HCI Prelim C2 P3

This document consists of 12 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 16S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9729/03 18 September 2017 2 hours READ THESE INSTRUCTIONS FIRST Write your name, CT group, centre number and index number clearly in the spaces above. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a new sheet of writing paper. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. Circle the question numbers for the questions that you have attempted on the cover page provided.

2 © Hwa Chong Institution 2017 9729 / 03 / C2 Prelim 2017 Section A Answer all the questions in this section. 1 (a) Azurite is a deep blue copper-containing mineral. For many centuries, finely ground rock containing azurite has been used as a pigment in blue paints. Azurite is a mixture of copper(II) carbonate and copper( II) hydroxide. The formula of pure azurite is Cu 3(CO3)2(OH)2, which has a relative formula mass of 344.5. The percentage by mass of pure azurite in a sample of finely ground rock can be determined by back titration. 3.70 g of the sample is added to 100 cm 3 of 0.425 mol dm–3 sulfuric acid. The resulting solution was made up to 250 cm3 with distilled water. 25.0 cm 3 of the diluted solution required 26.50 cm 3 of 0.100 mol dm –3 sodium hydroxide for neutralization. Dilute sulfuric acid reacts with pure azurite as shown in the following equation. Cu 3(CO3)2(OH)2(s) + 3H2SO4(aq)  3CuSO4(aq) + 4H2O(l) + 2CO2(g) Calculate the percentage by mass of pure azurite in the sample. You may assume that azurite is the only substance in the rock that reacts with sulfuric acid. [3] (b) The brick-red precipitate, Cu 2O, dissolves in concentrated ammonia solution to form a colourless complex ion P, [Cu(NH3)x]n+, which has a linear geometry about the central metal ion. When left exposed to air, the colourless complex ion P turns into a deep blue solution, containing the complex ion [Cu(NH3)4]2+. (i) State the value of x and write the formula of complex ion P. [2] (ii) With the help of the Data Booklet , write the two half equations, and hence the overall equation for the reaction of the colourless complex ion P to form [Cu(NH3)4]2+. [2] (c) Iodine and chlorine react together to give iodine trichloride. I2(s) + 3Cl2(g)  2ICl3(s) (i) Construct a fully labelled energy cycle relating the reactants and products of this reaction to their gas phase atoms. Use your cycle to calculate the average bond energy of the I–Cl bond in ICl3. Your cycle should include relevant data from the Data Booklet together with the following data. standard enthalpy change of formation (H f) of ICl3(s) = –81 kJmol–1 enthalpy change of sublimation of I2(s)  I2(g) = +38 kJmol–1 enthalpy change of sublimation of ICl3(s)  ICl3(g) = +60 kJmol–1 [4] (ii) The standard Gibbs free energy of formation, Gf, of ICl3(s) is –40.4 kJmol –1. Calculate Sf and comment on its sign with respect to the reaction. [2]

3 © Hwa Chong Institution 2017 9729 / 03 / C2 Prelim 2017 (d) The antipyretic (fever-reducing) drug antifebrin can be made via a 3-step synthetic route, starting from benzene. antifebrin (i) State the 3 types of reactions involved (in sequence) in the formation of antifebrin from benzene. [3] (ii) Draw the structures of the products formed when antifebrin was subjected to prolonged heating with dilute sulfuric acid. [1] [Total: 17]

4 © Hwa Chong Institution 2017 9729 / 03 / C2 Prelim 2017 2 Thiols are a group of organic compounds which may be represented as CnH2n+1SH or RSH, where R is an alkyl group. (a) In an experiment a sample of thiol was completely burnt in a stoichiometric amount of oxygen. The product mixture was collected in a 1.65 dm3 flask at 110 °C and 1 atm total pressure. The partial pressure of sulfur dioxide was found to be 16.9 kPa. (i) Write a balanced equation for the complete combustion of one mole of a thiol with n carbon atoms. [1] (ii) Show that when a thiol with n carbon atoms is completely burnt in a stoichiometric amount of oxygen, the mole fraction of sulfur dioxide in the product mixture is 1/(2n+2). Assume that all products are gaseous. [1] (iii) Use the above data, together with the information in (ii) to deduce the value of n for the thiol burnt in the experiment. [1] (iv) Hence find the mass of thiol burnt. [2] (b) Thiols are suitable starting materials for the synthesis of organic sulfides. An example is given below. (i) How would the acid strength of ethanethiol compare to that of ethanol, given that Reaction 1 is effectively complete? [1] (ii) State the type of reaction occurring in Reaction 2. [1] (iii) Suggest why Reaction 1 was carried out in the above synthesis. [1] (iv) Give two reasons to explain why ethylphenyl sulfide, C 6H5SCH2CH3, cannot be obtained by changing bromoethane in Reaction 2 to bromobenzene. [2]

5 © Hwa Chong Institution 2017 9729 / 03 / C2 Prelim 2017 (c) The following diagram shows a process which may be used for the removal and recovery of thiols from petroleum. The process utilizes the reaction between thiols and lead(II) oxide, as given in the equation below. 2RSH(l) + PbO(s) Pb(RS)2(s) + H2O(l) Reaction I As the equilibrium position for Reaction I lies very much towards the products, the overall recovery of thiols in the process is high. (i) State the role of lead(II) oxide in Reaction I. [1] (ii) Comment on the sign and magnitude of G  for Reaction I. [2] (iii) Suggest the identity of colourless solution A and brown gas B. [2] (iv) Suggest the temperature in the furnace, explaining your reasoning with the use of relevant data from the Data Booklet. For reference, copper( II) nitrate and barium nitrate decompose at 170 °C and 630 °C respectively. [4] Reactor (Reaction I) (rtp) Filtration Unit Settler Furnace Evaporator petroleum with thiols (RSH) petroleum with no