2022 SCGS Chem Prelims P2

CANDIDATE NAME CLASS 4 REGISTER NUMBER CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Theory 6092/02 23 August 2022 1 hour 45 minutes READ THESE INSTRUCTIONS FIRST Write your class, index number and name on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid/tape. Section A Answer all questions in the spaces provided. Section B Answer all three questions, the last question is in the form either/or. Answer all questions 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 21. The use of an approved scientific calculator is expected, where appropriate. This question paper consists of 21 printed pages and 1 blank page. SINGAPORE CHINESE GIRLS’ SCHOOL Preliminary Examination Secondary Four For Examiner’s Use Section A 50 Section B 30 Total 80

2 {Turn Over Section A Answer all questions in this section in the spaces provided. The total mark for this section is 50. A1 Use the substances in the list below to answer the following questions. Each option may be used once, more than once or not at all. aluminium iodide sodium chloride ammonium chloride zinc chlorine nitric acid sodium bromide potassium carbonate (a) Which substance in aqueous form reacts with aqueous chlorine to form a brown solution with black crystals? ………………………………………………………………………………………………... [1] (b) Which substance can reduce hydrochloric acid in a redox reaction? ………………………………………………………………………………………………... [1] (c) Which substance , when warmed in a mixture with aqueous sodium hydroxide and aluminium, produces a gas that turns moist litmus blue? ………………………………………………………………………………………………... [1] (d) Which substance produces a reddish brown vapour when it conducts electricity in the molten state? ………………………………………………………………………………………………... [1] (e) Which salt cannot be prepared by titration method? ………………………………………………………………………………………………... [1] [Total 5] A2 The table below lists some information about the elements in Period 3 in the Periodic Table. Element Na Mg Al Si P S Cl Ar Number of valence electrons 1 2 3 4 5 6 7 8 Common oxidation state +1 +2 +3 +4 / - 4 -3 -2 -1 0 Melting point /oC 98 650 660 1414 317 115 -101 -189 (a) Explain the variation in oxidation state(s) across the period. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. .…………………………………………………………………………………………………... [2]

3 [Turn Over (b) Explain the large difference in melting points between silicon and chlorine. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. …..……………………………………………………………………………………………….. …………………………………………………………………………………...…………… [3] (c) Sodium phosphide is a reagent that is used in the production of other chemicals and as a catalyst in polymerisation. Sodium phosphide can be produced by reacting sodium with phosphorus in the reaction shown below. 12Na + P4 → 4Na3P (i) Draw a dot and cross diagram to show the bonding in sodium phosphide. Show all the electrons in your diagram. [2] (ii) 5.75 g of sodium was reacted with 3.1 g of phosphorus. Show by calculation which reactant is in excess and c alculate the mass of sodium phosphide formed. [3] [Total 10]

4 [Turn Over A3 To mine copper from land t hat contains very low percentages of copper compounds, plants may be cultivated to absorb these compounds through their roots. These plants are then burnt to produce copper(II) oxide, which can then undergo reactions to produce copper or copper(II) sulfate solution as shown in the diagram below. (a) Excess copper(II) oxide was reacted with carbon. Suggest how the solid copper metal can be separated from the resulting mixture. ……………………………………………………………………………………………………………. ………………………………………………………………………………………………………… [2] (b) Name another non-metallic element that can be reacted with copper(II) oxide to obtain copper. ………………………………………………………………………………………………………… [1] (c) Copper can be obtained from the copper( II) sulfate solution in two ways. One way involves reacting the copper(II) sulfate solution with iron. (i) Write the ionic equation with state symbols for the reaction. …………………………………………………………………………………...……..……. [1] (ii) Describe two observations seen in the reaction. …………………………………………………………………………………………………. ……………………………………………………………………………………………..… [2] (d) Another method to obtain copper from the copper(II) sulfate solution involves the electrolysis of copper(II) sulfate solution using carbon electrodes. State an advantage of obtaining copper from copper( II) sulfate by the addition of iron compared to electrolysis. …………………………………………………………………………………………………………… ………………………………………………………………………………………………………… [1] (e) Suggest a reason why copper should be recycled. …………………………………………………………………………………………………………… ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………….. [1] [Total 8] Add carbon and heat Copper Copper(II) oxide produced from plants Add sulfuric acid Copper(II) sulfate solution

5 [Turn Over A4 A student carried out electrolysis on a solution of sodium chloride using the apparatus shown. (a) Write the half equation for the formation of (i) gas A: …………………………………………………………………………………………. (ii) gas B: ………………………………………………………...……………………………. [2] (b) Describe a test to confirm the identity of gas A. ……………………………………………………………………………………………………………. ………………………………………………………………………………………………………… [1] (c) Explain why the ratio of gas A to gas B is 1:2. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ………………………………………………………………………………………………………… [1] (d) The student carried out a second experiment using a different solution of sodium chloride and the gases she obtained were in the ratio of 1:1. The gas collected a t the positive electr ode turned moist litmus blue and bleached it. (i) How is the sodium chloride solution used in the second experiment different from that in the first experiment? ...…………………………………………………………………………………………………. ……………………………………………………...………………………………………… [1] Gas A Gas B

6 [Turn Over (ii) Describe and explain the changes in the elec trolyte as the electrolysis progressed in the second experiment. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. ...…………………………………………………………………………………………………. …………………………………………………………………………………………………[2] (e) What must the student do if she wants to obtain sodium by electrolysis from sodium chloride? ………………...………………………………………………………………………………………. [1] [Total 8]

7 [Turn Over A5 A student investigated the voltage produced by different chemical cells. The apparatus used is shown below. The student measured the voltage obtained using different metals as electrode X. The table below shows the results obtained. Electrode X Voltage / V cobalt +0.62 copper 0.00 magnesium +2.71 nickel +0.59 silver -0.46 tin +0.48 (a) List the six metals used for electrode X in decreasing order of reactivity. ……………………………………………………………………………………………………….. [1] (b) If X is zinc, (i) write a half equation for the reaction at the copper electrode. Is this reaction oxidation or reduction? Explain your answer. ……………………………………………………………………………