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Secondary 3 Chemistry Practice Paper 4

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TuitionGoWhere Practice Paper - Chemistry Secondary 3

Answer Key & Marking Scheme Topic: Acids, Bases and Salts
Version: 4 of 5

Section A: Structured Questions

1. (a) Solution B [1] (b) Solution A [1] (c)

  • Strong acid ionises/dissociates completely in water [1].
  • Weak acid ionises/dissociates partially in water [1]. (d) Calcium carbonate / Magnesium carbonate / Any insoluble carbonate or weak base [1]. (Note: Adding a strong alkali like NaOH is dangerous and hard to control; insoluble carbonates react until acid is used up).

2. (a) An amphoteric substance reacts with both acids and bases to form salts and water [1]. (b) (i) ZnO(s)+H2SO4(aq)→ZnSO4(aq)+H2O(l)ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l) [2] (1 for formulae, 1 for balancing/states) (ii) ZnO(s)+2NaOH(aq)→Na2ZnO2(aq)+H2O(l)ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l) [2] (1 for formulae, 1 for balancing/states)

3. (a) To ensure all the sulfuric acid is reacted/neutralised [1]. (b) Filtration [1]. (c)

  • Copper(II) sulfate is a hydrated salt [1].
  • Evaporating to dryness would remove the water of crystallisation, forming anhydrous white powder instead of blue crystals [1]. (d) Blue [1].

4. (a) A white precipitate is formed [1] which is insoluble in dilute nitric acid [1]. (b) To remove any carbonate or sulfite ions that might also form a white precipitate with barium ions [1]. (c) Ba2+(aq)+SO42−(aq)→BaSO4(s)Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) [2] (1 for correct ions, 1 for state symbols/balancing).

5. (a) Ammonia [1]. (b)

  • Test: Hold damp red litmus paper near the mouth of the test tube [1].
  • Observation: The red litmus paper turns blue [1]. (c)
  • Alkaline substances react with ammonium salts to release ammonia gas [1].
  • This results in the loss of nitrogen from the fertiliser, making it less effective [1].

6. (a) 13 [1]. (b) 25.0 cm325.0 \text{ cm}^3 [1]. (c)

  • Moles of NaOH = 25.01000×0.10=0.0025 mol\frac{25.0}{1000} \times 0.10 = 0.0025 \text{ mol} [1].
  • Equation: NaOH+HCl→NaCl+H2ONaOH + HCl \rightarrow NaCl + H_2O. Ratio is 1:1.
  • Moles of HCl = 0.0025 mol0.0025 \text{ mol} [1].
  • Concentration of HCl = 0.00250.025=0.10 mol/dm3\frac{0.0025}{0.025} = 0.10 \text{ mol/dm}^3 [1].

7. (a) Acid X (Hydrochloric acid) [1]. (b)

  • Hydrochloric acid is a strong acid and fully ionises, producing a higher concentration of H+H^+ ions [1].
  • Ethanoic acid is a weak acid and partially ionises, producing a lower concentration of H+H^+ ions [1].
  • Higher concentration of H+H^+ leads to a higher frequency of effective collisions with magnesium [1]. (c) No [1]. The total volume of hydrogen depends on the number of moles of acid available (since Mg is in excess). Since both acids have the same volume and concentration, they contain the same number of moles of potential H+H^+ ions [1].

8. (a) Precipitation / Double decomposition [1]. (b) Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq) [2] (1 for correct formulae, 1 for balancing/states). (c)

  • Filter the mixture to collect the precipitate [1].
  • Wash the residue with distilled water to remove soluble impurities [1].
  • Dry the residue between filter papers or in an oven [1].

9. (a) Calcium oxide (Quicklime) OR Calcium hydroxide (Slaked lime) OR Calcium carbonate (Limestone) [1]. (b) These compounds are bases/alkaline. They neutralise the excess acid (H+H^+ ions) in the soil [1], raising the pH towards neutral [1]. (c) Enzymes in plants/soil bacteria work best at specific pH levels; nutrients are more available at optimal pH [1].

10. (a) Zinc (Zn2+Zn^{2+}) OR Aluminium (Al3+Al^{3+}) [1]. (Accept either, as both dissolve in excess NaOH). (b) Copper(II) (Cu2+Cu^{2+}) [1]. (c) Ammonium (NH4+NH_4^+) [1].

Section B: Free-Response Questions

11. (a)

  • Add dilute HCl to each solid [1].
  • Sodium Carbonate: Effervescence/bubbles produced. Gas turns limewater milky (CO2CO_2) [1].
  • Calcium Carbonate: Effervescence/bubbles produced. Gas turns limewater milky (CO2CO_2) [1]. (Note: To distinguish Na2CO3 and CaCO3, one is soluble in water, one is not. But the question asks for a test using HCl. Both fizz. However, CaCO3 is a solid rock-like structure, Na2CO3 is a powder. If the question implies distinguishing all three, the student might note that NaCl does NOT fizz. To distinguish the two carbonates, solubility in water is better, but strictly using HCl: NaCl = no reaction. Carbonates = fizz. If forced to distinguish all 3 with ONLY HCl: It is difficult without observing solubility first. Accept: NaCl: No observation. Carbonates: Effervescence.)
    • Refined Answer for Marks:
      • Sodium Chloride: No observable change / No effervescence [1].
      • Sodium Carbonate: Effervescence / Bubbles of gas produced [1].
      • Calcium Carbonate: Effervescence / Bubbles of gas produced [1].
      • (Note: To fully distinguish the two carbonates, one would usually check solubility in water first. However, based strictly on HCl reaction, NaCl is the odd one out. If the question implies distinguishing the carbonates from each other, additional steps are needed. Given the marks, identifying NaCl as non-reactive is key.)

(b) (i) White precipitate [1]. (ii) Ag+(aq)+Cl−(aq)→AgCl(s)Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s) [2].

(c) To remove any carbonate ions that might interfere by forming a white precipitate of silver carbonate [1].

(d)

  • Hydrochloric acid contains chloride ions (Cl−Cl^-) [1].
  • Adding it would introduce chloride ions into the solution, causing a white precipitate of silver chloride to form regardless of whether the original solid contained chloride ions [1].
  • This gives a false positive result [1]. (Max 2 marks).