2019 NYJC Prelim P3 QP

[Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9729/03 Paper 3 Free Response 19 September 2019 2 hours Candidates answer Section A on the Question Paper Candidates answer on separate paper. Additional Materials: Answer Booklet 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. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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 4 5 This document consists of 12 printed pages.

2 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM Section A Answer all the questions from this section. 1 (a) When ethanoic acid reacts with ethanol to form ethyl ethanoate and water, the following equilibrium is established. CH3CO2H(l) + CH3CH2OH(l) CH3CO2CH2CH3(l) + H2O(l) H = 3.8 kJ mol1 A student mixed 1.48 g of ethanoic acid in a conical flask with 0.92 g of ethanol and 1.80 g of water. He then carefully added 0.98 g of concentrated sulfuric acid to catalyse the reaction. The flask was sealed and placed in a thermostatic water bath set at 25 C for 5 days. After 5 days, the student added phenolphthalein indicator to the flask and titrated the entire contents of the conical flask with 0.800 mol dm 3 Ba(OH)2. The indicator turned pink when 23.30 cm3 of Ba(OH)2 had been added. (i) Calculate the amount of Ba(OH)2 that has reacted with H2SO4. [1] (ii) Write a balanced equation for the reaction of ethanoic acid with Ba(OH) 2. Using your answer in (a)(i), calculate the amount of ethanoic acid present at equilibrium. [2] (iii) Write an expression for the equilibrium constant, Kc, for this reaction and calculate the value of Kc, stating its units. [3] (iv) Suggest and explain what would happen to the position of this equilibrium and the value of Kc if the temperature is decreased. [2] (b) (i) The addition of an acid to water generates heat and this process is termed as dilution. The dilution of ethanoic acid can be represented with the equation below. CH3COOH(l) CH3COO(aq) + H+(aq) Calculate the enthalpy change of dilution of ethanoic acid given the following: Hf / kJ mol1 CH3COO(aq) 486 H+(aq) 0 CH3COOH(l) 485 [1] (ii) In the presence of heat, ethyl ethanoate, CH3CO2CH2CH3 is able to react with Ba(OH)2 as seen in the equation below: 2CH3CO2CH2CH3(l) + Ba(OH)2(aq)  (CH3CO2)2Ba2+(aq) + 2C2H5OH(l) State the type of reaction occurring. [1]

3 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM [Turn over (iii) Construct a fully labelled energy level diagram to calculate the enthalpy change of the reaction in (b)(ii). Your diagram should include the data given in (a), (b)(i) and the following data. Standard enthalpy change of neutralisation of CH3COOH(aq) and Ba(OH)2 = 55 kJ mol1 [3] (c) Suggest a synthesis of phenyl propanoate starting from prop ane according to the scheme below. State the structure of compound A. Include reagents and conditions for all reactions, and the structures of all other intermediate compounds, in your answer. propane C3H6O2 more than one step + OH more than one step OCOCH2CH3 phenyl propanoate A [5] [Total: 18]

4 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM 2 Alanine is a non-essential -amino acid in humans and widely used for biosynthesis of proteins. NH2 C COOH H CH3 alanine Protonated alanine can be synthesised in the laboratory from ethanol. CH3CH2OH CH3CHO step 2 HCN, traces of NaOH(aq) cold B Br C CN H CH3 step 3 step 5NH3 + C COOH H CH3 step 1 K2Cr2O7, H2SO4(aq) heat with distillation step 4 excess ethanolic NH3 heat in sealed tubeNH2 C CN H CH3H2SO4(aq) heat under reflux 2-bromopropionitrile (a) (i) Heating with distillation in step 1 is required for the synthesis of aldehyde to prevent the aldehyde from further oxidising to carboxylic acid. Explain why heating with distillation is suitable in isolating the aldehyde. [1] (ii) Draw the displayed formula of compound B. [2] (iii) Describe the mechanism in step 2. [3] (iv) Protonated glycine can also be synthesised from methanol, CH3OH, using the reaction scheme shown above. NH3 + C COOH H H protonated glycine Suggest why the relative rate of step 2 will be higher in the synthesis of protonated glycine. [2] (v) Suggest the reagent and condition for step 3. [1]

5 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM [Turn over (b) To prove that the mechanism in step 4 is SN2, two separate experiments were performed and the rate equation was found to be rate = k[NH3][2-bromopropionitrile]  In experiment 1, the rate of reaction was monitored with increasing concentration of NH3.  In experiment 2, concentration of 0.1 mol dm–3 of 2-bromopropionitrile was monitored over time. (i) Copy the two graph axes below onto your answer booklet. Draw the graph for each experiment and explain the shape of your graphs. [2] (ii) Suggest a suitable concentration of NH3 in experiment 2. Explain your answer. [1] (iii) Suggest why the mechanism in step 4 is unlikely to be SN1. [1] rate / mol dm–3 s–1 [NH3] Experiment 1 [2-bromopropionitrile] time / s Experiment 2

6 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM (c) Protonated alanine (H2A) dissociates in two stages as shown below. NH3 + C COOH H CH3 NH3 + C COO H CH3 NH2 C COO H CH3 (H2A) (HA) (A2) pKa1 pKa2 H+ H+ The graph below shows how the percentage composition of H 2A, HA– and A2– change with pH. (i) State the values of pKa1 and pKa2 of alanine. [1] (ii) A buffer of pH 10.3 is prepared by adding 0.100 mol of protonated alanine (H 2A) in 1 dm3 of water and solid NaOH was subsequently added. Using the graph above, determine the amount of HA– and A2– present in the buffer and hence, calculate the mass of solid NaOH added. [2] (iii) Assuming 1 dm3 of water is added to the buffer prepared in (c)(ii), explain qualitatively if the new buffer will result in a greater or smaller change in pH compared to the original buffer, when same amount of HCl(aq) is added. [2]

7 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM [Turn over (iv) The solubility of alanine under different pH was investigated. The graph below shows the results of solubility against pH. Isoelectric point is the pH at which the amino acid has a net zero charge. With reference to the structures of H2A, HA– and A2–, explain  the low solubility of alanine at its isoelectric point of 6.0.  the high solubility of alanine at low and high pH. [4] [Total: 22] 6.0 solubility / mol dm–3 pH

8 H2 Chemistry 9729/03 NYJC J2/2019 PRELIM 3 This question looks at the chemistry of some compounds formed by transition metal ions and halides. (a) When aqueous potassium iodide is mixed with aqueous copper( II) sulfate