2024 CGS Chemistry Prelim P2 Mark Scheme

Page 1 of 22 CRESCENT GIRLS’ SCHOOL SECONDARY FOUR PRELIMINARY EXAMINATION 2024 Paper 2 READ THESE INSTRUCTIONS FIRST Candidates answer on the Question Paper. No Additional Materials are required. Write your name, index number and class in the spaces provided at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graph. Do not use staples, paper clips, and glue or correction fluid. Section A (70 Marks) Answer all questions Write your answers in the spaces provided Answer one question. Write your answers in the spaces provided. Answer one question. Write your answers in the spaces provided. The number of marks is given in brackets [ ] at the end of each question or part question. A copy of the Periodic Table is printed on page 23. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Section A Section B Deductions Significant Figures Units Total 80 This paper consists of 22 printed pages including the cover page. CHEMISTRY Class: Register No: Name: 6092/02 23 August 2024 1 hr 45 mins

Page 2 of 22 Section A Answer all questions in this section in the spaces provided. The total mark for this section is 70. A1 The figure below shows ‘dot-and-cross’ diagrams for molecules that contain elements from the first two periods of the Periodic Table. The elements are represented by the letters M, Q, R, T and Z. Each diagram shows outer electrons only. Use the letters M, Q, R, T and Z to answer the questions below. (a) Which element can form an ion with a charge of 1-? [1] Q or Z (b) Which element can gain, lose and share electrons? [1] Z (c) Which element can form an acidic oxide? [1] M or Q or R (d) Which element forms a triple covalent bond? [1] M

Page 3 of 22 (e) (i) Potassium reacts with element T to form a compound. Draw a dot-and-cross diagram of the compound formed between potassium and element T. Show only the valence electrons. [2] Correct charge – [1], correct ratio – [1] (ii) State one physical property of the above compound and explain the reason for the physical property. [2] High mp/bp (✓) – strong electrostatic FOA between oppositely charged ions (✓) and hence large amount of energy (✓) to overcome. OR Good electrical conductor (✓) in aqueous/molten (✓) state – ions are mobile (✓) to conduct electricity OR Poor electrical conductor (✓) in solid (✓) state – ions are in fixed positions (✓) and cannot conduct electricity 3 (✓) – [2], 1 – 2 (✓) – [1] Allow ecf if the properties match the dot and cross diagram [Total: 8 marks] K T

Page 4 of 22 A2 The table below shows information about the preparation of pure samples of some solid salts. Complete the table by filling in the missing information. Include state symbols with any formulae. formulae of salt formulae of reagent 1 formulae of reagent 2 method of preparation CaCO3 (s) Ca(NO3)2, CaCl2 (must be aqueous) (✓) Group I/ammonium carbonate (aq) (✓) Precipitation (✓) Ag2SO4 (s) Ag2CO3 (s)/ Ag2O (s), AgOH (s) (✓) H2SO4 (aq) Adding excess solid to acid evaporation and crystallisation NH4NO3 (s) HNO3 (aq) NH3 (aq) (✓) Titration (✓) evaporation and crystallisation [5] 6(✓) – [5], 5(✓) – [4], 3 – 4 (✓) – [3], 2 (✓) – [2], 1 (✓) – [1] Formula and state symbols must be correct to be given (✓). [Total: 5 marks] A3 Nitrogen dioxide is an acidic oxide. It dissolves in water to form two acids, nitric acid and nitrous acid, HNO2 in a single reaction (a) (i) Write a balanced chemical equation for the above reaction. [1] 2NO2 + H2O → HNO3 + HNO2 (ii) Disproportionation is a reaction when the same substance is oxidised and reduced in the same reaction. Explain why the reaction in (a)(i) is a disproportionation reaction. [2] NO2 is oxidised as the O.S of N increases from +4 to +5 in HNO3. [1] NO2 is reduced as the O.S of N decreases from +4 to +3 in HNO2 [1] (b) One of the main sources of nitrogen monoxide, NO is from the combustion engines of vehicles. (i) State how nitrogen monoxide, NO is formed in combustion engines of vehicles. [2]

Page 5 of 22 Oxides of nitrogen are formed at high temperatures (✓) when large amount of energy (✓) is absorbed to break the N-N triple bonds (✓) in N2. 3 (✓) – [2]; 1 – 2 (✓) – [1] (ii) Hence, explain with the aid of a chemical equation how nitrogen monoxide is removed by catalytic converters fitted in cars. [2] Oxides of nitrogen react with carbon monoxide to form nitrogen gas and carbon dioxide. [1] 2NO + 2CO → N2 + 2CO2 [1] The amount of air in the pollutant gases that enter the catalytic converter affects the reactions in the converter. The graph shows the percentage of nitrogen monoxide that the catalytic converter successfully removed. (iii) Using the equation in (ii) and the graph above, explain why the percentage of nitrogen monoxide successfully removed by catalytic converter decreases as the amount of air increases. [2] As amount of air increases, carbon monoxide will react with more oxygen to form carbon dioxide [1]. Lesser CO present to react with NO [1] and hence lesser NO will be successfully removed from catalytic converter. [Total: 9 marks]

Page 6 of 22 A4 A student electrolysed aqueous copper(II) sulfate using two sets-ups shown below. The electrodes used in each set-up are made of the same material. However, the electrodes used in set-ups A and B are made of different materials. He recorded the following observations in the two set-ups. set-up A set-up B mass of electrode A1 increased mass of electrode B1 increased mass of electrode A2 remained the same mass of electrode B2 decreased effervescence observed at electrode A2 blue copper(II) sulfate solution fades in colour no effervescence observed at B2 (blue copper(II) sulfate solution remains unchanged) (a) Name the particles which transfer charges through the [1] (i) copper wires electrons (ii) copper(II) sulfate solution Ions both (✓) to get [1] (b) State which electrode is the cathode in each set-up. Set-up A: A1 Set-up B: B1 both (✓) to get [1] [1] (c) Explain, with an appropriate equation, the increase in mass at electrodes at A1 and B1. [2] Copper(II) ions gain electrons OR are discharged/reduced preferentially to form copper solid. [1] Cu2+ (aq) + 2e → Cu (s) [1] set-up A set-up B copper wires

Page 7 of 22 (d) Write the half-equations of the reactions taking place at Electrode A2 and Electrode B2. [2] Half-equation at A2: 4OH- (aq) → 2H2O (l) + O2 (g) + 4e [1] Half-equation at B2: Cu (s) → Cu2+ (aq) + 2e [1] (e) Describe how the electrolyte of set-up A would change by the end of experiment in terms of its pH and explain why. [2] Cu2+ and OH- ions are preferentially discharged, leaving behind H+ ions [1] and hence pH of solution will become acidic/pH will decrease from pH 7 to below 7 [1]. (f) Suggest the materials that are used to make the electrodes in [1] (i) Set-up A: carbon/graphite (ii) Set-up B: copper both (✓) to ge