TJC 08 TJC Chem H2 Prelim P2

TEMASEK JUNIOR COLLEGE Preliminary Examinations Higher 2 CANDIDATE NAME CIVICS GROUP / CHEMISTRY 9746/02 Paper 2 Structured Questions 17 September 2008 1 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group and candidate name on all the work you hand in. Write in dark blue or black pen. 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. You are reminded of the need for good English and clear presentation in your answers. You may lose marks if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 3 4 5 6 Total This document consists of 16 printed pages.

2 1 (a) For decades, scientists were puzzled by why dinosaurs suddenly became extinct 65 million years ago. In studying core samples of rock dating back to that period, scientists found an unusual high level of iridium. This possibly came from an iridium- rich asteroid that struck the Earth’s surface and the mystery to dinosaurs’ extinction was solved. Iridium exists as two naturally occurring isotopes, iridium-191 and iridium-193. For Examiner’s Use (i) Define the term relative atomic mass. …………………………………………………………………………………………... …………………………………………………………………………………………... (ii) The relative atomic mass of iridium is 192.2. Calculate the natural abundance, in percentage, of each isotope. [3] TJC H2 Chemistry Paper 2 2008 [Turn over

3 (b) Aqua regia (a mixture of 75% nitric acid and 25% hydrochloric acid by volume) is highly corrosive. Only noble metals like iridium are inert to this solution. A 5 g sample of platinum-iridium alloy required 24.6 cm 3 of aqua regia for complete reaction. Platinum was completely oxidised to platinum( IV) ions by nitric acid and 0.5 g of metal was recovered. For Examiner’s Use (i) Find the percentage of each metal in the alloy. (ii) The concentration of nitric acid used to make aqua regia is 5.0 mol dm -3. Assuming that the reaction between the sample and aqua regia is complete, construct the balanced equation for the reaction. [4] [Total: 7] TJC H2 Chemistry Paper 2 2008 [Turn over

4 2 Propene readily undergoes electrophilic addition with bromine to give 1,2-dibromopropane. For Examiner’s Use (a) (i) Calculate the enthalpy change for this reaction. (ii) By referring to the electrophilic addition mechanism, sketch the reaction pathway diagram for this reaction. Label your graph, showing clearly the intermediate formed and the enthalpy change of the reaction. (iii) Bromine adds to propene readily under normal conditions whereas iodine adds only at low temperatures. The 1,2-diiodopropane that results is unstable, decomposing back to propene and iodine at room temperature. The enthalpy change for this addition reaction is – 69 kJ mol -1. Sketch the reaction pathway diagram for iodine addition on the same diagram above labeling your curve clearly. [5] Energy CH3 CH=CH2 + Br2 (or I2) Extent of reaction TJC H2 Chemistry Paper 2 2008 [Turn over

5 (b) 1,2-dibromopropane formed from electrophilic addition of bromine to propene undergoes a series of reactions as shown in the flow chart below. For Examiner’s Use 1,2-dibromopropane NaOH (aq) reflux AgNO3 (aq) No precipitate Acidify with HNO3 followed by addition of AgNO3 (aq) A (Cream precipitate) Evaporate to dryness Solid B Conc H2SO4, heat concentrated NH3 (aq) precipitate dissolves Mixture of Gases (i) Identify the compounds A and B. A : ……………………….………….. B : …………………………………… (ii) Explain why no precipitate is formed when aqueous silver nitrate was added to 1,2-dibromopropane while a cream precipitate is formed when 1,2-dibromopropane was first heated with sodium hydroxide then acidified, followed by the addition of aqueous silver nitrate. ……………………………………… ………………………………………………........... ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. TJC H2 Chemistry Paper 2 2008 [Turn over

6 (iii) Explain why the cream precipitate dissolves upon the addition of concentrated aqueous ammonia. For Examiner’s Use ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. (iv) Write balanced equation(s) for the reaction between solid B and concentrated H2SO4. ………………………………………………………………………………………………. [9] [Total: 14] TJC H2 Chemistry Paper 2 2008 [Turn over

7 3 (a) Trace amounts of elements in a sample can be determined by spectroscopy. In atomic spectroscopy, the concentration of an element in a solution can be determined by its absorbance. The relationship between the concentration of aluminium in a solution and its absorbance is given in the graph below. For Examiner’s Use Absorbance 0 0.001 0.002 0.003 0.004 0.005 Concentration of aluminium (where 1 μg = 1 x 10-6 g) (μg cm-3) 0.1 g of a soil sample was made up to 10 cm 3 of solution for the analysis of the amount of aluminium present. (i) From the graph, determine the concentration of aluminium in the soil sample given that it has an absorbance of 0.360. ………………………………