SAJC Prelim P3 Questions

1 Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry Higher 2 9647/03 14 September 2015 Paper 3 2 hours Candidates answer on separate paper. Additional Materials: Writing paper, Data Booklet 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 15 printed pages including this page. [Turn over

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3 1 (a) Organic compounds have various applications. For instance, Ketoprofen is a kind of nonsteroidal anti-inflammatory drug, which can be used as a substitute for aspirin. (i) Friedel Craft Acylation was first discovered by Charles Friedel and James Crafts in 1877. This reaction allowed for the acylation of an aromatic ring with acyl chloride. One such example is given below. Benzophenone, which can be used to synthesise Keto profen, has the structure below. Suggest the reagent needed to convert 2 moles of benzene into 1 mole of benzophenone. [1] [Turn over

4 1 (a) (ii) Ketoprofen may be synthesised from benzophenone in the following route. State the reagents and conditions for Steps 1 to 3. Draw the structural formula of A. [4] (iii) Ketoprofen is commonly sold as its sodium salt form for better absorption by the body, which is made up of 70% water. Explain, with the aid of a diagram, how this better absorption comes about. [2] (iv) Nitrogen-containing derivatives of Ketoprofen may be synthesised and some examples are as shown. Describe a simple chemical test to distinguish between compound D and compound E. [2] (v) Ketoprofen can react with cold alkaline HCN to form a product which exists as a mixture of 4 stereoisomers. Give the structure of the product formed. Hence, state the type of isomerism exhibited by this product and draw the stereoisomers. [4] [Turn over

5 1 (b) Another use of organic compounds is in the area of food preservation. To prevent food spoilage, the pH of food should not fluctuate too much. Hence, a mixture of lactic acid, CH3CH(OH)COOH, and sodium lactate is commonly added in yoghurt to act as a pH regulator. (i) Explain how the mixture of lactic acid and sodium lactate can function as a pH regulator when an acid is added. Include any relevant equation. [2] (ii) The pH of yoghurt is usually maintained at about 4.2. Calculate the ratio of the concentrations of lactate ion to lactic acid in yoghurt, given that the pKa of lactic acid is 3.86. [1] (iii) Given that the concentration of lactic acid in a sample of yoghurt is 0.05 mol dm -3 and using your ratio in (b)(ii), calculate the concentration of lactate ion. [1] (iv) Perfluorooctanoic acid is a common food contam inant. In yoghurt, this monobasic acid will dissociate completely. Calculate the new pH of the yoghurt when 0.05 mol of perfluorooctanoic acid is added to 5 dm 3 of the yoghurt in b(iii). Hence, calculate the change in pH. [3] [Total: 20 marks] [Turn over

6 2 (a) Transition metals are known to have many uses, ranging from materials, medicine to catalysts. Explain what is meant by the term transition metal. [1] (b) Aqueous copper( II) sulfate is blue and this is due to the presence of hexaaquacopper(II) ion, [Cu(H2O)6]2+. When 1,2-diaminoethane, H2NCH2CH2NH2, and ammonia are added separately to a sample of copper( II) sulfate solution, the following reactions occur. (Note: “en” represents H2NCH2CH2NH2) Reaction 1: [Cu(H2O)6]2+ (aq) + en (aq) [Cu(en)(H2O)4]2+ (aq) + 2H2O (l) ΔHƟ = - 52 kJ mol-1 Reaction 2: [Cu(H2O)6]2+ (aq) + 2NH3 (aq) [Cu(NH3)2(H2O)4]2+ (aq) + 2H2O (l) ΔHƟ = - 48 kJ mol-1 (i) State the full electronic configuration of Cu2+. [1] (ii) With reference to relevant bonds, s uggest why the two enthalpy changes are similar in value. [1] (iii) By considering the ent ropy and enthalpy changes of Reaction 1 and Reaction 2, suggest and explain the difference in magnitude of the standard Gibbs free energy change of the two reactions. [2] (iv) Copper(II) sulfate can combine with H 2NCH2CH2NH2 and sodium cyanide to form a compound, where the co -ordination number of copper is 6. The formula unit of this compound contains two sodium ions and an anion. Draw the structure of the anion to illustrate its shape. You may use to represent H2NCH2CH2NH2. [1] [Turn over

7 2 (c) Partition coefficient, K D, is an example of an equilibrium constant. It is the ratio of equilibrium concentrations of a species in a mixture of two immiscible solvents. From the value of K D, the difference in solubility of a species in two immiscible solvents can be determined. When an aqueous solution of ammonia was shaken with dichloromethane at room temperature, an equilibrium was established as shown. NH3 (aq) NH3 (in CH2Cl2) KD = 0.04 (i) State the solvent in which ammonia has a higher solubility. With reference to interactions involved, explain your answer. [2] (ii) Write the KD expression. [1] (iii) 100 c m3 of 1 .00 mol dm -3 aqueous ammoni a solution was mixed with dichloromethane. 100 cm3 of 0. 0133 mol dm -3 chromium(III) sulfate was then added to the water-dichloromethane mixture. This resulted in complex formation as shown below. All the Cr3+ reacted with some ammonia in the aqueous layer. Cr3+ + n NH3 [Cr(NH3)n]3+ The setup was then left to stand and the dichloromethane layer was found to contain 0.0184 mol dm-3 of ammonia. Using your answer to c(ii), c alculate the concentration of ammonia left in the aqueous layer after complex formation. [1] (iv) Using your answer to c(iii), calculate the value of n in the formula [Cr(NH3)n]3+. [1] [Turn over

8 2 (d) The production of “elepha nt toothpaste” involves the decomposition of hydrogen peroxide. The equation for this reaction is as given. 2 H2O2