NYJC H2 Chem 2012 Prelim P2 Soln

2 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer'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. HOC6H4NH2 (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; HOC6H4NH2 + (CH3CO)2O → CH3CONHC6H4OH + CH3CO2H (i i) Calculate the minimum masses of reactants needed to prepare 5 g of pure paracetamol. Mass of 4-aminophenol required = 109 ×70 100×151 5 = 5.16 g M ass of ethanoic anhydride required = 5 100× ×102151 70 = 4.82 g [ 3]

3 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer'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. Apparatus: - Conical flask for dissolving crude solid (accept beaker) - Hot plate for heating (accept oil bath but not water bath or heating mantle) - Filter funnel/ conical flask for hot filtration - Buchner apparatus for collecting pure crystals. Method: - Dissolve crude solid in minimum amount of hot water - Filter solution while hot - Cool hot solution (in ice bath) - Collects crystals using vacuum filtration/ reduced pressure (using Buchner apparatus) - Wash with small amounts of cold water - Dries crystals between filter paper/ under IR lamp. ................................................. ............................................................................. [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. - Melts sharply/ over small temperature range - Melting point agrees with data value (i.e. 169 oC) ................................................. ............................................................................. [1] (d) Identify two potential safety hazards in this experiment and the relevant safety precautions you would take. Hazard Pr ecaution 4-aminophenol is a skin irritant and is toxic. Ethanoic acid is corrosive Avoid skin contact; wear personal protective equipment (gloves, goggles); flood affected areas if there is spillage. Ethanoic anhydride can cause irritation of tissue, especially in nasal passages. Perform experiment in fume cupboard. Ethanoic anhydride is flammable Do not use naked flames. - Precautions must be relevant to the hazard - Do not allow “use fume cupboard as a precaution for toxicity. - Do not allow “do not eat/ consume, do not breathe in” as precautions ................................................ . ............................................................................. [2] [Total: 12]

4 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use C(graphite) + 2H2( g) + 2O2(g) CH4( g) + 2O 2(g) CO2( g) + 2H 2O(g) 2 Given the following thermochemical data: Reaction ∆∆ ∆∆ H/eshshortrev/eshshortrev /eshshortrev/eshshortrev/ kJ mol−−−− 1 C(graphite) + 2H 2(g) → CH 4(g) − 7 5.0 C(graphite) + O 2(g) → CO 2(g) − 3 93.5 H 2(g) + ½O2(g) → H 2O(l) − 285.9 (a) With the aid of an energy cycle, calculate the enthalpy change for the reaction CH4(g) + 2O2(g) → 2H 2O(l) + CO 2(g) Energy 0 −− −− 75 (−− −− 393.5) + 2(−− −− 285.9) ∆∆ ∆∆ H B y Hess’ Law: ∆∆ ∆∆ H = +75.0 + {2(−− −− 2 85.9) + (−− −− 393.5)} = −− −− 890.3 kJ mol−−−− 1 [3]

5 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use (b) The experimental enthalpy change is − 801.7 kJ mol− 1 for the following reaction CH 4(g) + 2O2(g) → 2H 2O(g) + CO2(g) …………….. ( I) (i) Calculate the enthalpy change of vaporisation of water at 298K. −− −− 890.3 CH4(g) + 2O2(g)    → → →→ 2H2O(l) + CO 2(g) −− −− 801.7 2∆∆ ∆∆ Hvap 2H2O(g) + CO2(g) 2 ∆∆ ∆∆ Hvap = +890.3 + (−− −− 801.7) 2∆∆ ∆∆ Hvap = +88.6 ∆∆ ∆∆ Hvap = +44.3 kJ mol−−−− 1 [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). ∆∆∆∆ H = 4(410) + 2(496) −− −− 4(460) −− −− 2(740) = −− −− 688 kJ mol−− −− 1 The value obtained from bond energy calculations is different because bond energies are only average values obtained from different molecules. They may not be the actual values for the molecules used. [3] [Total: 8]

6 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use 3 Mixtures of citric acid, C5H7O4CO2H (Ka = 7.40 x 10–4 mol dm–3 ), and its sodium salt a re 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 a nd 2.48 g of sodium citrate (Mr = 198) is 3.53. pH = pK a + log [C5H7O4CO2Na]/[C5H7O4CO2H] pH = - log (7.40 x 10 –4 ) + log 0.200 × 0.025 2.48/198 = 3.53 [2 ] (b) When 0.059 g of an unknown solid was added to the mixture prepared in (a), the pH o f the resultant solution is 3.73. Determine the molar mass of the solid and hence suggest its identity. S ince pH is raised on addition of the solid, the soli d added must be a base. C5H7O4CO2H + OH– /barb2right C5H7O4CO2 – + H2O 3 .73 = - log (7.40 x 10–4 ) + log [C5H7O4CO2Na]/[C5H7O4CO2H] [ C5H7O4CO2Na]/[C5H7O4CO2H] = 3.97 L et the molar mass of the solid be M n citric acid = 0.025 x 0.200 – 0.059/ M n citrate = 2.48/198 + 0.059/M 0.059/M0.200 × 0.025 0.059/M + 2.48/198 = 3.97 /barb2right M = 40.0 S olid is sodium hydroxide. [3]

7 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use (c) Determine the volume of 0.100 mol dm -3 of HCl(aq) that needs to be added to the s olution in (a) to obtain a buffer at its maximum buffering capacity. p H at max. buffering capacity = 3.13 C5H7O4CO2 – + H3O+ /barb2right C5H7O4CO2H + H2O L et the volume of 0.100 mol dm-3 HCl required be V n citric acid = 0.025 x 0.200 + 0.100 V n citrate = 2.48/198 – 0.100 V A t max. buffering capacity, [C 5H7O4CO2H] = [C5H7O4CO2 – ] /barb2right ncitric acid = ncitrate 0.025 x 0.200 + 0.100 V = 2.48/198 – 0.100 V V =