SAJC H2 Chem 2013 Prelim P3 QP

1 ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Chemistry Higher 2 9647/03 Paper 3 16th September 2013 2 hours Candidates answer on separate paper. Additional Materials: Writing paper, Data Booklet, Graph paper READ THESE INSTRUCTIONS FIRST: Write your name and civics group on all the work you hand in. Write in dark blue or black pen. 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. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 14 printed pages including this page. 1180

2 Answer 4 out of 5 questions. 1 Fructose or fruit sugar (C 6H12O6) is a simple monosaccharide, which is derived from sugar cane, sugar beets and corn. It is a common source of sweetness used in the candy industry. One isomer of fructose is shown below.   D-fructose (a) State the number of chiral centers present in D-fructose. [1] (b) Carbonyl compounds can undergo rapid and reversible reaction with alcohols to form hemiacetals: hemiacetal (i) State the type of reaction for the formation of hemiacetal. (ii) Outline the mechanism of the above reaction using propanone and phenol dissolved in aqueous sodium hydroxide. 1181

3 1 (b) (iii) D-fructose can form two cyclic hemiacetal structures via the mechanism in b(ii). One of the two isomers is α-D-fructopyranose.   An alternative cyclic hemiacetal structure is a five-membered ring, α-D-fructofuranose. Draw the structure of α-D-fructofuranose.   [5] (c) In solution, D-fructose exists as an equilibrium mixture of 70% α-D-fructopyranose and 22% α-D-fructofuranose. (i) Calculate the equilibrium constant, Kc. (ii) Suggest a method to change the numerical value of Kc. [3] (d) α-D-fructopyranose is very soluble in water and insoluble in organic solvents. Hence, they are very difficult to purify. The formation of ether allows the purification to be easier. Silver oxide with iodo-alkane works with alcohol in the formation of ether as shown below:   Rate = k [CH3I][R’OH] (i) State the type of reaction involved in the formation of ether. (ii) Draw the product formed when α -D-fructopyranose reacts with Ag 2O and CH3CH2CH2I. [Turn over] 1182

4 1 (d) (iii) Predict, with reasoning how the rate of d(ii) would differ if (I) CH3 C CH3 CH2I CH3 is used. (II) CH3CH2CH2Cl is used [6] (e) Sucrose undergoes the following reaction at room temperature. This reaction can be catalysed by insects such as honey bees which have protein-like enzymes called invertases. (i) State the type of reaction sucrose undergoes. (ii) When invertases were used in the sucrose reaction at 373 K, the experiment was not successful. Suggest a possible reason and explain your answer. (iii) The graph shows how the initial rate of this reaction varies with concentration of sucrose. Explain the shape of the graph provided. [5] [Total marks: 20] Initial rate [sucrose] 1183

5 2(a) A student investigates the action of hydrogen peroxide on a sample of hair to design an efficient test-kit for Asian customers. A fixed sample of hair was added to a bottle of hydrogen peroxide labelled “approximately 1.00 mol dm -3”. At regular intervals, 10.0 cm 3 portions were withdrawn at various times and titrated with acidified 0.100 mol dm-3 K2Cr2O7 (aq) to determine the change in concentration of hydrogen peroxide over time. The following results were obtained. Time / min Volume of 0.1 mol dm -3 of K2Cr2O7 (aq) / cm3 0 30.00 5 23.40 10 18.30 15 14.20 20 11.10 25 8.70 30 6.80 (i) Write a balanced equation for the reaction between potassium dichromate and hydrogen peroxide. Hence, with the aid of a Data Booklet, deduce the feasibility of the reaction. (ii) Calculate the initial concentration of hydrogen peroxide. (iii) The calculated concentration of hydrogen peroxide in (ii) is lower than the initial value of 1.00 mol dm -3. Explain why the concentration of hydrogen peroxide has changed over time. (iv) By plotting a suitable graph, deduce the rate order of reaction with respect to hydrogen peroxide. (v) Write an expression for the rate equation and calculate the rate constant, stating its units. [Turn over] 1184

6 2(a) (vi) The student reads the following information from a scientific article. “Ammonia is the alkaline chemical that opens the cuticle and allows the hair color to penetrate the cortex of the hair. It also acts as a catalyst when the permanent hair color comes together with the peroxide.” Using the same axes, sketch and label on your graph in a(iv) the curve when ammonia is added to the same experiment. [12] (b) The cultural emergence towards environmental consciousness has led to a re-emergence of organic products. One such compound that is derived from coconut is MEA (Mr = 61). In hair colour, MEA helps to eliminate odour and neutralizes the toxicity of hair colorants. MEA is a simple aliphatic organic compound that can dissolve in water to form a weakly basic solution. In the presence of concentrated sulfuric acid, 2 g of MEA reacts with 4.0 g of benzoic acid to form B, C 9H11NO2. However, 2 g of MEA require 9.2 g of benzoyl chloride to react completely to form C, C16H15NO3 and copious white fumes. 2 moles of MEA can also react with gaseous PCl5 to form a cyclic D, C4H10N2 as one of the products under room temperature and pressure. Deduce the structures of MEA, B, C and D. [8] [Total marks: 20]      1185

7 3 (a) Currently, mobile phones and laptops use lithium-ion batteries. The following cell diagram notation is a simplified illustration of the electrochemical cell. Li | Li + || Li+, CoO2 | LiCoO2 | Pt (i) Write balanced half-equations for the reactions occurring at each electrode. (ii) Explain why cobalt is able to exhibit variable oxidation states. [4] (b) In recent years, organic radical polymers have been developed as alternatives to inorganic materials because of their light weight, flexibility and their environmentally-friendly features. With the increase in demand for thinner mobile phones and tablet computers, there is a surge to invent film-like organic-derived electrode-active battery materials to power such devices. One such material is TEMPO-NO radical which is capable of repeated stable oxidation-reduction between N=O radicals and N=O cations. Its voltage when paired with a Li/Li + half-cell is able to reach +3.60 V which is comparable to a common lithium-ion battery. The following is a simplified illustration of the electrochemical cell set-up. [Turn over] 3 (b) (i) The following half-equation represents the reaction which occurs at the organic 1186

8 derived electrode. With the use of the Data Booklet and the information above, write the overall balanced equation for the electrochemical reaction and calculate the standard electrode potential for TEMPO-NO radical. (ii) In order for the radical polymer to be used as an electrode, it must not be soluble in the electrolyte. To improve the usability of TEMPO-NO radical, the material can be converted to PTMA which has the following structure: PTMA Explain how the structure of PTMA will help to reduce its solubility in polar solvents like water. (iii) Write an equation to illustrate the reaction when an aqueous medium is used at the lithium anode.    1187

9 3 (b) (iv) The synthesis of PTMA required the mon