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Secondary 4 Pure Chemistry Preliminary Examination Paper 3
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Questions
TuitionGoWhere Practice Paper – Pure Chemistry Secondary 4
TuitionGoWhere Secondary School (AI)
Subject: Pure Chemistry
Level: Secondary 4
Paper: PRELIMINARY EXAMINATION – Version 3 of 5
Duration: 1 hour 30 minutes
Total Marks: 75
Name: _________________________
Class: _________________________
Date: _________________________
Instructions to Candidates
- This paper consists of three sections: Section A, Section B, and Section C.
- Answer all questions in Section A and Section B.
- Section C contains two questions. Answer one question only.
- Write your answers in the spaces provided.
- You may use a calculator.
- The number of marks is given in brackets [ ] at the end of each question or part question.
- Show all working for calculation questions.
Section A: Multiple Choice (15 marks)
Answer all questions. Circle the letter of the correct answer.
1. Which of the following is a strong acid?
A. Ethanoic acid
B. Carbonic acid
C. Hydrochloric acid
D. Citric acid
[1]
2. A student adds excess zinc powder to dilute sulfuric acid. Which gas is produced?
A. Oxygen
B. Hydrogen
C. Carbon dioxide
D. Sulfur dioxide
[1]
3. Which salt is soluble in water?
A. Barium sulfate
B. Lead(II) chloride
C. Calcium carbonate
D. Sodium nitrate
[1]
4. The pH of a solution changes from 3 to 5. How has the hydrogen ion concentration changed?
A. Increased by a factor of 2
B. Decreased by a factor of 2
C. Increased by a factor of 100
D. Decreased by a factor of 100
[1]
5. Which method is most suitable for preparing a pure, dry sample of lead(II) sulfate?
A. Titration of lead(II) nitrate with sulfuric acid
B. Reacting lead with dilute sulfuric acid
C. Precipitation by mixing lead(II) nitrate and sodium sulfate solutions, followed by filtration and drying
D. Reacting lead(II) oxide with dilute sulfuric acid
[1]
6. A solution of ammonia in water is a weak alkali. This means that in aqueous solution, ammonia:
A. Ionises completely to produce hydroxide ions
B. Does not produce any hydroxide ions
C. Ionises partially to produce hydroxide ions
D. Reacts completely with water
[1]
7. Which of the following reactions is a neutralisation reaction?
A. Zn + 2HCl → ZnCl₂ + H₂
B. CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
C. NaOH + HCl → NaCl + H₂O
D. 2Mg + O₂ → 2MgO
[1]
8. A student adds aqueous sodium hydroxide dropwise to a solution containing aluminium ions. What is observed?
A. A blue precipitate forms which is insoluble in excess
B. A white precipitate forms which dissolves in excess
C. A green precipitate forms which turns brown on standing
D. No precipitate forms
[1]
9. Which statement about the Haber Process is correct?
A. The reaction is carried out at low pressure to increase yield
B. An iron catalyst is used to increase the rate of reaction
C. The reaction is endothermic in the forward direction
D. Ammonia is removed to shift equilibrium to the left
[1]
10. A farmer tests the soil in a field and finds it has a pH of 4.5. Which substance should be added to neutralise the soil?
A. Ammonium nitrate
B. Calcium oxide
C. Hydrochloric acid
D. Sodium chloride
[1]
11. Which equation represents the reaction between an acid and a carbonate?
A. 2HCl + Mg → MgCl₂ + H₂
B. HCl + NaOH → NaCl + H₂O
C. 2HCl + CaCO₃ → CaCl₂ + CO₂ + H₂O
D. H₂SO₄ + BaCl₂ → BaSO₄ + 2HCl
[1]
12. A solution of sulfuric acid has a concentration of 0.5 mol/dm³. What volume of this acid is required to neutralise 25.0 cm³ of 1.0 mol/dm³ sodium hydroxide solution?
A. 12.5 cm³
B. 25.0 cm³
C. 50.0 cm³
D. 100.0 cm³
[1]
13. Which gas turns damp blue litmus paper red and then bleaches it white?
A. Ammonia
B. Carbon dioxide
C. Chlorine
D. Sulfur dioxide
[1]
14. When aqueous silver nitrate is added to a solution containing chloride ions, a white precipitate forms. What is the formula of this precipitate?
A. AgCl
B. AgCl₂
C. Ag₂Cl
D. AgNO₃
[1]
15. A student prepares copper(II) sulfate crystals by reacting excess copper(II) oxide with warm dilute sulfuric acid. Which step is carried out first after the reaction is complete?
