DHS 2008 Prelim - H2 Chemistry Paper 3

This question paper consists of 8 printed pages and 0 blank page. [Turn over CANDIDATE NAME ____________________________ CLASS________ DUNMAN HIGH SCHOOL (SENIOR HIGH) YEAR 6 PRELIMINARY EXAMINATION 2008 H2 CHEMISTRY Paper 3 Free–Response 9746/03 15 September 2008 2 hours Additional materials: Data Booklet Cover Sheet Answer Paper INSTRUCTIONS TO CANDIDATES 1 Write your name and class on this question paper, the Cover Sheet and on all the work you hand in. 2 Answer ANY FOUR questions. 3 Write your answers on the separate answer paper provided. 4 Start each question on a fresh sheet of paper. 5 A Data Booklet is provided. 6 You are reminded of the need for good English and clear presentation in your answers. 7 The number of marks is given in brackets [ ] at the end of each question or part question. 8 You are advised to show all workings in calculations. 9 You may use a calculator. 10 At the end of the examination, fasten all your work securely together with the Cover Sheet on top.

2 © DHS 2008 9746/03 [Turn over Answer any four questions. 1(a) When under attack, the bombardier beetle uses ‘chemical warfare’. It defends itself from attack by mixing together solutions of hydrogen peroxide and hydroquinone (C 6H4(OH)2) in the presence of enzymes. This reaction releases free oxygen that produce enough energy to bring the mixture to boiling point. An audible ex plosion occurs as the beetle fires a hot spray at its attacker from its abdomen. Quinone, C 6H4O2 is one the products formed. + H2O2 (aq) hydroquinone OHHO OO + 2H2O (l) quinone (aq) (aq) (i) State Hess’ Law. (ii) Construct an energy cycle and use the following data to calculate the enthalpy change of reaction inside the beetle. C6H4(OH)2 (aq) Æ C6H4O2 (aq) + H2 (g) ΔHθ= +177 kJ mol–1 H2O2 (aq) Æ H2O (l) + ½O2 (g) ΔHθ= –95 kJ mol–1 H2 (g) + ½O2 (g) Æ H2O (l) ΔHθ= –286 kJ mol–1 (iii) Draw a labelled energy profile for the reaction above and indicate clearly the activation energy and enthalpy change of reaction. [7] (b) When developing exposed film from a came ra, aqueous alkaline hydroquinone reacts with silver ions in a light–activated reaction in I. + 2Ag+ (aq) hydroquinone OHHO OO + X quinone (aq) (aq) OH- II Y NaOH (aq), followed by CH3I I (i) Identify X and state the role of hydroquinone in reaction I. (ii) Draw the structural formula of Y and name the type(s) of reaction involved in II. (iii) Write a balanced equation for the reaction between quinone and each of the following reagents: (1) LiAlH4 in dry ether (2) HBr (iv) State what would be observed when 2,4–dinitrophenylhydrazine is added a solution of quinone. Draw the structural formula of the organic product formed. (v) One student attempts to synthesise hydroquinone from 1,4–dichlorobenzene using hot aqueous sodium hydroxide. State with reasons whether the student would be successful in his attempt. [9]

3 © DHS 2008 9746/03 [Turn over (c) Concentrated hydrogen peroxide is a dangerously reactive substance because the decomposition is very exothermic. H 2O2 (l) Æ H2O (l) + ½O2 (g) ΔHθ= – 98.1 kJ mol–1 (i) Use ΔHθ and the following data to calculate ΔGθ of the reaction. H 2O2 (l) H 2O (l) O 2 (g) Sf θ /J mol–1 K–1 109.6 69.9 205.0 (ii) Based on your answer in part (i), or otherwise, suggest two possible precautions to take for storing concentrated hydrogen peroxide. [4] [Total: 20]

