RI Prelim P3

Page 1 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 RAFFLES INSTITUTION (JUNIOR COLLEGE) PRELIMINARY EXAMINATION 2009 HIGHER 2 CHEMISTRY 9746/03 Paper 3 Free Response 16 September 2009 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, civics tutorial group and index nu mber in the spaces provided on the cover page on page 9 and the writing papers. Write in dark blue or black pen on both sides of pa per. 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 any four questions. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good English and c lear presentation in your answers. 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, with the cover page on top. ________________________________________________________________________ This question booklet consists of 8 printed pages and one cover page.

Page 2 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 Answer any four questions. Begin each question on a fresh sheet of paper. 1 Lactic acid (2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. (a) 25.0 cm 3 of an aqueous solution of lactic acid is titrated against 0.120 mol dm − 3 of aqueous sodium hydroxide and the following graph is obtained. 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 0 10 20 30 40 Volume of aq NaOH added/ cm pH 3

Page 3 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 (i) Calculate the concentration of the lactic acid used. (ii) Determine the acid dissociation constant of lactic acid. (iii) With the aid of an equation, explain why the pH at equivalence point is above 7. [4] (b) The table below lists some properties of three samp les of lactic acid prepared from different natural sources. Sample Source Effect on plane− polarised light Melting point/ oC A Meat extract Rotate to the right 26 B Fermentation of sucrose Rotate to the left 26 C Sour milk No effect 18 (i) With the help of suitable structural formulae, exp lain why samples A and B behave differently towards plane− polarised light. (ii) Explain why sample C has no effect on plane−polarised light and suggest why it has a different melting point from A. [4] (c) When lactic acid is refluxed with a catalytic amoun t of concentrated sulfuric acid, compound D with the molecular formula of C 6H8O4 is formed. It does not react with sodium. Deduce the structural formula of D, giving reasons for your answer. [3] (d) E is an isomer of lactic acid. It rotates plane-pola rised light and reacts with Fehling’s solution to give a reddish brown precipit ate. 0.01 mol of E reacts with sodium to give 0.24 dm 3 of hydrogen gas under room temperature and pressur e conditions. (i) Suggest a structural formula of E, giving reasons for your answer. (ii) Explain why E is less acidic than lactic acid. [5] (e) Maleic acid and fumaric acid are naturally occurrin g acids found in fruits. They have the same molecular formula of C 4H4O4 and are dibasic acids. They both decolourise bromine at room temperature and give th e same product upon hydrogenation. Maleic acid has a melting point of 1 31 oC while fumaric acid has a melting point of 286 o C. (i) Draw the structural formulae of maleic acid and fu maric acid. (ii) With the help of a suitable diagram, account for t he differences in their melting points. [4] [Total: 20] 2 Cyclobutane is a colourless gas at room temperatur e. It decomposes to ethene as shown by the following equation: , ∆ H 1 (a) To determine ∆ H1, the enthalpy change of reaction of the decomposit ion of cyclobutane to ethene, the enthalpy change of combu stion of cyclobutane is first found by the following experiment: CH 2 CH 22

Page 4 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 1.00 g of cyclobutane was completely combusted. T he heat evolved increased the temperature of 200 g of water by 46.9 oC. The heat transfer was known to be only 80% efficient. (i) Calculate the enthalpy change of combustion of cyclobutane. (ii) Given that enthalpy change of combustion of ethene is − 1422 kJ mol − 1 and using your answer in (a)(i) , show that ∆ H1 has a value of +99 kJ mol − 1 . (iii) Explain briefly why the decomposition of cyclobutan e is favoured by a high temperature. [6] (b) The bond energy of C − C bond in cyclobutane is different from the bond en ergy of C− C bond in straight chain alkanes. (i) Define the term bond energy . (ii) With reference to the Data Booklet and using ∆ H 1 = +99 kJ mol −1 , calculate the bond energy of C − C bond in cyclobutane. (iii) Explain why the bond energy of C − C bond in cyclobutane is lower than the bond energy of C − C bond found in the Data Booklet. [5] (c) Ethene reacts with bromine in tetrachloromethane t o form 1,2 − dibromoethane as shown by the equation: CH 2=CH 2 + Br 2 → CH 2BrCH 2Br To find out the orders of reaction with respect to ethene and bromine, ethene and bromine were first dissolved separately in tetrachl oromethane. Various volumes of these solutions and tetrachloromethane were mixed a nd the time taken for the colour of bromine to disappear was recorded. The re sults are shown in the table below: Experiment Volume of ethene solution/ cm 3 Volume of bromine solution/ cm 3 Volume of tetrachloromethane / cm 3 Time taken for colour of bromine to disappear/ s 1 20 20 0 15 2 12 20 8 25 3 20 10 10 15 4 40 20 20 t 4 Discuss (i) to (iii) with reference to experiments 1 to 3. (i) Explain why varying volumes of tetrachloromethane were used. (ii) State the relationship between the rate of reaction and /square4 time taken for the colour of bromine to disappear /square4 volume of bromine used (iii) Deduce the order of reaction with respect to ethen e and show that the order of reaction with respect to bromine is 1.

Page 5 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 (iv) Suggest a value for t 4, time taken for the colour of bromine to disappear in experiment 4. (v) When methanol is used as a solvent instead of tetra chloromethane, another product with the formula of C 3H7OBr is formed. Draw its displayed formula and explain how it is formed. [9] [Total: 20] 3 Vanadium is a silvery grey metal and forms stable coloured ions of various oxidation state in aqueous solutions: Formula of ion of vanadium VO 2 + VO 2+ V 3+ V 2+ Colour of ions in aqueous solution Yellow Blue Gree n Violet (a) Explain why aqueous solutions of vanadium ions such as V 3+ are coloured. [3] (b) 0.300 g of a vanadium(V) salt is dissolved in wate r. The aqueous solution formed is then treated with sulfur dioxide gas until reaction is completed. The colour of the reaction mixture changes from yellow to green and f inally to blue. The excess sulfur dioxide gas is boiled out. The solution is found to require 33.00