SAJC Prelim P3 QP

1 [Please Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Chemistry (9729) Paper 3 Free Response 14 September 2018 2 hours Additional Materials: Data Booklet, Writing Paper READ THESE INSTRUCTIONS: Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Marks [60] Section B Answer one question. Marks [20] The use of an approved scientific calculator is expected, where appropriate. 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 16 printed pages including this page.

2 Section A Answer all the questions in this section. 1 Electrosynthesis in organic chemistry is the synthesis of compounds in an electrolytic cell using inert electrodes. One important process is the Kolbe reaction. The Kolbe reaction is the decarboxylative dimerization of two carboxylate ions. Alkanes can be formed at the anode by the following reaction. (a) Explain why alkanes are generally unreactive. [1] (b) (i) Calculate the volume of carbon dioxide produced when a steady current of 5A is passed through the electrolyte for 32 minutes at standard temperature and pressure. [2] (ii) Methanoate ions undergo the Kolbe reaction to form only 2 gases. Using the axes given below, sketch the variation of pV/RT against p for one mole of each gas at the same temperature. Briefly explain your answer. [2] pV/RT p 0 1.0 ideal gas

3 [Please Turn Over 1 (c) The cathodic reaction for the Kolbe reaction can be represented by the following half equation: 2H2O + 2e  H2 + 2OH-. Write an overall equation for the Kolbe reaction involving ethanoate ions. Hence, calculate ΔGƟ for the Kolbe reaction given that the E Ɵcell value for the reaction is +2.28 V. [2] (d) The reaction mechanism of the Kolbe reaction involves a 4-step reaction mechanism as described below: Step 1: Dissociation of carboxylic acid to form a carboxylate ion. Step 2: Oxidation of carboxylate ion to form a carboxyl radical, RCOO•. Step 3: Decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R•. Step 4: Formation of a covalent bond between two alkyl radicals. (i) Using ethanoic acid as a starting reagent, suggest suitable equations for Steps 1, 2 and 4 on how ethane can be produced using the Kolbe reaction. [3] (ii) The decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R •, in Step 3 can be represented by the following equation: RCOO•  CO2 + R• Outline the mechanism of this step by copying the diagram below and include relevant curly half arrows. [1] (e) In another separate experiment, a new alkane A, C6H12, was produced. When reacted with bromine under ultraviolet light, A produced only one isomeric monobromo compound, B, which does not have a chiral centre. Draw the skeletal formulae of A and B. [2]

4 1 (f) (i) Using monohalogenoethane, C 2H5X, as examples, describe and explain the relative reactivities of chloro- and bromo-compounds in hydrolysis reactions. [2] (ii) Halogenoalkanes such as chlorofluoroalkanes, CFCs, were once used as refrigerant fluids and aerosol propellants. In many applications, they have now been replaced by alkanes. This is because CFCs contribute to the destruction of the ozone layer. Explain how CFCs destroy the ozone layer and suggest one potential hazard of using alkanes instead of CFCs. [2] (iii) Halogenoalkanes can react with lithium to give organolithium compounds. R-Br + 2Li  R-Li + LiBr These organolithium compounds can react with carbonyl compounds to form alcohols. (R is alkyl, R’ and R’’ are either alkyl or H) Using the above reaction sequence, deduce the structure of a suitable bromoalkane, R-Br, and a suitable carbonyl compound to synthesise hexan-2-ol. [2] [Total: 19]

5 [Please Turn Over 2 Wastewater has to be treated carefully to remove all harmful contaminants before it can be released into water bodies. (a) Industrial wastewater contains high levels of heavy metal ions. Electrocoagulation is used to remove heavy metal ions from wastew ater. In this method, heavy metal ions are oxidised to form hydroxides, so t hat they can coagulate easily and be removed easily in subsequent steps. A sample of wastewater containing Fe 2+ is treated by electrocoagulation. 4Fe2+ (aq) + 10H2O (l) + O2 (g) 4Fe(OH)3 (s) + 8H+ (aq) (i) By considering the interaction it forms with water, explain why Fe2+ cannot be easily removed from wastewater. [1] (ii) State a suitable physical method to remove Fe(OH)3 from the wastewater. [1] (iii) Using Le Chatelier’s Principle, suggest why the wastewater has to be kept alkaline for a more effective removal of Fe2+. [1] (iv) With an appropriate sketch of the Boltzmann distribution, explain how an increase in temperature would affect the rate of the electrocoagulation process. [3] (b) Ammonia is a toxic substance present in wastewater from the mining industry. Nitrification is a common biological treatment method to convert ammonia into less toxic nitrate. NH3 (aq) + 2O2 (g) NO3− (aq) + H+ (aq) + H2O (l) (i) Write the expression for the equilibrium constant, Kc, for this reaction given that Kc has units of mol−1 dm3. [1] (ii) 12 mol of oxygen gas was pumped into 1000 dm 3 of wastewater containing 5 mol of NH3. After nitrification, the resulting treated wastewater had a pH of 4.2. Calculate the value of Kc. Leave your answer to 3 significant figures. [3] (iii) State how the Kc in (ii) would change when the amount of NH3 in wastewater is increased to 10 mol. [1] (iv) The nitrification process was carried out at room temperature. It was observed that the outer surface of the reaction vessel was cold after some time. By considering the effect of temperature on the equilibrium, suggest and explain how you can change the reaction temperature to increase the conversion of ammonia. [2]

6 2 (b) (v) Construct a suitable energy cycle using the following equations to show how the enthalpy change of reaction, ∆Hr, can be determined. [3] (c) Wastewater treatment plants that treat pharmaceutical wastewater produces sludge that contains a high amount of organic substances. These organic substances can be combusted to harness energy for other processes. Tonalide, C18H26O, is one such organic compound found commonly in sludge. It has a melting point of 54.5OC. tonalide (i) Write an equation which describes the standard enthalpy change of combustion of tonalide. [1] (ii) The enthalpy change of combustion of tonalide is −865 kJ mol−1. Given that the combustion is 70% efficient, determine the amount of energy that can be harnessed from the combustion of sludge, which contains 900 g of tonalide. [2] (iii) The sign of the entropy change of combustion of tonalide is positive. Briefly explain the effect of temperature on the spontaneity of combustion of tonalide. [1] [Total: 20]

7 [Please Turn Over 3 Geraniol, C10H18O, is commonly used in perfumes and food flavourings. It is produced from geranyl diphosphate as shown in the following equation. (a) The mechanism of this reaction consists of the following: (i) State the mechanism in reaction I. [1] (ii) Explain why the mechanism in reaction I takes place in two steps instead of one step. [1] (iii) State the role of water in reaction II. [1]

8 3 (a) (iv) The following compound can be synthesised from geraniol. Propose a reaction scheme for this synthesis. [4] (v) Explain what is mean