), so no H⁺ ions are produced and the solution is NOT acidic (1 mark)
• Therefore, it is the presence of H⁺ ions in aqueous solution that makes a compound acidic, not simply the presence of hydrogen atoms in the formula

(b) [2 marks]

• The claim is incorrect / partially correct (1 mark for evaluation)
• Explanation: Acid strength refers to the degree of ionisation/dissociation, not the number of H⁺ ions per molecule (1 mark)
• Both HCl and H₂SO₄ are strong acids that fully ionise in water (first ionisation of H₂SO₄ is complete; second is partial but still extensive), so they have similar strengths at 1.0 mol/dm³; being diprotic does not make H₂SO₄ a stronger acid, it just means it can donate two protons.

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14. [2 marks]

• Mix solutions of silver nitrate and sodium chloride; a white precipitate of silver chloride forms (1 mark)
• Filter the mixture, wash the residue with distilled water, and dry it between sheets of filter paper / in a warm oven (1 mark)
• Accept: Any valid method for precipitation, filtration, washing, and drying.

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15. Water acts as a Brønsted-Lowry base / proton acceptor, accepting a proton from HCl to form H₃O⁺ and Cl⁻. [1 mark]

• Accept: Water ionises/dissociates the HCl molecules.

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Section D: Consolidation (0 marks)

16. No answer – placeholder for structural alignment.

17. No answer – placeholder for structural alignment.

18. No answer – placeholder for structural alignment.

19. No answer – placeholder for structural alignment.

20. No answer – placeholder for structural alignment.

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END OF ANSWER KEY