A. Crystallisation
B. Evaporation to dryness
C. Filtration
D. Titration
[1]
Section B: Structured Questions (40 marks)
Answer all questions in the spaces provided.
16. A student investigates the reaction between magnesium ribbon and excess dilute hydrochloric acid.
(a) Write a balanced chemical equation, with state symbols, for the reaction. [2]
(b) State two observations the student would make during the reaction. [2]
(c) The student repeats the experiment using the same mass of magnesium powder instead of magnesium ribbon. Explain why the reaction is faster. [2]
(d) The student bubbles the gas produced through limewater. State and explain the expected observation. [2]
17. The diagram below shows the apparatus used to prepare a salt by titration.
[Diagram: Burette containing dilute sulfuric acid above a conical flask containing
sodium hydroxide solution and a few drops of methyl orange indicator]
(a) Name the salt that would be produced in this titration. [1]
(b) State the colour change of the methyl orange indicator at the end-point. [1]
(c) Explain why this method is suitable for preparing sodium sulfate but not suitable for preparing lead(II) sulfate. [3]
(d) The student uses 25.0 cm³ of 0.40 mol/dm³ sodium hydroxide solution. Calculate the volume of 0.25 mol/dm³ sulfuric acid required for complete neutralisation.
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O [3]
18. A student tests four unknown solutions labelled W, X, Y, and Z. The results are shown in the table.
| Test | Solution W | Solution X | Solution Y | Solution Z |
|---|---|---|---|---|
| Add NaOH(aq) dropwise, then in excess | White ppt, soluble in excess | Blue ppt, insoluble in excess | Green ppt, turns brown on standing | No ppt |
| Add NH₃(aq) dropwise, then in excess | White ppt, insoluble in excess | Blue ppt, soluble in excess (deep blue) | Green ppt, insoluble in excess | No ppt |
| Add AgNO₃(aq) followed by dilute HNO₃ | No ppt | No ppt | No ppt | White ppt |
(a) Identify the cation present in solution W. Explain your reasoning. [2]
(b) Identify the cation present in solution X. Explain your reasoning. [2]
(c) Identify the cation present in solution Y. Explain your reasoning. [2]
(d) Identify the anion present in solution Z. Write an ionic equation, with state symbols, for the reaction that occurs. [3]
19. Ammonia is manufactured industrially by the Haber Process.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol
(a) State the source of nitrogen and hydrogen used in the Haber Process. [2]
(b) Explain why a temperature of 450 °C is used rather than a lower temperature, even though the forward reaction is exothermic. [3]
(c) Explain why a pressure of 200 atm is used rather than atmospheric pressure. [2]
(d) State the role of the iron catalyst in the process. Explain how a catalyst works in terms of activation energy. [3]
20. A student prepares a sample of barium sulfate by mixing aqueous barium chloride and aqueous sodium sulfate.
(a) Write a balanced chemical equation, with state symbols, for the reaction. [2]
(b) Name the type of salt preparation method used. [1]
(c) Describe how the student can obtain a pure, dry sample of barium sulfate from the reaction mixture. [3]
(d) The student starts with 50.0 cm³ of 0.20 mol/dm³ barium chloride solution. Calculate the mass of barium sulfate that can be obtained. (Mᵣ of BaSO₄ = 233) [4]
Section C: Free-Response Questions (20 marks)
Answer one question only from this section. Indicate clearly which question you are answering.
Question 21
(a) Define the terms acid and base in terms of the ions they produce in aqueous solution. [2]
(b) Explain the difference between a strong acid and a weak acid, using hydrochloric acid and ethanoic acid as examples. Include equations in your answer. [4]
(c) Describe, with the aid of a labelled diagram, how you would prepare a pure, dry sample of copper(II) sulfate crystals starting from copper(II) oxide and dilute sulfuric acid. Include all steps and explain why each step is necessary. [8]
(d) A student adds excess calcium carbonate to 50.0 cm³ of 0.50 mol/dm³ hydrochloric acid.
(i) Write a balanced chemical equation, with state symbols, for the reaction. [2]
(ii) Calculate the volume of carbon dioxide gas produced at room temperature and pressure. (Molar volume of gas at r.t.p. = 24 dm³/mol) [4]
Question 22
(a) Describe the characteristic reactions of acids with the following substances, giving a balanced chemical equation for each:
(i) A reactive metal [3]
(ii) A metal carbonate [3]
(b) A farmer needs to determine whether a sample of fertiliser contains ammonium ions. Describe a chemical test, including the expected observation and a balanced equation, to confirm the presence of ammonium ions. [4]
(c) Explain why the pH of soil is important for plant growth. Describe how a farmer can adjust soil pH if it is too acidic. Include a balanced chemical equation in your answer. [4]
(d) A student carries out a titration to determine the concentration of a solution of potassium hydroxide. 25.0 cm³ of the potassium hydroxide solution required 20.0 cm³ of 0.10 mol/dm³ sulfuric acid for complete neutralisation.
