NYJC H2 Chem 2012 Prelim P2 QP

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS 1 1 INDEX NO. TUTOR CHEMISTRY 9647/02 Paper 2 Structured 10 September 2012 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 / 8 3 / 7 4 / 14 5 / 6 6 / 4 7 / 8 8 / 9 /609 / 4 This document consists of 16 printed pages and 0 blank page. [Turn over

2 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use Answer all questions in the spaces provided. 1 Planning (P) Paracetamol (acetaminophen) is commonly used for the relief of headaches and is a major ingredient in numerous cold and flu medicine. It is a white solid (m.p. = 169 oC) which can be prepared by a reaction between 4-aminophenol and ethanoic anhydride. HOC 6H4NH2 (CH3CO)2O CH 3CONHC6H4OH 4-aminophenol (Mr = 109) ethanoic anhydride (Mr = 102) paracetamol (Mr = 151) A typical yield, based on 4-aminophenol, is 70%. The crude product can be purified by recrystallisation from water. The purity of the recrystallised product can be confirmed by determining its melting point. Ethanoic anhydride can cause irritation of tissue, especially in nasal passages. 4-aminophenol is a skin irritant and is toxic. (a) Using the information above: (i) Write a balanced equation for the formation of paracetamol; (ii) Calculate the minimum masses of reactants needed to prepare 5 g of pure paracetamol. [3]

3 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use (b) Write a full description of the purification of the crude paracetamol. List the essential apparatus you would use in the process. You do not have to describe the preparation of paracetamol. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .............................................................................................................................. [6] (c) Write a brief description showing how you would use the melting point of the purified product to confirm its purity. You do not have to describe how you would obtain the melting point. .................................................................................................................................. .............................................................................................................................. [1] (d) Identify two potential safety hazards in this experiment and the relevant safety precautions you would take. .................................................................................................................................. .................................................................................................................................. .................................................................................................................................. .............................................................................................................................. [2] [Total: 12]

4 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use 2 Given the following thermochemical data: Reaction ∆∆ ∆∆ H/eshshortrev/eshshortrev /eshshortrev/eshshortrev/ kJ mol−− −− 1 C(graphite) + 2H 2(g) → CH4(g) − 75.0 C(graphite) + O 2(g) → CO2(g) − 393.5 H 2(g) + ½O2(g) → H2O(l) − 285.9 (a) With the aid of an energy cycle, calculate the enthalpy change for the reaction CH4(g) + 2O2(g) → 2H2O(l) + CO2(g) [3] Energy

5 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use (b) The experimental enthalpy change is − 801.7 kJ mol− 1 for the following reaction CH 4(g) + 2O2(g) → 2H2O(g) + CO2(g) …………….. (I) (i) Calculate the enthalpy change of vaporisation of water at 298K. [2] (ii) Using bond energy data from the Data Booklet, calculate another value for ∆ H for reaction (I) in (b). Account for any differences between your answer and the given experimental value in (b). ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... [3] [Total: 8]

6 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use 3 Mixtures of citric acid, C5H7O4CO2H (Ka = 7.40 x 10–4 mol dm–3 ), and its sodium salt are often used as acidity regulators for food. The mixture regulates the pH of food by acting as a buffer. (a) Prove that the pH of a mixture formed from 25.0 cm 3 of 0.200 mol dm –3 citric acid and 2.48 g of sodium citrate (Mr = 198) is 3.53. [2] (b) When 0.059 g of an unknown solid was added to the mixture prepared in (a), the pH of the resultant solution is 3.73. Determine the molar mass of the solid and hence suggest its identity. [3]

7 H2 Chemistry 9647/02 NYJC J2/12 PX [Turn over For Examiner's Use (c) Determine the volume of 0.100 mol dm -3 of HCl(aq) that needs to be added to the solution in (a) to obtain a buffer at its maximum buffering capacity.