HCI H2 CHEM P3 Prelim

This document consists of 10 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 13S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9647/03 22 September 2014 2 hours INSTRUCTIONS TO CANDIDATES Write your name and class 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. Begin each question on a new piece of paper. A Data Booklet is provided. 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. At the end of the examination, fasten all your work securely together. Write down the question numbers for the questions that you have attempted on the cover page provided.

2 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 Answer any four questions. 1 (a) Describe and explain the trend observed for the reactions of chlorine, bromine and iodine with hydrogen. [2] (b) Bromoalkanes can be synthe sised by reacting alkanes and bromine in the presence of UV light or heat. One example of such reaction is shown below: + Br2 UV or heat Br + HBr reaction 1 (i) Describe the mechanism for reaction 1. Indicate any unpaired electrons using a dot (). (ii) State how monosubstituted product can be favoured for reaction 1. [4] (c) Another way to synthesise bromoalkanes is by reacting alkenes with hydrogen bromide, as shown in reaction 2 below. This is known as a radical addition reaction, where HBr is added across the alkene in a reaction involving radicals. + HBr Br reaction 2ROOR (R = alkyl group) major product (i) By considering the electrophilic addition of HBr on alkenes, s uggest why the major product of reaction 2 is unusual. The compo und ROOR in reaction 2 is used to generate a bromine radical, which will attack the alkene C=C bond to produce a radical intermediate as shown below: + Br Br intermediate The radical intermediate will then abstract a hydrogen atom from HB r to form the major product in a subsequent step. (ii) Explain why the bromine radical would attack the C=C bond. (iii) Draw the structure of another possible radical intermediate . C ompare the relative stabilities of the two radical intermediates to account for the major product formed. (iv) Hence, propose a three-stage reaction scheme for the following transformation, naming the reagents and conditions used in each step. Draw the structures of the intermediate compounds. CO2H [7]

3 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 (d) In the atmosphere, the ozone gas O 3 absorbs UV light and undergoes gas phase decomposition through the following mechanism: Step 1: O3  O2 + O Step 2: O3 + O  2O2 The rate equation for the decomposition of ozone has the following form, rate = k[O3]2[O2]1 where k is the rate constant. (i) State the units of k. (ii) With an appropriate sketch of the B oltzmann distribution, explain how a rise in temperature affects the value of k. The presence of a chlorine radical accelerates the gas phase decomposition of ozone through the following mechanism: O3 + Cl  ClO + O2 ClO + O  Cl + O2 (iii) State the role of the chlorine radical in the mechanism above. Explain your answer. (iv) Explain how chlorofluoroalkanes (CFCs) destroy the ozone layer. [7] [Total: 20]

4 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 2 Longifolene is a naturally occurring oily liquid found in pine trees. longifolene (a) Explain why longifolene is not miscible with water. [1] (b) Hydrazine, ethylamine and ammonia are common bases used in chemical synthes es. The structure of hydrazine is shown below. hydrazine Describe and explain the relative basicities of hydrazine, ethylamine and ammonia. [3] (c) Part of the laboratory synthesis of l ongifolene by American Nobel Laureate Elias James Corey is shown below. S S O S S OH OH I Na(s) hydrazine II A B OHCl III1. CH3Li 2. Reagent X IV longifolene O V (i) Suggest reagents and conditions needed for reactions I, II, IV and V. (ii) Suggest a simple chemical test to distinguish between compounds A and B. [6]

5 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 (d) (i) In reaction III, CH 3Li can be treated as a source of a CH 3 – anion. Draw a dot-and-cross diagram for the CH3 – anion and state the geometry of the anion. (ii) Suggest the identity of reagent X used in reaction III. (iii) Name and draw out the full mechanism of reaction III. Show relevant lone pairs and dipoles, and use curly arrows to indicate the movement of electron pairs. (You may use R to represent parts of the molecule not involved in the reaction.) [6] (e) The natural synthesis of longifolene in pine trees involves the folding of a long chain farnesyl carbocation into longifolene. farnesyl carbocation longifolene (i) Deduce whether the ∆S of the folding process is positive or negative. (ii) State the number of chiral carbons on longifolene. (iii) Suggest why the smell of chemical ly synthesi sed longifolene in the laboratory is different from the samples of longifolene extracted from pine trees. [4] [Total: 20]

6 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 3 (a) The wide temperature range over which water is a liquid (m.p. 0 C, b.p. 100 C) and its polarity have given it a dominant position as a solvent for ionic species. In the search for alternative polar solvents, ammonia (m.p. 78 C, b.p. 33 C) is a logical choice. (i) Explain in terms of electronegativity and the shape of the m olecule, why the water molecule is polar. (ii) Give two reasons why the boiling point of water is higher than that of ammonia. [4] (b) Solid copper(II) hydroxide dissolves readily in excess aqueous ammonia as well as in liquid ammonia, forming similar complex ions. (i) Explain what is meant by the term complex ion. (ii) Give one reason why transition metal ions have a strong tendency to form complex ions. (iii) Suggest an equation for the reaction between solid cop per(II) hydroxide and liquid ammonia, and the colour of the solution formed. [5] (c) Like pure water, pure liquid ammonia is weakly ionised. The ionic product of water, Kw, is 1 .0  10–14 mol2 dm–6 at 25 C while that of liquid ammonia, KNH3, is 1.0  10–30 mol2 dm–6 at 33 C. Write an equation for the auto -ionisation of liquid ammonia and use it to calculate a value for the concentration (in mol dm –3) of ammonium cations in liquid ammonia at 33 C. [2] (d) A student plans to prepare an ammonia / ammonium chloride buffer with pH 9. He is given the following solutions.  100 cm3 of an aqueous solution containing 0.28 mol of ammonia  1.00 mol dm–3 dilute hydrochloric acid Determine the volume of hydrochloric acid he could add to the 100 cm3 aqueous ammonia to obtain the required buffer. (Kb of NH3 = 1.8  10–5 mol dm–3) [2]

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