2KOH + H₂SO₄ → K₂SO₄ + 2H₂O
(i) Calculate the number of moles of sulfuric acid used. [2]
(ii) Calculate the concentration of the potassium hydroxide solution in mol/dm³. [2]
(iii) Calculate the concentration of the potassium hydroxide solution in g/dm³. (Mᵣ of KOH = 56) [2]
END OF PAPER
Answers
TuitionGoWhere Practice Paper – Pure Chemistry Secondary 4
PRELIMINARY EXAMINATION – Version 3 of 5
ANSWER KEY AND MARKING SCHEME
Total Marks: 75
Section A: Multiple Choice (15 marks)
| Question | Answer | Mark |
|---|---|---|
| 1 | C | [1] |
| 2 | B | [1] |
| 3 | D | [1] |
| 4 | D | [1] |
| 5 | C | [1] |
| 6 | C | [1] |
| 7 | C | [1] |
| 8 | B | [1] |
| 9 | B | [1] |
| 10 | B | [1] |
| 11 | C | [1] |
| 12 | B | [1] |
| 13 | C | [1] |
| 14 | A | [1] |
| 15 | C | [1] |
Section B: Structured Questions (40 marks)
Question 16 (8 marks)
(a) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) [2]
Marking:
- Correct formulae: 1 mark
- Correct state symbols: 1 mark
- Accept: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g) for ionic equation
(b) Any two from: [2]
- Effervescence / bubbles of gas produced (1)
- Magnesium ribbon dissolves / disappears (1)
- Solution becomes warm / temperature increases (1)
- Colourless solution formed (1)
(c) Magnesium powder has a larger surface area than magnesium ribbon [1]. This increases the frequency of effective collisions between magnesium atoms and hydrogen ions, increasing the rate of reaction [1].
(d) No change / limewater remains colourless [1]. Hydrogen gas is produced, not carbon dioxide. Hydrogen does not react with limewater [1].
Question 17 (8 marks)
(a) Sodium sulfate [1]
(b) Yellow to orange / peach [1]
Accept: yellow to pink
(c) Titration is suitable for preparing soluble salts of Group I metals and ammonium salts [1]. Sodium sulfate is soluble in water, so titration can be used [1]. Lead(II) sulfate is insoluble in water, so precipitation must be used instead. Titration would not produce a visible end-point because lead(II) sulfate would precipitate during the titration [1].
(d) Moles of NaOH = 0.40 × (25.0/1000) = 0.0100 mol [1]
Moles of H₂SO₄ required = 0.0100 ÷ 2 = 0.00500 mol [1]
Volume of H₂SO₄ = 0.00500 ÷ 0.25 = 0.0200 dm³ = 20.0 cm³ [1]
Question 18 (9 marks)
(a) Al³⁺ (aluminium ion) [1]. A white precipitate forms with NaOH(aq) which dissolves in excess NaOH(aq), characteristic of Al³⁺ forming Al(OH)₃ which is amphoteric and dissolves to form [Al(OH)₄]⁻ [1].
(b) Cu²⁺ (copper(II) ion) [1]. A blue precipitate forms with NaOH(aq) which is insoluble in excess. With NH₃(aq), the blue precipitate dissolves in excess to form a deep blue solution (formation of [Cu(NH₃)₄]²⁺ complex) [1].
(c) Fe²⁺ (iron(II) ion) [1]. A green precipitate forms with NaOH(aq) which turns brown on standing due to oxidation of Fe(OH)₂ to Fe(OH)₃ by air [1].
(d) Cl⁻ (chloride ion) [1]. White precipitate of AgCl forms with AgNO₃(aq) [1].
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) [1]
Question 19 (10 marks)
(a) Nitrogen: from fractional distillation of liquid air [1]
Hydrogen: from cracking of hydrocarbons / from natural gas (methane) reacting with steam [1]
(b) At lower temperatures, the rate of reaction would be too slow to be economical [1], even though the equilibrium yield of ammonia would be higher [1]. 450 °C is a compromise temperature that gives a reasonably fast rate of reaction while still producing an acceptable yield [1].
