SAJC 2008 H2 Prelim Paper3

ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations CHEMISTRY Higher 2 9746/03 Paper 3 Free Response 9 September 2008 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet, Graph Paper, Answer Paper READ THESE INSTRUCTIONS FIRST Write your name and civics group 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. 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 examination, fasten all your work securely together. This document consists of 10 printed pages including this page.

Answer any four questions. 1(a) The compound K 3CrO4 is a green solid. When mixed with dilute H 2SO4, it disproportionates to yield Cr 3+(aq) and Cr 2O7 2-(aq) ions. The mixture produced was mixed with excess K I and the iodine liberated required 32.00 cm 3 of 0.30 mol dm -3 Na2S2O3 for reaction. Write equations for the following changes: (i) CrO 4 3- to Cr3+ (ii) CrO 4 3- to Cr2O7 2- (iii) the overall equation for the disproportionation of CrO 4 3- in acid. (b) 1 mole of Cr 2O7 2- reacts with excess K I to liberate 3 moles of iodine. Hence, by using the equation in (a)(iii), calculate the mass of K 3CrO4 used in the disproportionation reaction. [6] (c) Chromium plating can be carried out on an object in an acidified solution of 2.0 mol dm -3 sodium dichromate (V I). The process is carried out on an object of surface area of 0.5 m2 and the current used is 40 A. Calculate the time needed to plate the object to a thickness of 1.0 x 10-5 m. [Density of chromium is 7.3 g cm -3.] [4] (d) Explain briefly why it would not be possible to electroplate the object with aluminium from an aqueous solution of aluminium nitrate. [1] (e) Explain why Cr 3+(aq) ion is coloured. [3] (f) (i) A cyclic di-ester can be synthesized from reacting a dicarboxylic acid with a diol. Hence, describe how 2, 3-dichlorobutane can be converted to compound A in 3 steps. Include in your answer, the reagents and conditions for each step, including the structures of the intermediate compounds formed. C C C C O O O O CH2 H H H3C H3C Compound A (ii) Name the isomerism exhibited by but-2-ene and draw the isomers. (iii) A by-product in the reaction between ethylamine, CH 3CH2NH2 and chloromethane, CH 3Cl, has the formula C 5H14NCl and it gives an immediate precipitate with aqueous silver nitrate. Suggest a structure for this compound and write an equation for its formation. [6] [Total : 20] 2

2(a) Carbon dioxide is often used as a coolant gas in nuclear reactor. It is observed that carbon dioxide shows significant deviations fr om the ideal behaviour that is predicted by the kinetic theory of gases. (i) State two assumptions of the kinetic theory of gases. [2] (ii) From the following sets of conditions, which set of conditions would you expect carbon dioxide to deviate most from ideal gas behaviour? Explain your answer. [ 3 ] Set I 100K and 1 atm Set II 100K and 5 atm Set III 300K and 2 atm (iii) Carbon dioxide can be liquefied at room te mperature just by pressurising them. Why does the application of pressure cause the gas to liquefy? [1] (b) The solubility product of calcium dihydrogen phosphate, Ca(H 2PO4)2, at 250C is 1.0 x 10-5 mol3 dm-9. (i) Calculate the number of moles of Ca(H 2PO4)2 that can be dissolved in 100 cm 3 of water. [2] (ii) Will precipitation occur when 30 cm 3 of 3.0 x 10 -3 mol dm-3 Ca(NO3)2 is added to 60 cm3 of 3.0 x 10-2 mol dm-3 NaH2PO4? [2] (c) (i) Beryllium resembles aluminium in its chemical properties. Both beryllium oxide and aluminium oxide are amphoteric. With reference to either beryllium oxide or aluminium oxide, explain the term amphoteric with appropriate equations. [2] (ii) Write a balanced equation to show the action of heat on beryllium nitrate. [1] (iii) Describe and explain how the thermal stability of beryllium nitrate compares with that of strontium nitrate. [3] (d) State what you would observe in each of the following experiments. Explain the observations by using relevant redox potentials in the Data Booklet. (i) Solid manganese( IV) oxide is added to aqueous iron(II) sulphate solution; (ii) Chlorine gas is passed through aqueous potassium bromide solution. [ 4 ] [ T o t a l : 2 0 ] [Turn over 3

3(a) Dinitrogen pentoxide is a colourless gas which decomposes on heating according to the equation: N2O5(g) Î 2NO2(g) + ½ O2(g) ΔH > 0 The data shown in the table below were obtained in an investigation of the rate of decomposition of N2O5 at 373K. Time/ min [N2O5] / mol dm-3 0 2.00 20 1.06 40 0.56 60 0.32 80 0.18 100 0.10 120 0.06 (i) State another method by which the rate of decomposition could be followed. [ 1 ] (ii) Use a graphical method to find the order of reaction with respect to N 2O5 and construct the rate equation for this reaction. [3] (iii) Determine the rate constant of the reaction, stating the units. [1] (iv) The following reaction mechanism has been proposed: Step 1: N2O5 (g) Î 2NO (g) + 23 O2 (g) slow Step 2: 2NO (g) + O 2 (g) Î 2NO2 (g) fast Sketch a labelled energy profile diagram for the proposed mechanism represented by Step 1 and Step 2. [2] (b) The N 2O5 molecule is present in the vapour state, but the structure is ionic in the solid state, existing as NO2 +.NO3 -. (i) Draw the dot-and-cross diagrams of NO 2 + and NO3 -. [2] (ii) Draw the displayed formula for the N 2O5 molecule and suggest the values for the bond angles in this molecule. [2] 4

3(c) D , C 12H16O3N2, is a neutral compound. Upon reaction with hot aqueous sodium hydroxide, E and F are formed. On acidification, E forms C 2H4O2, which gives effervescence with solid sodium carbonate. 1.52 g of F, C8H12N2O, required 25.0 cm3 of 0.40 mol dm-3 of sulphuric acid for complete reaction at room temperature. 1 mole of F is able to decolourise 3 moles of bromine water and give a white suspension G. When acidified orange potassium dichromate ( VI) is added to F, compound H is formed and the solution turned green. A yellow precipitate is also formed when F and H is reacted with alkaline aqueous iodine respectively. Suggest the identities of D, E, F, G and H, explaining your reasoning. [9] [ T o t a l : 2 0 ] [Turn over 5

4(a) Nitroglycerin, C 3H5(NO3)3, is a flammable oil commonly used to manufacture dynamite. The atomisation of nitroglycerin is represented by the equation: C3H5(NO3)3(l) Î 3C(g) + 5H(g) + 3N(g) + 9O(g) Given: Standard enthalpy change of formation of nitroglycerin(l) / kJ mol-1 -364 Standard enthalpy change of atomisation of carbon (graphite) / kJ mol-1 +715 Standard enthalpy change of formation of H2O(g) / kJ mol-1 -242 Standard enthalpy change of formation of CO2(g) / kJ mol-1 -394 (i) Using the data above and relevant values from the Data Booklet, draw a labelled energy cycle and use it to calculate the standard enthalpy change of atomisation of nitroglycerin.