TJC H2 CHEM P2 Prelim

1 9647/02/TJC Prelim 2011 [Turn Over CANDIDATE NAME CIVICS GROUP / CENTER NUMBER S INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 14th September 2011 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. 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. This document consists of 23 printed pages including 1 blank page. For Examiner’s Use 1 /12 2 /18 3 /10 4 /16 5 /16 Total PRELIMINARY EXAMINATIONS HIGHER 2

2 9647/02/TJC Prelim 2011 [Turn Over 1 The literature value of standard enthalpy change of reaction between solid copper and aqueous hydrochloric acid is +64.8 kJ mol-1. Cu(s) + HCl(aq)  CuCl2(aq) + H2(g) HO reaction = +64.8 kJ mol-1 HO reaction = +64.8 kJ mol -1 is a hypothetical value as copper does not react with hydrochloric acid under normal room conditions. Hence, to determine the hypothetical value for the enthalpy change, a suitable Hess cycle needs to be constructed. To construct a suitable Hess cycle, the enthalpy change of the following two reactions needs to be determined experimentally. Reaction 1: Mg(s) + 2HC l(aq)  MgCl2(aq) + H2(g) HO 1 = x kJ mol -1 Reaction 2: Mg(s) + CuC l2(aq)  Cu(s) + MgCl2(aq) HO 2 = y kJ mol-1 (a) Draw an enthalpy cycle diagram that shows the relationship between HO reaction, HO 1 andHO 2. Hence, show how HO reaction can be calculated. [2] For Examiner’s Use

3 9647/02/TJC Prelim 2011 [Turn Over (b) An online website reported that the enthalpy change of Reaction 1 , HO 1, is about -460 kJ mol-1. Write a plan to verify if the enthalpy change of Reaction 1 , HO 1, is -460 kJ mol-1. Reaction 1: Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) HO 1 = x kJ mol-1 You may use the apparatus normally found in the school laboratory and the following apparatus and chemical reagents.  200 cm 3 Styrofoam cup  Magnesium powder  HC l(aq) of concentration 5 mol dm-3 Note: 4.2 J of heat energy raise the temperature of 1 cm 3 of water or aqueous solution by 1 oC Density of HCl = 1 g cm-3 In your plan you should give the essential details of the procedures, including quantities used, for the determination of HO 1. For Examiner’s Use

4 9647/02/TJC Prelim 2011 [Turn Over [5] (c) Draw up tables with appropriate headings to show the data you would record in (b). [1] For Examiner’s Use

5 9647/02/TJC Prelim 2011 [Turn Over (d) Outline how you would use the data collected to determine enthalpy change of Reaction 1, HO 1. You may use letters such as a, b, c and the like to represent the various data collected. [2] (e) Identify one potential safety hazard in this experiment and state how you would minimize this risk. [1] For Examiner’s Use

6 9647/02/TJC Prelim 2011 [Turn Over (f) Copper does not react with hydrochloric acid under normal room conditions. In thermodynamic terms, explain if copper is abl e to react with hydrochloric acid at high temperature. [1] [Total: 12] For Examiner’s Use

7 9647/02/TJC Prelim 2011 [Turn Over BLANK PAGE For Examiner’s Use

8 9647/02/TJC Prelim 2011 [Turn Over 2 Iodine can be produced through various processes. (a) In the laboratory, a common method of obtaini ng iodine is to react sodium iodide with concentrated sulfuric acid. Sample procedures could be as follows: Procedures A1. Add about 15 g of sodium iodide into a 250 cm 3 beaker. A2. Add enough deionised water to just dissolve all the sodium iodide. A3. Place the beaker in an ice bath. A4. Using a 50 cm 3 measuring cylinder, add 50 cm 3 of concentrated sulfuric acid to the sodium iodide solution and stir for 5 minutes. A5. Scrape off the black crystalline so lid iodine formed on the inside of the beaker, and transfer to a tall glass jar. A6. Heat the iodine crystals with a gentle flame and collect the purified iodine at the top of the jar. (i) Write a balanced equation for the reaction of the iodide with concentrated sulfuric acid to form iodine. (ii) Describe what you would observe during the 5 minutes of stirring in procedure A4. (iii) Suggest why an ice bath is necessary. (iv) When the iodine crystals came into contact with sodium iodide, a reddish brown colouration would be observed. Explain the observation. For Examiner’s Use

9 9647/02/TJC Prelim 2011 [Turn Over (v) Procedure A6 is done in order to obtain a higher-purity sample of iodine. A research assistant suggested that instead of performing procedure A6, the iodine crystals could be washed with cold aqueous sodium hydroxide to remove any sulfuric acid impurity. With the aid of a relevant chemical equation, evaluate how this suggestion might affect the yield of iodine obtained. [6] For Examiner’s Use

10 9647/02/TJC Prelim 2011 [Turn Over (b) In the 18 th and 19 th centuries, iodine was industrially produced from kelp, a large seaweed. Combustion of the kelp converts the organic substances to ash, and sodium halides (mainly sodium iodide) are obtained. In the laboratory, a similar process can be done according to the following procedures. Procedures B1. Fill a large crucible on a tripod with the seaweed. Heat with a strong Bunsen flame until all the seaweed has been turned to ash. B2. Boil the ash with about 20 cm 3 of purified water in a beaker, and filter while hot. Collect the clear filtrate in a second beaker and allow to cool. B3. Add about 2 cm 3 of dilute sulfuric acid to the solution, followed by hydrogen peroxide solution. B4. Transfer the mixture to a separating funnel and