(c) The forward reaction produces fewer moles of gas (4 moles → 2 moles) [1]. According to Le Chatelier's principle, increasing pressure shifts the equilibrium to the right, increasing the yield of ammonia [1].
(d) The iron catalyst increases the rate of reaction [1] without being consumed in the process [1]. A catalyst works by providing an alternative reaction pathway with a lower activation energy, so more particles have energy greater than or equal to the activation energy, increasing the frequency of effective collisions [1].
Question 20 (10 marks)
(a) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) [2]
Marking: Correct formulae (1), correct state symbols (1)
(b) Precipitation [1]
(c)
- Filter the mixture to separate the solid barium sulfate (residue) from the solution (filtrate) [1].
- Wash the residue with distilled water to remove any soluble impurities (NaCl) [1].
- Dry the residue between sheets of filter paper or in a warm oven [1].
(d) Moles of BaCl₂ = 0.20 × (50.0/1000) = 0.0100 mol [1]
Moles of BaSO₄ = 0.0100 mol (1:1 ratio) [1]
Mass of BaSO₄ = 0.0100 × 233 [1]
= 2.33 g [1]
Section C: Free-Response Questions (20 marks)
Question 21
(a) An acid is a substance that produces hydrogen ions (H⁺) in aqueous solution [1]. A base is a substance that produces hydroxide ions (OH⁻) in aqueous solution [1].
(b) A strong acid ionises completely in aqueous solution to produce H⁺ ions [1].
Example: HCl(aq) → H⁺(aq) + Cl⁻(aq) [1]
A weak acid ionises partially in aqueous solution to produce H⁺ ions [1].
Example: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) [1]
Accept: use of reversible arrow for weak acid essential
(c) Diagram (2 marks): Labelled diagram showing beaker with CuO and H₂SO₄, Bunsen burner/tripod/gauze for heating, filter funnel and filter paper, evaporating dish for crystallisation.
Procedure (6 marks):
- Add excess copper(II) oxide to warm dilute sulfuric acid and stir [1]. Excess CuO ensures all acid is neutralised [1].
- Filter the mixture to remove unreacted CuO [1]. This separates the insoluble excess CuO from the copper(II) sulfate solution [1].
- Heat the filtrate to evaporate some water until a saturated solution is obtained (crystals form on cooling rod / first signs of crystallisation) [1].
- Allow the saturated solution to cool slowly. Blue crystals of CuSO₄·5H₂O will form [1].
- Filter the crystals and wash with a little cold distilled water to remove impurities [1].
- Dry the crystals between sheets of filter paper [1].
Note: Award marks for clear, logical sequence with explanations.
(d)(i) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) [2]
Marking: Correct formulae (1), correct state symbols (1)
(d)(ii) Moles of HCl = 0.50 × (50.0/1000) = 0.0250 mol [1]
Moles of CO₂ = 0.0250 ÷ 2 = 0.0125 mol [1]
Volume of CO₂ = 0.0125 × 24 [1]
= 0.30 dm³ (or 300 cm³) [1]
Question 22
(a)(i) Acids react with reactive metals to produce a salt and hydrogen gas [1].
Example: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) [2]
Marking: Correct observation (1), correct balanced equation with state symbols (2)
(a)(ii) Acids react with metal carbonates to produce a salt, carbon dioxide, and water [1].
Example: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) [2]
Marking: Correct products (1), correct balanced equation with state symbols (2)
(b) Add aqueous sodium hydroxide to the fertiliser sample and warm gently [1]. Test any gas produced with damp red litmus paper [1]. If ammonium ions are present, ammonia gas is produced which turns damp red litmus paper blue [1].
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l) [1]
(c) Soil pH affects the availability of nutrients to plants [1]. If soil is too acidic, essential nutrients may become unavailable or toxic metals may dissolve [1]. The farmer can add calcium oxide (quicklime) or calcium carbonate (limestone) to neutralise the acidic soil [1].
CaO(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) [1]
Accept: CaCO₃ + 2H⁺ → Ca²⁺ + CO₂ + H₂O
(d)(i) Moles of H₂SO₄ = 0.10 × (20.0/1000) = 0.00200 mol [2]
Marking: Correct substitution (1), correct answer (1)
(d)(ii) Moles of KOH = 2 × 0.00200 = 0.00400 mol [1]
Concentration of KOH = 0.00400 ÷ (25.0/1000) = 0.160 mol/dm³ [1]
(d)(iii) Concentration in g/dm³ = 0.160 × 56 [1]
= 8.96 g/dm³ [1]
END OF ANSWER KEY