NYJC H2 Chemistry P2 Answers Prelim

H2 Chemistry 9647/02 NYJC J2/14 PX [Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME Teachers’ Mark Scheme CLASS 1 3 INDEX NO. TUTOR CHEMISTRY 9647/02 Paper 2 Structured 22 September 2014 2 hours Candidates answer on the Question Paper Additional Materials: 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 all questions in the spaces provided. 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. For Examiner’s Use 1 P /12 2 /26 /60 3 /18 4 / 6 5 /10 This document consists of 17 printed pages and 0 blank page.

2 H2 Chemistry 9647/02 NYJC J2/14 PX For Examiner's Use Answer all questions in the spaces provided. 1 Planning (P) H A is a solid monoprotic acid. It is a weak organic acid with p K a value of 4.15. Its solubility in water is only about 3.44 g dm 3 under normal laborator y temperature. Its sodium salt, NaA (Mr = 144.0), is soluble in water. FA 1 is a fine powder of HA with NaA as an impurity. FA 2 is 0.975 mol dm 3 aqueous sodium hydroxide. The reaction between H A and NaOH(aq) is exothermic. A preliminary experiment was conducted using 2.50 g of FA 1 and 40 cm 3 of FA 2. The sample of FA 1 dissolved completely to give a temperature rise of 6.3 C. The experiment was repeated two more times with different masses of FA 1. FA 1 dissolved completely when 5.00 g were used but only partially when 5.50 g were used. To determine the percentage by mass of H A in FA 1, m g of FA 1 can be reacted with V cm 3 of FA 2 to obtain the temperature rise, T. By repeating the experiments for different sets of m and V values, a suitable graph can be plotted to determine the set of m and V values required for complete neutralisation (the end-point). (a) If a series of five experiments is to be carried out, suggest suitable masses and volumes so that T is at least 10 C for each experiment. Show relevant calculations. Experiment I II III IV V Mass of FA 1, m / g Volume of FA 2, V / cm3 Minimum mass of FA1 = 50.23.6 10  = 3.97 g [1] m / g = 4.00, 4.50, 5.00, 5.50, 6.00 or other suitable sets V / cm3 = 40.00 (constant) [1] 2/3 points on either side of end-point Note: For 4.00-5.00 g, NaOH is in excess; 5.50 –6.00 g HA is in excess. [1] [3]

3 H2 Chemistry 9647/02 NYJC J2/14 PX [Turn over For Examiner's Use (b) Using the information given above, you are required to write a plan to determine the temperature rise for Experiment I. You may assume that you are provided with:  FA 1;  FA 2;  polystyrene cup;  the apparatus and chemicals normally f ound in a school or college laboratory. In your plan, you should give essential details and include a labe led diagram of the experimental set-up. Diagram of experimental set-up: [2] Plan for Experiment I: Plan (Accept workable alternative plans.) 1. Place 40.00 cm 3 FA2 into plastic cup using burette. (Accept 50 cm 3 measuring cylinder.) 2. Measure the initial temperature of the solution using thermometer (0.2 C graduation). 3. Weigh accurately about 4.00 g of FA1 in a weighing bottle. 4. Pour FA1 into the FA2 in the cup. 5. Stir with thermometer. 6. Record highest temperature reached. 7. Reweigh weighing bottle with any residual solid. [3] ............................................................................................................................ .... [5]

4 H2 Chemistry 9647/02 NYJC J2/14 PX For Examiner's Use (c) The end-point can be determined from a suitable plot of data from the five experiments in (a). Label the axes on the grid bel ow and sketch the graph. Show how the end-point can be determined from the graph. m / g T / °C 0 End-point Axes [1] Shape of graph and end-point [1] [2] (d) FA 3 is another solid sample of H A and Na A with a different percentage composition from FA 1. The following data at the end-point we re obtained from a series of eight experiments using FA 3 and FA 2. Mass of FA 3 = 4.04 g Volume of FA 2 = 30 cm3    T = 13.1 C Calculate the percentage by mass of H A in FA 3. n(HA) = n(NaOH) = 1000 30  0.975 = 0.02925 mol m(HA) = 0.02925  122.0 = 3.569 g [1] % HA = 04.4 569.3  100 = 88.3% [1] %H A = ......................................................................................................... [2] [Total: 12]

5 H2 Chemistry 9647/02 NYJC J2/14 PX [Turn over For Examiner's Use 2 Mannich reaction is an organic reaction whic h consists of an amino alkylation of the hydrogen atom adjacent to a carbonyl func tional group, by methanal and a primary amine, secondary amine or ammonia, in the presence of acid. (a) Predict the structures of V and W for each of the following Mannich reactions.       [2] [1] V (C7H15NO) + H2O [1] W (C13H17NO) + H2O

6 H2 Chemistry 9647/02 NYJC J2/14 PX For Examiner's Use (b) Tramadol is a narcotic-like pain reli ever. It could be synthesised from bromocyclohexane via a four step synthetic route. The third st ep of the reaction scheme involves a Mannich reaction.   (i) Draw the structures of X and Y in the space above. (ii) Suggest the reagents and conditions for I and II. I Aqueous NaOH, heat under reflux [1] .......................................................... II KMnO4(aq) or K2Cr2O7(aq), H2SO4(aq), heat under reflux [1] ..................... [4] (c) When aqueous silver nitrate is added to a so lution containing sodium chloride, a white precipitate is formed. When ex cess aqueous ammonia is added to the resulting solution, the white precipitate dissolves to give solution Z. (i) Suggest the identity of the white precipitate and complex ion found in solution Z. white precipitate AgCl [1] .................................................................................. complex ion [Ag(NH3)2]+ [1] ............................................................................... (ii) Solution Z is added separately to compounds V and W from (a) and warmed. State the observation and write equations for any reactions that occur. Solution Z containing [Ag(NH3)2]+ is Tollen’s reagent. V will give a silver mirror. W will not give a silver mirror. [1]    or +2 [ A g ( N H3)2]+ +3 O H +2 A g+2 H2O+4 N H3H N O H O N O H [1] for structure of final product; [1] for balancing the equation      [5] [1] X [1] Y

7 H2 Chemistry 9647/02 NYJC J2/14 PX [Turn over For Examiner's Use This equation for Mannich reaction is repeated from page 6. The proposed mechanism for Mannich reaction: Step 1: Step 2: Step 3: (d) Suggest the types of reaction that occurred in steps 1 and 2. Step 1 Nucleophilic addition [1] ................................................................................... Step 2 Elimination [1] .................................................................................................. [2]

8 H2 Chemistry 9647/02 NYJC J2/14 PX For Examiner's Use (e) The rate equation for the Mannich reaction can be expressed as rate = k [Carbonyl U]p [Methanal]q [Amine]r [H+]s A chemist carried out a series of experiments to investigate the order of reaction with respect to each of the reactants.