4 © DHS 2008 9746/03 [Turn over 2(a) Ammonium cyanate, NH 4NCO undergoes thermal decomposition to form a covalent compound urea, (NH2)2C=O, which is a common fertilizer. Describe the type of bonding found in ammonium cyanate and hence suggest a dot–and–cross diagram of cyanate ion, NCO–, stating its shape. [3] (b) On heating urea above its melting point produces an organic cyclic compound N, and ammonia. N has the composition by mass of C, 28%; N, 33%; H, 2.3%; O, 37% and a relative molecular mass of 129. (i) Determine the molecular formula of N. (ii) Suggest a possible structure of N. [2] (c) Cyanic acid (NCOH), with an acid dissociation constant of 3.5 x 10 –4 mol dm –3, is the simplest chemical compound that contains carbon, hydrogen, nitrogen, and oxygen. A buffer solution can be prepared by mixing 500 cm 3 of 0.200 mol dm –3 of ammonium cyanate and 500 cm3 of 0.100 mol dm–3 of cyanic acid solution. (i) With the help of an equation, explain how the mixture can act as a buffer on addition of H+. (ii) Calculate the pH of the prepared buffer solution. (iii) A 20.0 cm 3 sample of 0.100 mol dm –3 of cyanic acid is titrated with 0.100 mol dm –3 aqueous barium hydroxide, Ba(OH) 2. Use the data to sketch the pH changes that occur during the titration when a total of 30 cm 3 of Ba(OH) 2 is added. Label the following points on your sketch: (1) Initial pH (2) End–point (3) Point of maximum buffer capacity (iv) At certain temperature, cyanic acid is able to form an isomer which is also an acid, via the migration of a proton. Suggest the structural formula of this isomer. [8] (d) Another ammonium salt, ammonium nitrate, sublimes reversibly with heat to produce ammonia and nitric acid until dynamic equilibrium is reached: NH4NO3 (s) Ý NH3 (g) + HNO3 (g) (i) Explain what is meant by the term dynamic equilibrium. (ii) Write an expression for the equilibrium constant, Kp for the above reaction. (iii) 9.00 g of ammonium nitrate is heated in a sealed container and the equilibrium constant is found to be 15.7 atm 2. Calculate the partial pressure of ammonia present at equilibrium. (iv) Hence, determine the percentage of ammonium nitrate which has dissociated, given that one mole of gas under these conditions exerts a pressure of 50 atm.

5 © DHS 2008 9746/03 [Turn over (v) At a higher temperature, the equilibrium constant is 16.8 atm 2. Compare this with the value given in part (iii). Deduce with reasons, whether the sublimation reaction is endothermic or exothermic. [7] [Total: 20]

6 © DHS 2008 9746/03 [Turn over 3(a) The electrolysis of aqueous potassium chlori de containing methyl orange indicator is carried out in the following apparatus using platinum electrodes. The volume of gases liberated at the electrodes is shown below: (i) It is found that 12 cm 3 of gas is collected at the cathode after passing the current for two minutes under room conditions. Calculate the size of current used. (ii) Determine the volume ratio of gases collected at the anode and cathode. (iii) State and explain the colour of the methyl orange indicator around the electrodes. (iv) The electrolysis is continued over a long time and a greenish–yellow gas is liberated at the anode. Explain this observation. [7] (b) (i) When potassium chloride and potassium bromide are separately added to concentrated sulfuric acid, different observations are noted. Explain the difference in behaviour of both salts towards concentrated sulfuric acid using relevant data from the Data Booklet. Write balanced equations for the reactions that occur. (ii) Potassium bromide in concentrated sulfuric acid is used as a reagent in the following reaction scheme. It is found that organic compound Q produces white fumes when subjected to PC l5, and has no reaction with alkaline aqueous iodine. Deduce with reasons, the structures for compounds P – T. [10] (c) Give a reagent, other than concentrated sulfuric acid, that can be used to distinguish between separate solutions of potassium chlori de and potassium bromide. State what would be observed and write an equation for one of the reactions. Identify another reagent that could be added to the mixtures from the first test to confirm the identity of the halide ions. State what would be observed i