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Secondary 4 Pure Chemistry Acids Bases Salts Quiz
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Secondary 4 Pure Chemistry Quiz - Acids Bases Salts (Answer Key)
Section A: Short Answer Questions [15 marks]
1. Complete the following word equation: [1 mark]
Answer: Zinc carbonate + Hydrochloric acid → Zinc chloride + Water + Carbon dioxide
2. State the colour change observed when universal indicator is added to: [2 marks]
(a) Dilute hydrochloric acid: Red [1 mark]
(b) Sodium hydroxide solution: Blue/Purple [1 mark]
3. Name the gas produced when magnesium reacts with dilute sulfuric acid. [1 mark]
Answer: Hydrogen (gas)
4. Write the balanced chemical equation for the neutralization of nitric acid with potassium hydroxide. Include state symbols. [2 marks]
Answer: HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l) [1 mark for correct formula, 1 mark for state symbols]
5. State two differences between a strong acid and a weak acid of the same concentration. [2 marks]
Possible answers (any 2):
- Strong acid ionizes completely/weak acid ionizes partially [1 mark]
- Strong acid has lower pH/weak acid has higher pH [1 mark]
- Strong acid conducts electricity better/weak acid conducts poorly [1 mark]
- Strong acid reacts faster with metals/weak acid reacts slower [1 mark]
6. Name a suitable indicator for the titration of ethanoic acid with sodium hydroxide. Explain your choice. [2 marks]
Answer: Indicator: Phenolphthalein [1 mark] Explanation: The equivalence point is above pH 7 (weak acid-strong base), and phenolphthalein changes colour at pH 8.3-10 [1 mark]
7. Complete the following ionic equation: [1 mark]
Answer: H⁺(aq) + OH⁻(aq) → H₂O(l)
8. State the solubility rule for sulfates and give one exception. [2 marks]
Answer: Rule: All sulfates are soluble in water [1 mark] Exception: Barium sulfate/Lead sulfate/Calcium sulfate (any one) [1 mark]
9. Name the type of reaction occurring when barium chloride solution is added to sodium sulfate solution. [1 mark]
Answer: Precipitation (reaction)
10. Write the formula of the salt formed when copper(II) oxide reacts with nitric acid. [1 mark]
Answer: Cu(NO₃)₂
Section B: Structured Questions [20 marks]
11. A student wants to prepare pure, dry crystals of zinc sulfate from zinc oxide and dilute sulfuric acid. [6 marks]
(a) Write the balanced chemical equation for this reaction. Include state symbols. [2 marks]
Answer: ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) [1 mark for correct equation, 1 mark for state symbols]
(b) Describe the method the student should use to prepare the salt. [4 marks]
Answer: Step 1: Add excess zinc oxide to dilute sulfuric acid and stir [1 mark] Step 2: Filter to remove unreacted zinc oxide [1 mark] Step 3: Heat the filtrate gently to concentrate the solution [1 mark] Step 4: Cool to allow crystals to form, then filter and dry [1 mark]
12. The diagram shows the pH changes during the titration of 25.0 cm³ of sodium hydroxide solution with hydrochloric acid. [8 marks]
(a) State the pH at the equivalence point. [1 mark]
Answer: pH 7 (or 7.0)
(b) Explain why the pH changes rapidly near the equivalence point. [2 marks]
Answer: At the equivalence point, all the base has been neutralized. A small addition of acid causes a large change in H⁺ ion concentration, resulting in a rapid pH change. [2 marks for complete explanation]
(c) 22.5 cm³ of 0.100 mol/dm³ hydrochloric acid was needed to neutralize the sodium hydroxide. Calculate the concentration of the sodium hydroxide solution. [3 marks]
Answer: Moles of HCl = 0.100 × 22.5/1000 = 0.00225 mol [1 mark] Moles of NaOH = 0.00225 mol (1:1 ratio) [1 mark] Concentration of NaOH = 0.00225 × 1000/25.0 = 0.0900 mol/dm³ [1 mark]
(d) Name a suitable indicator for this titration and state the colour change observed. [2 marks]
Answer: Indicator: Methyl orange (or phenolphthalein) [1 mark] Colour change: Yellow to red (or colourless to pink for phenolphthalein) [1 mark]
13. Ammonia gas can be prepared in the laboratory by heating ammonium chloride with calcium hydroxide. [6 marks]
(a) Write the balanced chemical equation for this reaction. [2 marks]
Answer: 2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2H₂O(l) + 2NH₃(g) [1 mark for correct reactants and products, 1 mark for balancing]
(b) Describe a chemical test to identify ammonia gas. [2 marks]
Answer: Test: Hold a glass rod dipped in concentrated hydrochloric acid near the gas [1 mark] Observation: White smoke/fumes of ammonium chloride are formed [1 mark]
(c) Ammonia dissolves in water to form an alkaline solution. Explain why this solution is alkaline. [2 marks]
Answer: Ammonia accepts protons from water molecules: NH₃ + H₂O → NH₄⁺ + OH⁻. The formation of OH⁻ ions makes the solution alkaline. [2 marks for complete explanation with equation]
Total: 35 marks
Marking Notes:
- Accept alternative correct answers where appropriate
- Award partial marks for incomplete but correct working
- Deduct marks for missing or incorrect state symbols where specified
- Accept common chemical names (e.g., "hydrogen gas" for "hydrogen")