CJC H2 Chem 2013 Prelim P3 QP

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2 9647/03/CJC JC2 Preliminary Exam 2013 Answer any four questions. 1. Use of the Data Booklet is relevant to this question. (a) (i) Define the term standard electrode potential, E o . (ii) For the element M (where M is Co or Ni), suggest an explanation for the difference in the E o values between M 2+ + 2e– M and [M (NH3)6]2+ + 2e– M + 6NH3 [4] (b) A student set up the following electrochemical cell. (i) What device is used to measure the e.m.f. of the cell? (ii) Name a compound that could be used to prepare a workable salt-bridge for this cell and explain how it helps to maintain electrical neutrality. (iii) Amongst other factors, the electrode potential is dependent on the temperature and the concentration of the ions. At 298 K, the reduction potential, E, of Ag +(aq)/Ag(s) can be obtained from the equation E = E o + 0.059 log10 [Ag+(aq)] The student found that the e.m.f. of the cell was +0.17 V at 298 K. Calculate the reduction potential, E, of each half-cell and hence the concentration of Ag +(aq) ions in the saturated AgCl(aq). [Given: E o of Ag+(aq)/Ag(s) = +0.80 V at 298 K] (iv) State and explain the effect on the cell e.m.f. of adding a small volume of NH3(aq) to the Ag+(aq)/Ag(s) half-cell. [9] salt-bridge + Ag(s) – AgNO3(aq) 0.010 mol dm–3 Ag(s) saturated AgCl(aq) AgCl(s) 434

3 9647/03/CJC JC2 Preliminary Exam 2013 (c) Putrescine (1,4-diaminobutane) is a foul-smelling compound found in rotting flesh and in "bad breath". It is produced by the breakdown of amino acids in living and dead organisms. Like ammonia, putrescine is a weak base. While ammonia has a p Kb value of 4.74, putrescine has two p Kb values which are found to be 3.20 (p Kb1) and 4.65 (p Kb2) respectively. (i) Explain why putrescine is a stronger base than ammonia. (ii) Write equations to represent Kb1 and K b2 of putrescine. Hence explain why pKb1 of putrescine is smaller than pKb2. A chemist proposed the following synthetic routes to synthesise putrescine. Route 1: Route 2: (iii) By considering the type of reaction taking place, comment briefly on the yield of putrescine synthesised via route 1. (iv) Route 2 is found to give only 10 % yield of putrescine. Suggest another synthetic route to obtain a higher yield of putrescine, starting from ethene. State clearly the reagents and conditions required. [7] [Total: 20] CH3CH2CH2CH2Cl ClCH2CH2CH2CH2Cl Cl2, uv light ClCH2CH2CH2CH2Cl H2NCH2CH2CH2CH2NH2 putrescine conc. NH3 (excess) heat in sealed tube 435

4 9647/03/CJC JC2 Preliminary Exam 2013 2. (a) Sodium hypochlorite (NaClO) is a chemical commonly found in bleaching agents. It was developed in the 18 th century and was initially used to bleach cotton. It became a popular compound for bleaching clothes when it was found that sodium hypochlorite can remove stains from clothes at room temperature. Sodium hypochlorite is also used as a disinfectant in water treatment and in swimming pools. As compared to chlorine which can cause respiratory problems, sodium hypochlorite is a safer choice of disinfectant. The original method of producing sodium hypochlorite involved passing chlorine gas through a solution of sodium carbonate. The addition of chlorine to water results in the equilibrium shown below. Cl 2 + H2O HOCl + H+ + Cl – (i) Some chlorine gas was bubbled through 100 cm 3 of water at 25 °C and allowed to reach equilibrium. The initial concentration of chlorine was 0.10 mol dm –3. At equilibrium, the concentration of chlorine decreased by 30 %. Write an expression for Kc for the above equilibrium and use the data provided to calculate its value. You may assume that the [H 2O] is 55.5 mol dm –3 throughout. (ii) A typical household bleach contains a mixture of sodium hypochlorite, calcium hypochlorite and sodium hydroxide. Suggest why sodium hydroxide is added to household bleach. (iii) HOCl is a weak acid (pKa = 7.5) which partially dissociates at a pH range of 5 to 8, thus giving rise to the equilibrium below. Cl2 + H2O 2H+ + OCl – + Cl – Using this information and your answer in (a)(i), calculate the Kc value for this equilibrium. Include its units in your answer. (iv) A more effective method to manufacture sodium hypochlorite is through the reaction of chlorine gas with sodium hydroxide. Write an equation for this reaction. Explain why the resulting mixture has to be kept at temperatures below 40 °C. [9] 436

5 9647/03/CJC JC2 Preliminary Exam 2013 (b) Elements X, Y and Z are found in periods 2 and 3 of the Periodic Table. The proton numbers of these elements fall within the range of 4 to 15.  Element X reacts readily with water. 1 mol of the resulting compound is neutralised by 1 mol of ethanoic acid.  The oxide of Y is amphoteric but the chloride of Y is acidic. 1 mol of the chloride of Y reacts with 2 mol of ammonia to give a single compound.  Element Z forms two chlorides, ZCla and ZClb. 1 mol of ZCla reacts with water in the cold to give two chlorine-containing products in the mole ratio of 1: 2. Identify X, Y and Z . Explain your reasoning and give relevant equations to support your answer. [5] (c) Benzoic acid can react with alkyl halides to form esters. (i) Why is it necessary to react benzoic acid with silver(I) hydroxide in Step 1? (ii) What will be observed at the end of step 2? (iii) Describe the mechanism for the reaction that occurred in step 2. (iv) A carboxylic acid reacted with silve r(I) hydroxide followed by dibromomethane to give the following product. Deduce the structure of this carboxylic acid. [6] [Total: 20] C O O N H 437

6 9647/03/CJC JC2 Preliminary Exam 2013 3. During any vigorous physical activity, sweating is the primary means by which the body maintains its core temperature of 37 oC. However, this will result in the loss of body fluid and electrolytes like chloride, phosphate, calcium, magnesium, sodium and potassium ions. If the levels of electrolytes are not adequately replenished, this can potentially lead to dehydration followed eventually by heat st roke if the percentage bodyweight lost as sweat exceeds 10 % or more. An isotonic sports drink contains electrolytes, which aid fluid absorption, and carbohydrates that provide the necessary fuel for the body to maintain peak performance and enhance post exercise recovery. (a) Magnesium and calcium ions are found in many isotonic drinks. A student decides to investigate the amount of magnesium ion in a certain brand of isotonic drink by precipitating the ion with aqueous ammonia. K sp / mol3 dm–9 Decomposition Temperature / oC Mg(OH)2 1.5 x 10–11 350 Ca(OH)2 4.7 x 10–6 580 (i) Given