AJC H2 CHEM P3

AJC JC2 PRELIM 2014 9647/03/H2 [Turn over  ANDERSON JUNIOR COLLEGE 2014 JC2 PRELIMINARY EXAMINATIONS   CHEMISTRY 9647/03 Higher 2 16 September 2014 Paper 3 Free Response 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Writing paper READ THESE INSTRUCTIONS FIRST Write your name, PDG and register number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Start each question on a fresh sheet of paper. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 12 printed pages.

2 AJC JC2 PRELIM 2014 9647/03/H2 [Turn over  1 (a) An alloy containing only aluminium and magnesium was used in boat–building. A 1.75 g sample of the alloy was dissolved in excess hydrochloric acid. The solution was then made alkaline by the addition of aqueous sodium hydroxide until no further reaction occurred. The resultant mixture was filtered to obtain residue C and filtrate D. Carbon dioxide was passed into D and a white solid E, which contained aluminium, was obtained. Heating E to constant mass gave water vapour and a residue F. F can dissolve in both acid and alkali. (i) Suggest the identity of C, D, E and F. (ii) What type of reaction occurs when E is formed from D? (iii) Residue C obtained above was found to have a mass of 0.18 g. Determine the percentage composition by mass of magnesium in the alloy. [6] (b) Aluminium and beryllium exhibit similar pr operties due to their similarities in electronegativities. Be B A l electronegativity value 1.5 2.0 1.5 (i) Beryllium chloride, BeC l2, has properties similar to those of aluminium chloride, A lCl3. Write a balanced equation to show the reaction of beryllium chloride with limited amount of water. When equimolar amounts of aluminium chloride and ammonia are mixed, compound G is formed. (ii) Explain why these two molecules form compound G. (iii) Draw a dot–and–cross diagram to show the bonding in a molecule of compound G. (iv) Suggest the structural formula of a similar product formed when beryllium chloride reacts with ammonia. [4]

3 AJC JC2 PRELIM 2014 9647/03/H2 [Turn over  (c) Aluminium chloride is used as a catalyst in electrophilic substitution reactions. The chlorination of benzene is represented by the following overall equation. + Cl2 AlCl3 Cl + HCl The reaction occurs in several steps.  The first step is the reaction between C l 2 and AlCl3. Cl2 + AlCl3  Cl+ + AlCl4 –  The benzene ring is then attacked by the C l+ cation in the second step. AlCl3 reacts in a similar way with alkyl chlorides, producing a carbocation that can then attack a benzene ring. (i) Suggest the mechanism for the reaction between benzene and chloromethane, in the presence of AlCl3 catalyst, to form methylbenzene. Compound N can be synthesised in the laboratory from phenol and a dichloro compound H, forming J, C8H9OCl, in the first step. J then undergoes a series of transformations to form N as shown below. OH O O + H J, C8H9OClstep 1 step 2 K step 3 L, C8H8O3M step 4 N r.t. One mole of J reacts with two moles of aqueous bromine. (ii) Suggest reagents and conditions for steps 2 to 4 , and draw the structural formulae of H, J, K, L and M. [10] [Total: 20]

4 AJC JC2 PRELIM 2014 9647/03/H2 [Turn over  2 (a) Almost all of the hydrogen generated today come s from steam reforming of natural gas as shown in reaction (1). CH4(g) + H2O(g) 3H2(g) + CO(g) H1 > 0 –––––– (1) (i) Reaction (1) is reversible. Predict and explain the effect of increasing temperature and pressure on the partial pressure of hydrogen at equilibrium. (ii) Hydrogen behaves most like an ideal gas among the gases present in the equilibrium mixture in (1). State two assumptions of the kinetic theory, and use these to explain why you might expect the behaviour of steam to be less ideal than that of hydrogen under the same conditions of temperature and pressure. Pure hydrogen gas is an important chemical widely used in Polymer Electrolyte Membrane (PEM) fuel cell. However, if the gases produced from reaction (1) are introduced directly into the fuel cell, the carbon monoxide present will ‘poison’ the catalysts used in the fuel cell. A team of research scientists from Singapore developed a method to remove the carbon monoxide as shown in reaction (2). This involves passing the mixture of hydrogen and carbon monoxide from reaction (1), with oxygen gas, over a heterogeneous catalyst made up of gold nanoparticles and –iron(III) oxide at 1400 °C and 101 kPa. 2CO(g) + O 2(g) 2CO2(g) H2 < 0 –––––– (2) When a mixture of 5.00 mol of CO and 2.50 mol of O 2 was allowed to reach equilibrium, it was found that 4.95 mol of CO2 was formed. (iii) Write an expression for the equilibrium constant, Kp, for reaction (2) and calculate its value. (iv) Suggest a method whereby CO 2 could be removed from the product gas stream and explain how this process would help to improve the efficiency of the purification of H2. (v) Explain how the gold nanoparticles can act as a heterogeneous catalyst. [11] (b) In one such PEM fuel cell (PEMFC), hydrogen is oxidised at the anode. The protons produced migrate across the PEM, to the cathode where oxygen is reduced to water. (i) Construct an equation for the reaction that takes place at each electrode. (ii) Write the overall balanced equation for the reaction that occurs in the PEMFC, and by using appropriate values from the Data Booklet, calculate its standard cell potential.

5 AJC JC2 PRELIM 2014 9647/03/H2 [Turn over  One method for the construction of a PEMFC involves electroplating a layer of platinum onto the surface of the PEM. The electrolyte for this process consists of a solution of tetraammineplatinum(II) chloride, Pt(NH 3)4Cl2, and the PEM is the cathode in the electrolytic cell. (iii) State the shape and bond angle of the tetraammineplatinum(II) ion, [Pt(NH3)4]2+. (iv) Suggest the half–equation for the cathode reaction that deposits platinum on the PEM. [5] (c) Ether is a substance that has two organic groups bonded to the same oxygen and the oxygen can either be in an open chain or a ring as shown. ROR '                          O Discovered in 1850, the Williamson ether synthesis is still the best method for preparation of ethers, where metal alkoxides react with alkyl halides via an S N2 mechanism to yield the ethers. The synthesis proceeds through the following steps. NaH I II SROH ether alkyl halide (i) In step I, the alkoxide ion, S, required in the Williamson reaction is normally prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH, via an acid–base reaction. Write a balanced equation for the reaction between ethanol and sodium hydride. (ii) Using 2–chloropropane as the alkyl halide and S from your answer in (c)(i), suggest a mechanism for step II of the Williamson ether synthesis. In your answer show any relevant charges, dipoles or lone pairs of electrons you consider important in this mechanism. (iii) A reaction occurs in competition with the S N2 reaction you have described in (c)(ii) to form a by–product, T