Secondary 4 Pure Chemistry Acids Bases Salts Quiz

Secondary 4 Pure Chemistry Quiz - Acids Bases Salts (Answer Key)

Total Marks: 40
Topic: Acids, Bases & Salts

Section A (1 mark each)

1. SO₂
Teaching note: Sulfur burns in air: S(s) + O₂(g) → SO₂(g). SO₂ dissolves in rain to form sulfurous acid (H₂SO₃), contributing to acid rain. Common mistake: writing SO₃ (formed only on further catalytic oxidation).

2. Strong base
Teaching note: pH 13 is high on the 0–14 scale; values above 7 are alkaline, and 13 indicates a strong base (almost complete dissociation of OH⁻).

3. Potassium nitrate (KNO₃)
Teaching note: Acid (HNO₃) + base (KOH) → salt + water. Salt name: metal from base (potassium) + nitrate from nitric acid.

4. Litmus
Teaching note: Litmus is a natural dye from lichen; red in acid, blue in alkali.

5. H⁺ (or H₃O⁺)
Teaching note: Acidity in water is due to hydrogen ions (often written H₃O⁺, hydronium). Award H⁺ or H₃O⁺.

Section B (2 marks each)

6. Add NaOH(aq) dropwise.

• Al³⁺: white precipitate forms, dissolves in excess NaOH to colourless solution.
• Pb²⁺: white precipitate forms, insoluble in excess NaOH.
  (2m: 1 for reagent + method, 1 for correct observations for both)

7. CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)
(2m: 1 equation, 1 state symbols)
Teaching: Acidic oxide neutralises alkali. Common error: CO₂ + NaOH → NaHCO₃ (only with limited NaOH).

8. Two of: vigorous effervescence (H₂ gas); metal dissolves; solution may become warm; bubbles.
(2m: 1m each observation)

9. HCl dissociates fully into H⁺ and Cl⁻ in water (strong acid); ethanoic acid only partially dissociates (CH₃COOH ⇌ CH₃COO⁻ + H⁺), so fewer H⁺ present (weak acid).
(2m: 1m each concept)

10. pH about 6–7 (slightly acidic to neutral); rainwater is naturally slightly acidic due to dissolved CO₂, not strongly polluted.
(2m: 1m pH estimate, 1m nature)

11. Method: add excess insoluble base to acid, warm, filter off excess solid, evaporate filtrate, crystallise.
(2m: 1m method name, 1m steps)

12. H⁺(aq) + OH⁻(aq) → H₂O(l)
(2m: correct ions 1m, correct water 1m)

13. Calcium oxide (quicklime) or calcium hydroxide (slaked lime); cheap, neutralises acid, improves soil structure.
(2m: 1m substance, 1m reason)

14. ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l)
(2m: 1m products, 1m balanced + states)

15. Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)
(2m: 1m equation, 1m states)
Teaching: Chlorine is an acidic gas/oxide equivalent, disproportionates in cold alkali.

Section C (3 marks each)

16. (3m)

• Potassium is very high in reactivity series; reaction with dilute HCl is violent, explosive, releases much heat (1m observation).
• Unsuitable because uncontrolled, dangerous, difficult to collect pure KCl safely (1m suitability).
• Conclusion: not suitable; use less reactive metal e.g. Mg or Zn (1m).
  Teaching: Template 3 – safety and control over feasibility.

17.
(a) 25 cm³ (1m) from graph inflection at pH 7.
(b) n(NaOH) = 0.100 × (25/1000) = 0.00250 mol.
HCl + NaOH → NaCl + H₂O, 1:1, so n(HCl)=0.00250 mol in 25 cm³.
Conc = 0.00250 / 0.025 = 0.100 mol/dm³. (2m: 1m moles, 1m concentration)
Teaching: equivalence = neutralisation; use C = n/V with dm³.

18.
(a) Na₂CO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + CO₂(g) + H₂O(l) (1m)
(b) Heat solution to evaporate water until saturated, leave to crystallise, filter crystals, dry between paper towels. (2m)
Teaching: soluble salt from carbonate + acid; crystallisation method.

19. (3m)

• Acid has high [H⁺(aq)]; base provides OH⁻(aq). (1m)
• H⁺ + OH⁻ → H₂O reduces free H⁺ concentration. (1m)
• Lower [H⁺] means higher pH (less acidic). (1m)

20.
(a) 2SO₂(g) + O₂(g) → 2SO₃(g) (1m)
(b) SO₃(g) + H₂O(l) → H₂SO₄(aq) (1m) [or SO₂ + H₂O → H₂SO₃]
(c) Kills fish / damages trees / corrodes buildings. (1m)
Teaching: Template 1 gas formation and acid rain link.