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O Level Chemistry Practice Paper 1
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TuitionGoWhere Practice Paper - Chemistry O-Level - MARKING SCHEME
Total Marks: 80
Section A [40 marks]
Question 1 [16 marks]
(a) Complete the following table about acids and bases. [4]
| Property | Strong Acid | Weak Acid | Strong Base |
|---|---|---|---|
| Degree of ionization | Complete | Partial [1] | Complete |
| pH range (0.1 mol/dm³) | 0-2 [1] | 2-6 | 12-14 [1] |
| Example | HCl | CH₃COOH/ethanoic acid [1] | NaOH/KOH [1] |
Marking notes: Accept any correct weak acid or strong base example.
(b)(i) Write a balanced chemical equation for this reaction. [2]
Answer: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O [2]
Marking notes: Correct formulae [1], balanced equation [1]. Accept state symbols.
(b)(ii) Write the ionic equation for neutralization. [2]
Answer: H⁺ + OH⁻ → H₂O [2]
Marking notes: Correct ions with charges [1], correct product [1].
(c)(i) Calculate the concentration of the sulfuric acid. [4]
Answer: Equation: H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O Moles of KOH = 0.100 × 25.0/1000 = 0.00250 mol [1] From equation: 1 mol H₂SO₄ reacts with 2 mol KOH Moles of H₂SO₄ = 0.00250/2 = 0.00125 mol [1] Concentration of H₂SO₄ = 0.00125/(12.5/1000) [1] = 0.100 mol/dm³ [1]
Marking notes: Award marks for correct method even if arithmetic errors.
(c)(ii) Suggest a suitable indicator for this titration and explain your choice. [2]
Answer: Indicator: Methyl orange/phenolphthalein [1] Explanation: This is a strong acid-strong base titration, so any indicator that changes color around pH 7 is suitable [1]
Marking notes: Accept universal indicator. Must explain suitability for strong acid-strong base.
Question 2 [17 marks]
(a)(i) Write a balanced chemical equation for this reaction. [2]
Answer: ZnCO₃ + 2HCl → ZnCl₂ + H₂O + CO₂ [2]
Marking notes: Correct formulae [1], balanced equation [1].
(a)(ii) Describe the complete method to prepare pure, dry crystals of zinc chloride. [6]
Answer: Step 1: Add excess zinc carbonate to dilute hydrochloric acid [1] Step 2: Stir until no more zinc carbonate dissolves/effervescence stops [1] Step 3: Filter to remove unreacted zinc carbonate [1] Step 4: Heat the filtrate gently to evaporate water until crystals start to form [1] Step 5: Allow to cool and crystallize [1] Step 6: Filter the crystals and dry between filter papers [1]
Marking notes: Key steps: excess reactant, filtration, evaporation, crystallization, drying.
(b)(i) Write a balanced chemical equation for this reaction. [2]
Answer: AgNO₃ + NaCl → AgCl + NaNO₃ [2]
Marking notes: Correct formulae [1], balanced equation [1].
(b)(ii) Explain why a different method is needed to prepare silver chloride compared to zinc chloride. [2]
Answer: Silver chloride is insoluble in water [1], so it forms a precipitate and can be prepared by precipitation method, whereas zinc chloride is soluble [1].
Marking notes: Must mention solubility difference and relate to method.
(c) State the solubility of the following salts in water: [3]
Answer: Potassium sulfate: Soluble [1] Lead(II) chloride: Insoluble [1] Calcium carbonate: Insoluble [1]
Marking notes: Accept "slightly soluble" for lead(II) chloride.
Question 3 [7 marks]
(a) Name the gas collected in this experiment. [1]
Answer: Hydrogen [1]
(b) Write the test for this gas. [2]
Answer: Test: Apply a lighted/burning splint [1] Positive result: Burns with a squeaky pop [1]
Marking notes: Must mention both test and result.
(c)(i) Write a balanced chemical equation for this reaction. [2]
Answer: Zn + 2HCl → ZnCl₂ + H₂ [2]
Marking notes: Correct formulae [1], balanced equation [1].
(c)(ii) Calculate the volume of gas produced at RTP. [3]
Answer: Moles of Zn = 0.65/65 = 0.010 mol [1] From equation: 1 mol Zn produces 1 mol H₂ Moles of H₂ = 0.010 mol [1] Volume of H₂ = 0.010 × 24 = 0.24 dm³ [1]
Marking notes: Award method marks even with arithmetic errors.
(d)(i) State what would be observed. [1]
Answer: No reaction/no gas evolved/no change [1]
(d)(ii) Explain this observation using the reactivity series. [2]
Answer: Copper is below hydrogen in the reactivity series [1], so it cannot displace hydrogen from acids [1].
Marking notes: Must mention reactivity series and position relative to hydrogen.
