MJC H2 CHEM P2

No part of this paper should be repro duced for commercial use without the prior written permission of Meridian Junior College. Class Reg Number Candidate Name _____________________________ Meridian Junior College 2009 JC 2 Preliminary Examination H2 Chemistry 9746 22 September 2009 1 hr 30 min Paper 2 STRUCTURED QUESTIONS Additional Materials Data Booklet INSTRUCTION TO CANDIDATES Write your name, class and register number in the spaces provided at the top of this page. Write in dark blue or black ink. You may use a soft pencil for any diagrams or graphs. You are reminded on the need for good English and clear presentation of your answers. INFORMATION FOR CANDIDATES FOR EXAMINER’S USE The number of marks is given in brackets [ ] at the Q1 / 8end of each question or part of question. Q2 / 8 Q3 / 11 Q4 / 11 Q5 / 12 Q6 / 10 Total / 60 This question paper consists of 18 printed pages 1

Answer all the questions in the space pro vided 1 Early Periodic Tables, such as that devised by Mendeleev, listed the then known elements in order of their relative atomic mass. (a) (i) When Mendeleev created the Periodic Table, there were uncertainties regarding the relative atomic mass of tellurium, Te. It is now known that there are eight isotopes of tellurium. isotope percentage abundance isotopic mass x percentage abundance tellurium-120 0.09 11 tellurium-122 2.46 300 tellurium-123 0.87 107 tellurium-124 4.61 572 tellurium-125 6.99 874 tellurium-126 18.71 2357 tellurium-128 31.79 tellurium-130 Complete the above table and give your answers to two decimal places. Hence, calculate the relative atomic mass of tellurium to five significant figures. 2

(ii) TeO2 resembles aluminium oxide in its acid-base properties. TeO 2, SiO2 and P 4O10 cannot be distinguished based on their physical appearance. If a sample of one of the oxides was provided as a fine powder, describe the reactions you could ca rry out on the powder to determine which of the three oxides it was. In clude all relevant equations in your answer. [5] (b) Another chemist looking for patterns in the properties of the elements around the time of Mendeleev was Julius Lothar Meyer. Lothar Meyer looked at how the atomic volume of an elem ent varies with its relative atomic mass. A simplified version of the graph he plotted is shown below. I II 3

There are 2 circled regions on the graph. Explain why there is only a slight decrease in atomic volume bet ween the elements in region II compared to region I. [3] [Total: 8] 2 ‘Hard’ water is water that has high mineral content such as calcium ions. A common washing-up liquid contains sodium dodecylbenzenesulphonate, C18H29SO3Na [M r = 348]. In ‘hard’ water, it is ineffective as a detergent because it reacts with calcium ions to gi ve a precipitate. The solubility product, Ksp, for (C18H29SO3)2Ca (s) is 1.20  10-17 mol3 dm-9. The washing-up liquid contains 17.4% of sodium dodecylbenzenesulphonate by mass. In a factory production, 0.500 dm3 of 0.102 g dm-3 of the washing-up liquid was accidentally mixed with 0.200 dm3 of ‘hard’ water. A typical sample of ‘hard’ water has a concentration of calcium ions of 2.5010-4 mol dm-3. (a) Determine whether a prec ipitate is formed when the common washing-up liquid was accidentally mixed with 0.200 dm3 of ‘hard’ water. [2] 4

(b) In order for the detergent to be used in ‘har d’ water, sodium tripolyphosphate, Na5P3O10, is added as a water softening agent. The sodium tripolyphosphate ‘softens’ water by complexing with calcium ions. The complexation reaction is as follows: Ca2+ (aq) + P3O10 5- (aq) CaP3O10 3- (aq) For this reaction with calcium ions, the equilibrium co nstant is 7.7  108 mol-1 dm3. (i) Write the Kc expression for the reaction. (ii) Hence, calculate the concentration of tripolyphosphate ions required to reduce the calcium ion concentration in a typical sample of “hard water” to 1.0  10 -6 mol dm-3. [3] 5

(c) ‘Hard’ water also contains magnesium ions which can form a precipitate with the detergent. For example, magnesiu m ions form magnesium iodide, Mg I2, in the presence of aqueous potassium iodide. (i) The lattice energy of Mg I2 is –2327 kJ mol -1 while the values of the enthalpy change of hydration are listed below: Ions Hhyd / kJ mol-1 Mg2+ –1920 I- –295 Calculate the enthalpy change of solution of magnesium iodide. (ii) By using your answer from (c)(i), and given the entropy change of solution of magnesium iodide is posit ive, predict whether magnesium iodide is soluble in water at room temperature and pressure. Give your reasoning. [3] [ T o t a l : 8 ] 6

3 Cobalt is a bluish-white element that can form various cobalt compounds wit h ligands such as ethylenediamine, H2NCH2CH2NH2, and ethanedioate, C2O4 2-. Ethylenediamine is commonly expressed with the short form “en”. (a) Passing air through an aqueous soluti on containing CoCl 3, en and HC l produces a green complex cation, [Co( en)2Cl2]+. Another complex, [Co(C2O4)3]3- is obtained on passing air through an aqueous solution of CoCl3 and C 2- 2O4 . (i) State the coordination number of Co in [Co(en)2Cl2]+. (ii) Explain why transition element complexes such as [Co( en) 2Cl2]+ are coloured. 7

(iii) The standard electrode potential of the [Co(C 2O4)3]3- / [Co(C2O4)3]4- is given below. [Co(C2O4)3]3- (aq) + e– [Co(C2O4)3]4- (aq) E o = +0.57 V Draw a fully labelled diagram to show how the standard electrode potential of the [Co(C 2O4)3]3-(aq) / [Co(C 2O4)3]4-(aq) system can be measured. (iv) In general, the presence of ethanedioate ions, C 2O4 2-, significantly decreases the oxidising ability of Co (III). Explain this effect by using the information in (a)(iii) and relevant data from the Data Booklet. [9] 8

(b) Other than being used as silica gel, cobalt ( II) chloride can also be used as a homogeneous catalyst for the reaction of 2,3-dihydroxybutanedioate ions and hydrogen peroxide to form carbon dioxide, methanoate ions and water. H Suggest equations for a plausible mechani sm for the catal ysis of the above reaction using Co 2+ as the catalyst. [2] [ T o t a l : 1 1 ] 4 MTBE as well as its isomer MTBE A is added to car petrol as oxygenates. There has been concern ov er the leakage of MTBE and MTBE A into water supplies, mainly because of the foul taste it gives to water supplies. In the manufacture of f uel for industries, liquid MTBE A is added as a solvent to petroleum to reduce pollution. CH3 C CH3 CH3 OCH3 CH3 C CH2 CH3 CH3 CH3 MTBE MTBE A (a) What is meant by the standard enthalpy change of formation of MTBE A? [1] + 3H 2O2 2CO2 + 2HCO2 - + 4H2O C C HO CO2 - CO2 -HO H CC H 2CH3 CH3 CH3 O H 9

(b) Using relevant information from the Data Booklet and the data provided, obtain a value for the average bond energy for the O–H bond in MTBE A, b y means of an appropriate energy cycle. Data provided: Hat (C) = +715 k