Zhonghua Prelim P2

ZHONGHUA SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY 4 EXPRESS Candidate’s Name Class Register Number CHEMISTRY 6092 /02 Paper 2 11 September 2024 1 hour 45 minutes Additional Materials: NIL READ THESE INSTRUCTIONS FIRST Write your name, class and register number in the spaces at the top of this page and on all separate answer paper used. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Write your answers in the spaces provided on the question paper. Section B Answer one question. For Examiner’s Use Write your answers in the spaces provided on the question paper. Section A 70 Section B 10 Total 80 At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. All essential working must be shown clearly. A copy of the Periodic Table is printed on page 24. The use of an approved scientific calculator is expected, where appropriate. This document consists of 24 printed pages.

2 Section A Answer all the questions. 1 The table below consists of four different experimental procedures. (a) Put a tick (✓) if a redox reaction had occurred. experimental procedure Put a tick (✓) if a redox reaction has occurred. (i) a copper strip added to zinc nitrate solution (ii) aqueous chlorine added to potassium iodide solution (iii) hydrogen gas passed over heated copper(II) oxide (iv) dilute sulfuric acid added to lithium hydroxide [2] (b) A student repeated (a)(ii) by replacing aqueous chlorine with aqueous bromine. Describe a difference in the rates of the two reactions and explain your answer. [2] (c) Write the chemical equation for the reaction in (a)(iii). [1] [Total: 5]

3 [Turn over 2 (a) Glasses are mainly made of silicates. Soda-lime glass is made by heating a mixture of calcium carbonate, sodium carbonate and sand in a furnace to a high temperature. The simplified structures of silicate and soda-lime glass are shown in Fig. 2.1 and Fig. 2.2. silicates soda-lime glass Fig. 2.1 Fig. 2.2 (i) Describe in terms of bonding two differences in silicate and soda-lime glass. [2] (ii) Predict and explain the electrical conductivity of soda-lime glass. [2]

4 (b) Potassium hydroxide is used as an electrolyte in alkaline batteries and in electroplating. Table 2.1 shows the melting and boiling points of potassium hydroxide and chlorine. Table 2.1 melting point/ oC boiling point/ oC chlorine -101 -35 potassium hydroxide 360 1327 (i) Explain, in terms of structure and bonding, why potassium hydroxide has a much higher melting point than chlorine gas. [3] (ii) Draw a “dot-and cross” diagram to show the bonding in a hydroxide ion. Show outer shell electrons only. [2] [Total: 9]

5 [Turn over 3 Lavender flowers contain a variety of pigments including X, Y and Z. These pigments were separated using paper chromatography with a mixture of ethanol and water as a solvent. Fig. 3.1 shows how the Rf value of the pigments varies with different compositions of ethanol. Fig. 3.1 (a) Explain the principle behind paper chromatography. [1] (b) (i) Using Fig. 3.1, state and explain the number of spots obtained from the chromatography using lavender flowers, when the solvent consisted of 40% ethanol. [2] (ii) Using Fig. 3.1, explain why a pure solvent of water or ethanol is not suitable in separating the pigments effectively. [2] [Total: 5] pigment Z pigment X pigment Y

6 4 Electrolysis is commonly used to give an object an attractive appearance or to prevent corrosion of a metal. The apparatus shown in Fig. 4.1 was prepared to electroplate an iron object using 200 cm3 of aqueous copper(II) sulfate as the electrolyte. Fig. 4.1 Electrolysis was carried out and the graph below shows the concentration of aqueous copper(II) sulfate against time. (a) Explain the shape of the graph by using ionic half-equation to support your answer. [2] (b) After the electrolysis has been running for some time, describe a test that the student can perform to ensure that all the copper(II) ions has been discharged. [1] 200 cm3 of aqueous copper(II) sulfate concentration of CuSO4(aq)/ mol dm-3 time/min

7 [Turn over (c) Using data from the graph, calculate the increase in mass of the iron object after the electrolysis was conducted for 80 minutes. increase in mass of the iron object [2] (d) Fig. 4.2 was modified from Fig.4.1. Electrolysis was conducted using the same quantity of electricity as in Fig. 4.1. Fig. 4.2 (i) Sketch a line on the graph below to show the concentration of aqueous copper(II) sulfate against time in cell A. The initial concentration of aqueous copper(II) sulfate in cell A was 1.00 mol/dm3. [1] concentration of CuSO4/ moldm-3 time/min

8 (ii) Complete Table 4.3 to describe the reactions in cell B. Table 4.3 solution name of products of electrolysis ionic equation for reaction at each electrode silver nitrate solution at anode at cathode [2] (iii) Calculate the increase in mass of the iron object in cell B. increase in mass of iron object [2] [Total: 10]

9 [Turn over 5 Ammonia can be used to manufacture fertilisers and hydrazine. The reaction between nitrogen and hydrogen to form ammonia is a reversible process. The chemical reaction is shown below. N2 + 3H2 ⇌ 2NH3 ΔH = – 92.4 kJ / mol Table 5.1 shows how the percentage yield of ammonia at equilibrium varies with both temperature and pressure. Table 5.1 pressure/ atm % yield at 200 oC % yield at 300 oC % yield at 400 oC % yield at 500 oC 30 68 32 11 4 100 81 51 25 10 200 86 63 36 18 300 88 69 40 24 Ammonia can be used to manufacture hydrazine, N2H4 in a reaction called the Raschig process. The process involves a two-step process in the presence of sodium hypochlorite, NaOCl to convert ammonia to hydrazine. In step 1, ammonia acts as a ‘hydrogen donor’ as it ‘donates’ on