ACJC H2 Chem 2012 Prelim P3 QP

1 © ACJC 2012 9647/03/Prelim/12 [Turn over ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY H2 9647/03 Paper 3 Free Response 29 August 2012 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Answer Paper Cover Page READ THESE INSTRUCTIONS FIRST Write your index number, name, form class and tutorial class on all the work you hand in. 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 fluids. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question of part question. At the end of the examination, fasten all your work securely together. This document consists of 12 printed pages and 1 blank page. 9647/Prelim/12 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2012 Department of Chemistry [Turn over

2 © ACJC 2012 9647/03/Prelim/12 [Turn over Answer any four questions. 1 (a) When hydrochloric acid is added to sodium thiosulfa te, Na 2S2O3, a fine precipitate of sulfur appears. This gradually makes the solution opaque. Na 2S2O3(aq) + 2 HC l(aq) S(s) + SO 2(g) + 2 NaC l (aq) + H 2O( l) The rate of the reaction may be determined by measuring how long it takes for the solution to become opaque such that a cross marked on a piece of paper placed under the beaker containing the reaction mixture just becomes obscured. A series of experiments were carried out to study t he order of reaction with respect to Na 2S2O3 and the following results were obtained. Experiment Number Volume / cm 3 Time / s HC l Na 2S2O3 H 2O 1 20.0 40.0 40.0 39 2 20.0 30.0 50.0 50 3 20.0 20.0 60.0 83 4 20.0 10.0 70.0 170 (i) Explain why the total volume of the reaction mixture is kept constant for all the four experiments. (ii) Calculate the values of 1s / )Time 1( − for the various volumes of Na 2S2O3 used. (iii) Hence or otherwise, state how the rate changes with the volume of Na 2S2O3 used. (iv) Explain why the rate changes with the volume of Na 2S2O3 used. [5] (b) Sulfur reacts with chlorine gas to form a compound A containing 47.4% sulfur by mass. When compound A is reacted with water, a yellow precipitate was fo rmed together with a solution containing a mixture of su lfurous acid, H 2SO 3 and hydrochloric acid, HC l. (i) Given that the relative molecular mass of A is 135, determine the empirical formula and the molecular formula of A. (ii) Explain why compound A can react with water. (iii) If 1.00 g of compound A was dissolved in water, calculate the volume of 1.00 mol dm -3 NaOH that would be required to completely neutrali se the resultant solution. [5]

3 © ACJC 2012 9647/03/Prelim/12 [Turn over (c) The Wittig reaction was discovered by German chemis t Georg Wittig. This reaction is significant because it uses a phosphoro us-containing compound to produce an alkene molecule. In 1979, Georg Wittg w as awarded the Nobel Prize for his work in discovering the Wittig reaction. The Wittig reaction is thought to proceed through the following stages. CH 3Cl P + CH3 Ph Ph Ph P CH2 Ph Ph Ph C C H H C 2H5 CH 3 P(Ph) 3 I PhLi II CH 3COC 2H5 III where Ph refers to phenyl substituent. (i) What type of reaction takes place in stage I? (ii) Describe the mechanism for stage I. In your answer, show any relevant charges, lone pair of electrons and movement of electrons. (iii) State the role of PhLi in stage II . (iv) Suggest the other product of stage III and hence construct the balanced equation for this stage. (v) When butane reacts with chlorine under ultraviolet light, a mixture of monochlorobutane is produced. P(Ph) 3, PhLi and ethanal are then added in succession to the resultant mixture. This results i n the formation of two pairs of geometric isomeric alkenes. Suggest the structures of the four alkenes, and the ratio in which they might be produced. Explain your suggested ratio. [10] [Total: 20 marks]

4 © ACJC 2012 9647/03/Prelim/12 [Turn over 2 (a) Silver chloride is a white crystalline solid well k nown for its low solubility in water. It is often used in photography as it darkens quickly when it is exposed to light. (i) Explain what is meant by the lattice energy of silver chloride . Write an equation to represent the lattice energy of silver chloride. (ii) Using the following data, and relevant data from the Data Booklet, construct a thermochemical cycle to calculate the lattice energy of silver chloride. Include state symbols in your cycle. Value / kJ mol -1 Standard enthalpy change of formation of AgCl -127 Standard enthalpy change of atomization of Ag +285 First ionization energy of Ag +731 First electron affinity of Cl -349 (iii) Explain how you would expect the numerical magnitude of the lattice energy of silver chloride to compare with the lattice energy of silver iodide. [7] (b) Equal volumes of 0.020 mol dm -3 of aqueous silver nitrate and an unknown concentration of aqueous sodium chloride were mixed in a beaker. A white precipitate was observed. The precipitate formed wa s filtered and washed. A colourless solution was seen upon addition of aqueo us ammonia to the white precipitate. Solubility product, K sp , of silver chloride is 1.77 x 10 -10 mol 2 dm -6. (i) Write an expression for the solubility product of silver chloride. (ii) Calculate the maximum concentration of aqueous sodium chloride such that no precipitate of silver chloride is formed upon mixing the two solutions. (iii) Explain why a colourless solution is obtained when aqueous ammonia is added to the white precipitate. [4] (c) Iodine, a member of Group VII element, reacts with alkali at room temperature to form iodide and iodate(V) ions. Group II metals for m iodate(V) compounds which can undergo decomposition to give metal oxide, iodine vapour and oxygen gas. (i) Name the reaction between iodine and alkali and wri te balanced ionic equation for the reaction. A student conducted an experiment to find out the t hermal stability of Ca(IO 3)2, Sr(IO 3)2 and Ba(IO 3)2. He took 2.0 g of each compound and heated them separately in a test-tube at a temperature T ºC. He recorded the time of the appearance of purple iodine vapour. (ii) Briefly explain the relationship between thermal st ability of iodate compounds with the time of appearance of purple iodine vapour.

5 © ACJC 2012 9647/03/Prelim/12 [Turn over (iii) Arrange Ca(IO 3)2, Sr(IO 3)2 and Ba(IO 3)2 in the order of increasing thermal stability. Explain the trend as fully as you can. (iv) Solid sodium iodide undergoes reaction with concent rated sulfuric acid State the observations and write balanced equations for the reactions that take place. [9] [Total: 20 marks] 3 (a) Histidine is an essential α-amino acid in humans and other mammals. Histidine is utilized by the body to develop and maintain health y tissues. It is especially important in the myelin sheath that coat nervous ce lls to ensure the transmission of messages from the brain to organs throughout the body. Therefore, adequate histidine levels are es