Section B [40 marks]
Question 4 [15 marks]
(a)(i) Write a balanced chemical equation for the Haber process. [2]
Answer: N₂ + 3H₂ ⇌ 2NH₃ [2]
Marking notes: Correct formulae [1], balanced with reversible arrow [1].
(a)(ii) State two conditions used in the Haber process. [2]
Answer: Condition 1: High temperature (400-500°C) [1] Condition 2: High pressure/iron catalyst [1]
Marking notes: Accept any two correct conditions from: high temperature, high pressure, iron catalyst.
(b)(i) Write an equation to show how ammonia forms an alkaline solution. [2]
Answer: NH₃ + H₂O → NH₄⁺ + OH⁻ [2]
Marking notes: Correct formulae and charges [1], correct products [1].
(b)(ii) State the pH range of aqueous ammonia. [1]
Answer: 8-11/above 7 [1]
Marking notes: Accept any range above 7.
(c)(i) Describe the test for ammonia gas. [2]
Answer: Test: Hold damp red litmus paper in the gas [1] Positive result: Red litmus turns blue [1]
Marking notes: Must specify damp red litmus and color change.
(c)(ii) Write a balanced chemical equation for the reaction between ammonium chloride and sodium hydroxide. [2]
Answer: NH₄Cl + NaOH → NaCl + NH₃ + H₂O [2]
Marking notes: Correct formulae [1], balanced equation [1].
(d)(i) Write a balanced chemical equation for this reaction. [2]
Answer: 4NH₃ + 5O₂ → 4NO + 6H₂O [2]
Marking notes: Correct formulae [1], balanced equation [1].
(d)(ii) Name the catalyst used in this reaction. [1]
Answer: Platinum [1]
(d)(iii) State one large-scale use of nitric acid. [1]
Answer: Manufacture of fertilizers/explosives [1]
Marking notes: Accept any correct industrial use.
Question 5 [14 marks]
(a)(i) Identify the metal ion in each solution. [3]
Answer: Solution A: Al³⁺/aluminum ion [1] Solution B: Cu²⁺/copper(II) ion [1] Solution C: Fe²⁺/iron(II) ion [1]
Marking notes: Accept correct names or formulae.
(a)(ii) Write an ionic equation for the formation of the precipitate in solution B. [2]
Answer: Cu²⁺ + 2OH⁻ → Cu(OH)₂ [2]
Marking notes: Correct ions and charges [1], correct precipitate [1].
(b)(i) Describe the test for chloride ions. [3]
Answer: Reagent: Dilute nitric acid followed by silver nitrate solution [1] Observation: White precipitate forms [1] Confirmatory test: Precipitate dissolves in dilute ammonia solution [1]
Marking notes: Must include acidification step and confirmatory test.
(b)(ii) Describe the test for sulfate ions. [2]
Answer: Reagent: Dilute nitric acid followed by barium nitrate/barium chloride solution [1] Observation: White precipitate forms [1]
Marking notes: Must include acidification step.
(c) Describe how you would distinguish between these two compounds using chemical tests. [4]
Answer: Test: Add dilute hydrochloric acid [1] Observation with sodium carbonate: Effervescence/bubbles/CO₂ gas evolved [1] Observation with sodium sulfate: No reaction/no gas evolved [1] Conclusion: The compound that produces gas is sodium carbonate [1]
Marking notes: Must include test, observations for both compounds, and conclusion.
Question 6 [11 marks]
(a)(i) State what happens at the cathode during this electrolysis. [2]
Answer: Copper ions are reduced/gain electrons [1] and copper metal is deposited [1]
Marking notes: Must mention both reduction and deposition.
(a)(ii) Write the electrode equation for the reaction at the anode. [2]
Answer: Cu → Cu²⁺ + 2e⁻ [2]
Marking notes: Correct formulae and charges [1], balanced electrons [1].
(a)(iii) Explain what happens to the concentration of copper(II) sulfate solution during this process. [2]
Answer: The concentration remains constant [1] because copper ions are removed at the cathode but replaced by copper ions from the anode [1]
Marking notes: Must explain why concentration stays constant.
(b)(i) Name the products formed at each electrode. [2]
Answer: Cathode: Hydrogen [1] Anode: Oxygen [1]
(b)(ii) Write electrode equations for the reactions at both electrodes. [4]
Answer: Cathode: 2H⁺ + 2e⁻ → H₂ [2] Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ [2]
Marking notes: Award 2 marks for each correct equation (formulae and balancing).
(c) State one industrial use of electrolysis. [1]
Answer: Electroplating/purification of metals/extraction of metals [1]
Marking notes: Accept any correct industrial application.
END OF MARKING SCHEME
Grade Boundaries (Suggested):
- A: 72-80 marks (90-100%)
- B: 64-71 marks (80-89%)
- C: 56-63 marks (70-79%)
- D: 48-55 marks (60-69%)
- E: 40-47 marks (50-59%)