NYJC H2 Chem 2012 Prelim P3 QP

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 18 September 2012 2 hours Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. 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. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. 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. Section B This document consists of 9 printed pages and 1 blank page. [Turn over

2 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over Answer any four questions 1 Aldehydes and ketones react with primary amines in slightly acidic solution to form imines which have the C=N functional group. For example, propanal reacts with methylamine to give N-methyl-1-propanimine. CH3CH2 C H O CH3NH2 H+ N CH3 H CH3CH2 C propanal N-methyl-1-propanimine (a) Aldehydes and ketones also react with 2,4-dinitrophenylhydrazine (2,4-DNPH) to give compounds that are closely related to imines. (i) Write a balanced equation for the reaction between benzaldehyde and 2,4- DNPH. (ii) State what you would observe in the reaction with 2,4-DNPH. What is the type of reaction? [4] (b) The mechanism for the reaction between benzaldehyde and methylamine is shown below. + H2O – C O H + CH3NH2 C O H N H H CH3 + C OH H N H CH3 carbinolamine H+ C OH 2 H N H CH3 + G + step I step II step III step IV −− −− H+ −− −−imine iminium ion G step V

3 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (i) What is the type of reaction in step I? (ii) The positive charge on the iminium ion, G+, does not reside on a carbon atom. Suggest the structure of G+. (iii) Write equations for step I and step IV to show the movement of electrons, using curved arrow notation. Show the lone pairs of electrons, if any, that are involved in each step. (iv) The maximum rate of formation of imines occurs at a pH of about 4.5. Explain why the rate is slow under very acidic conditions and under alkaline conditions. [6] (c) Acrylamide is a carcinogen. It is formed when potato chips are heated to above 120 °C. Its formation has been linked to the presence of glucose (C 6H12O6) and an α -amino acid L found in relatively high amounts in potato. C C H CONH2 H H H C C OH H C OH H C OH H C OH H C OH H H O acrylamide glucose Glucose reacts with L to form an imine M. At high cooking temperatures, M decomposes to produce only three compounds in equimolar amounts: acrylamide, carbon dioxide and compound N (C6H13NO5). (i) Name the functional groups in acrylamide. (ii) Deduce the structure of L. Show clearly how you obtained your answer. [5] (d) Suggest the reagents and conditions required to synthesise the following imine derivative. Identify all the intermediate compounds. O N N [5] [Total: 20]

4 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over 2 (a) A container holds a gaseous mixture of nitrogen and propane. The pressure in the container at 200 ° C is 4.5 atm. At − 40 ° C, the propane completely condenses and the pressure drops to 1.5 atm. Calculate the mole fraction of propane in the original gaseous mixture. [3] (b) The graph below shows the variation in electromotive force (e.m.f.) of the following electrochemical cell with lg [Ag+(aq)] at 298 K. Cu(s) | Cu2+(aq) || Ag+(aq) | Ag(s) 0 0.1 0.2 0.3 0.4 0.5 0.6 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 (i) Using the information from the graph, calculate the standard electrode potential of the half-cell, Ag+(aq) | Ag(s), at 298 K. (ii) If the Ag +(aq) solution of the electrochemical cell is replaced by a saturated solution of silver bromate(V), AgBrO 3, in 0.1 mol dm − 3 potassium bromate(V) and the e.m.f. of the cell measured at 298 K is +0.27 V, determine (I) the concentration of Ag+(aq) ions in the saturated solution, and (II) the solubility product of silver bromate(V) at 298 K. [5] lg [Ag+(aq)] e.m.f / V

5 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (c) Solid silver nitrate was slowly dissolved in a solution Q containing ethanedioate, C2O4 2− , and chromate(VI), CrO 4 2− , ions of concentrations 2.50 x 10 − 2 mol dm − 3 and 1.44 x 10− 5 mol dm− 3 respectively. (i) When a permanent precipitate of silver ethanedioate first appeared, the concentration of silver ions in the solution was 2.10 x 10 − 5 mol dm − 3. Calculate the solubility product of silver ethanedioate. (ii) The dissolving of solid silver nitrate in Q was continued until a permanent red precipitate of silver chromate(VI) first appeared. Calculate the concentrations of silver ions and ethanedioate ions at that instant. (Ksp of silver chromate(VI) is 1.2 x 10− 12 mol3 dm− 9.) ( iii) What is the amount of silver ethanedioate precipitated from 1.00 dm 3 of the solution? [7] (d) An electrochemical cell containing an oxygen cathode and a hydrogen anode is shown below. The pistons above the gas chambers are frictionless. (i) Write balanced equations for the half reactions and for the overall reaction in the cell. (ii) How does the concentration of sulfuric acid affect the equilibria of the half reactions? (iii) If weights are added to the pistons of both chambers, how would the reading of the voltmeter change? Explain your answer. [5] [Total: 20] high resistance voltmeter pistons 1 atm 25 ° C H2 O2 1 mol dm− 3 H2SO4 Pt Pt

6 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over 3 (a) (i) Alkanes are generally considered to be unreactive compounds, showing an inertness to common reagents such as NaOH, H2SO4 and K2Cr2O7. Suggest a reason why these reagents do not attack an alkane such as CH 4. (ii) It is found by experiment that, during free-radical substitution of alkane, primary, secondary and tertiary hydrogen atoms are replaced by chlorine atoms at different rates, as shown in the following table. type of hydrogen atom reaction relative rate primary RCH3 → RCH2Cl 1 secondary R2CH2 → R2CHCl 7 tertiary R3CH → R3CCl 21 Using this information, and considering the number and type of hydrogen atoms within the molecule, predict the relative ratio of the two possible products J and K from the monochlorination of 2-methylpropane. Explain your answer. [3] (b) Compounds A and B are amines with the same molecular formula. Both have a molecular mass of 73.0 and the following composition by mass: C, 65.8 % and H, 15.1 %. Compound A has a pKb of 3.23 and contains a tri-substituted N atom. The pH of an aqueous solution of 0.0100 mol dm − 3 of compound B is 11.5. When B undergoes free-radical substitution reaction, it only gives one mono-substituted organic product. (i) Calculate the molecular formulae of compounds A and B. (ii) Calculate the pKb of compound B. (iii) Suggest the structural formulae of compounds A and B. (iv) State and explain the relative basicity of compounds A and B. [7]

7 H2 Chemistry 9647/03 NYJC J2/12 PX [Turn Over (c) (i) The hydrolysis of 2-bromo-2-methylpropane takes place as follows. (CH3)3CBr + H2O → (CH3)3COH + H+ + Br− An experiment was conducted to determine the order of reaction with respect to 2-bromo-2-methylpropane. The following results were obtained. time/s 19 28 50 69 154 [(CH3)3COH]/mol dm− 3 0.0030 0.0040 0.0060 0.0072 0.0095 It was found that the order of reaction with respect to 2-bromo-2- methylpropane is one and the half-life of the reaction is 34.5 s. Using a non-graphical method, show that the concentratio