TJC Prelim P3

TEMASEK JUNIOR COLLEGE CHEMISTRY 9746/03 Higher 2 Paper 3 Free Response Thursday 24th SEPTEMBER 2009 2 hours Candidates answer on separate paper. Additional materials: Answer paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, centre/i ndex number & CG in the spaces provided on the cover page provided and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. 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. You may use a calculator. A Data Booklet is provided. At the end of the examination, fasten all your work securely together 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. This question paper consists of 8 printed pages.

2 Answer any four questions 1 (a) The melting points of two halides are given below. Compound Formula Melting Point/ oC Aluminium fluoride AlF3 1291 Aluminium chloride AlCl3 178 Briefly relate these melting points to the structure of, and bonding in, each of these halides. [2] (b) Compound P, C 6H6O, was first used as a surgical antiseptic till it was later found to be carcinogenic. In the presence of anhydrous aluminium chloride, P can react with 3-chloropropene to yield Q, C9H10O. Both P and Q are found to decolourise aqueous bromine to form white precipitates R and S, respectively. Both R and S contain the same number of bromine atoms per molecule and S is found to be optically active. (i) Identify and draw the structural formulae of compounds P, Q, R and S explaining the reactions described. (ii) Write equations to illustrate the mechanism for the reaction of P with 3-chloropropene and anhydrous aluminium chloride. [9] (c) The trichlorides of nitrogen and phosphorus each react with water to give two products, but they react in different ways. Phosphorus trichloride, PC l 3, reacts to give HC l as the only chlorine- containing product. Nitrogen trichloride, NC l3, produces HOC l as the only chlorine-containing product. In each case, predict the other product and write a balanced equation for its production. Suggest an explanation for this difference in behaviour. [ 4 ] (d) Chrome Yellow is a natural yellow pigment made of lead(II) chromate. It is practically insoluble in water and is used in paints. The solubility product of PbCrO 4 at 15 0C is 1.69  10-14 mol2 dm-6. (i) Write an expression for the solubility product, K sp of PbCrO 4 and calculate the solubility of PbCrO 4, in mol -3 dm . (ii) Concentrated aqueous lead(II) nitrate is added dropwise to 0.010 mol dm -3 potassium chromate (VI). What is the concentration, in mol dm -3 of lead (II) ions when the first trace of precipitate appears? [ 5 ] [Total: 20] [Turn over

3 2 Compounds E and F are structural isomers with a molecular formula of C 4H7OCl. Only isomer F is optically active. Neither E nor F reacts with sodium metal. When heated with silver nitrate in excess ammonia, both E and F produced a silvery deposit. On heating with dilute sodium hydroxide, E and F produced G and H respectively both of which has a molecular formula of C4H8O2. A number of experiments at a constant pressure of 101 kPa and temperature of 90 oC were performed in which the concentrations of E, F and sodium hydroxide were varied. The results are shown below: Experiment Number Initial Concentration of E / mol dm-3 Initial Concentration of NaOH / mol dm-3 Initial rate of formation of G / mol dm-3 s-1 1 0.20 0.30 0.0150 2 0.30 0.30 0.0225 3 0.20 0.20 0.0150 Experiment Number Initial Concentration of F / mol dm -3 Initial Concentration of NaOH / mol dm-3 Initial rate of formation of H / mol dm-3 s-1 1 0.30 0.40 0.0240 2 0.10 0.40 0.0080 3 0.20 0.60 0.0240 (a) The rate equations when E and F react with NaOH are as shown below R a t e = k 1 [E][OH-]x R a t e = k 2 [F][OH-]y Deduce the values of x and y [2] (b) State and explain how the rate of reaction would change (if at all) when the experiment was conducted at (i) a pressure of 101 kPa and a temperature of 120 oC (ii) a pressure of 121 kPa and a temperature of 90 oC [4] (c) (i) Identify and draw the structural formulae of compounds E, F, G and H, explaining the reactions described. (ii) Write the balanced equation when E reacts with silver nitrate in excess ammonia. [11] (d) State and explain clearly the mechanism when F reacts with NaOH. [3] [Total: 20] [Turn over

4 3 (a) It has been suggested that the amount of carbon dioxide in the atmosphere could be reduced by injecting the gas into the sea at a depth of 300 m. (i) The maximum solubility of carbon dioxide in sea water at normal atmospheric pressure, is 3.29 x 10-2 mol dm-3. The pressure increases by 1.0 atm every 10 m below sea level. By considering the equilibrium, C O 2(g) + H2O(l) CO 2(aq) estimate the maximum solubility of carbon dioxide 300 m under the surface of the sea. Assume that the solubility of CO 2 is directly proportional to the p r e s s u r e . [2] (ii) The acid dissociation constant, K a1, for the reaction C O 2(aq) + H2O(l) HCO 3 -(aq) + H+(aq) is 4.5 x 10 -7 mol dm-3. (I) Calculate the pH of sea water at a depth of 300 m, saturated with carbon dioxide, assuming that the acidity is solely due to the carbon dioxide present. (II) Draw the dot-and-cross diagram of the hydrogen carbonate ion. (III) State the shape of the hydrogen carbonate ion with respect to the carbon atom [4] (iii) An equilibrium is established between hydrated carbon dioxide and carbonic acid: C O 2(aq) + H2O(l) H 2CO3(aq) The measured ratio of free aqueous carbon dioxide to carbonic acid is 400. (I) Carbonic acid can act as a weak acid as shown in the equation below: H 2CO3(aq) H +(aq) + HCO3 -(aq) Give the expression for the equilibrium constant of carbonic acid Ka2. (II) Calculate the value of Ka2 for carbonic acid, using the value of Ka1 for aqueous carbon dioxide given in (a)(ii). (III) State and explain why carbonic acid is a stronger acid compared to propanoic acid. [5] [Turn over

5 (b) The nerve gas sarin has a similar structure to an ester as shown below: CH H3C H3C OP F O CH3 It is an extremely dangerous substance as it interferes with the action of acetylcholine as a neurotransmitter. Many tons of the substance have been made for use in chemical warfare and the complete destruction of these stockpiles is of international importance. In order to destroy stockpiles of sarin, it is necessary to break the substance down by hydrolysis. It r eacts in a similar way to esters and acyl halides. (i) What are the most effective conditions to breakdown an ester by hydrolysis? (ii) Using one of the conditions specified in (b)(i), suggest the likely breakdown products when sarin is hydrolysed. (iii) Suggest a reason, whether you think sarin is more or less resistant to hydrolysis than methylethyl ethanoate. CH H3C H3C OC O CH3 methylethyl ethanoate (iv) Suggest a reason why methylethyl ethanoate is unable to form the following compound below. CH H3C H3C OC F O CH3 (v) Sarin is made by a series of chemical reactions. The first stage is to treat methanol with phosphorus trichl