DHS H2 Chem 2013 Prelim P3 QP

This question paper consists of 10 printed pages and 0 blank page. © DHS 2013 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/03 Paper 3 24 September 2013 Paper 2 2 hours Additional Materials: Data Booklet, Graph Paper & Writing Paper IMPORTANT INSTRUCTIONS TO CANDIDATES 1 Answer any four questions. 2 Begin each question on a fresh sheet of paper. 3 At the end of the examination: • Staple or fasten all your work securely together with the Cover Sheet on top. • Hand in the question paper separately. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings and calculations. You are reminded of the need for good English and clear presentation in your answers. 524

2 © DHS 2013 9647/03 [Turn Over Answer any four questions. Begin each question on a fresh sheet of paper. 1 Dinitrogen pentoxide, N2O5, can be produced by the following reaction sequence in the car engine: I N2(g) + O2(g) → 2NO(g) ∆Hθ = +180 kJ mol–1; ∆Sθ = +24.8 J mol–1 K–1 II NO(g) + 1/2O2(g) → NO2(g) ∆Hθ= –57.0 kJ mol–1 III 2NO2(g) + 1/2O2(g) → N2O5(g) ∆Hθ = –110 kJ mol–1 (a) (i) Explain why reaction I does not take place at room temperature but occurs in car engine. (ii) Predict, with reasons, the sign of ∆Sθ in reaction II. (iii) Nitrogen dioxide is a pollutant that is often produced in car engine. Suggest how the pollutant can be removed in the car engine. (iv) By drawing a suitable energy cycle and using the data above, calculate the standard enthalpy change of formation of dinitrogen pentoxide. [8] (b) The rate of reaction for reaction II was investigated and the following kinetics data was obtained. Time/minutes Experiment 1, with [NO] = 0.10 mol dm –3 Experiment 2, with [NO] = 0.05 mol dm–3 [O2] / mol dm–3 [O 2] / mol dm–3 0 0.0050 0.0050 5 0.0031 0.0045 10 0.0019 0.0040 15 0.0011 0.0036 20 0.0007 0.0032 25 0.0005 0.0029 30 0.0004 0.0026 (i) Explain why nitrogen monoxide is used in large excess. (ii) Using the same axes, plot graphs of [O2] against time for the two experiments. (iii) Use your graphs to determine the order of reaction with respect to O 2 and NO, showing your workings clearly. (iv) State the rate equation of reaction II and hence calculate the rate constant, including its units. (v) Explain how the half–life of oxygen will be affected when the concentration of nitrogen monoxide is doubled. [10] (c) The nitrogen dioxide produced in reaction II is able to form dinitrogen tetraoxide as shown below: 2NO2(g) N2O4 (g) brown colourless 525

3 © DHS 2013 9647/03 [Turn Over Explain whether the enthalpy change of dimerisation is endothermic or exothermic. Hence, predict the colour change of the reaction mixture when the temperature is increased. [2] [Total: 20] 2 Transition elements, such as cobalt, copper and chromium, have different properties that can distinguish themselves from the main group element such as magnesium. With their unique properties, transition elements are capable of having variable oxidation states and forming coloured complexes. (a) What do you understand by the term transition element? [1] (b) When air is bubbled through an aqueous solution containing CoC l 2, NH4Cl and NH3, and the resulting solution evaporated, crystals of a salt X can be isolated. X has the following composition by mass: Co, 25.2 %; N, 24.0 %; H, 5.1 %; Cl, 45.7 % On adding an excess of aqueous silver nitrate to an aqueous solution containing 0.01 mol of X, 1.43 g of silver chloride is precipitated. Determine the formula of the octahedral cation in X. Hence, calculate the oxidation number of the cobalt atom in X. [4] (c) Haemocyanin is a copper containing oxygen transport molecule in horseshoe crabs. Both oxygenated and deoxygenated forms of haemocyanin contain copper ions. Haemocyanin is colourless when deoxygenated and blue when it is exposed to oxygen. Suggest the oxidation states of copper in the oxygenated and deoxygenated forms of haemocyanin. Hence explain the difference in colour observed. [3] (d) Reagents containing transition elements are commonly used in organic synthesis. A common reagent used is potassium dichromate(VI). In the organic synthesis below, observations are made when an optically active organic halogen derivative A, C 5H11ClO is reacted with several reagents to yield the final organic product E, C10H16O4. Compound A, C 5H11ClO is an organic halogen derivative that fumes with thionyl chloride in pyridine. When A reacts with aqueous sodium hydroxide, it yields equimolar of B and C with the same molecular formulae, C5H12O2. When B and C are separated, each compound is found to rotate plane–polarised light in opposite directions with equal magnitude. C is then heated under reflux with acidified potassium dichromate( VI) to yield D which produces effervescence with sodium carbonate. When D is further refluxed in the presence of concentrated sulfuric acid, a neutral organic product E, C 10H16O4 is obtained. (i) Suggest, with explanations, the structures for compounds A, B, C, D and E. 526

4 © DHS 2013 9647/03 [Turn Over (ii) Describe the mechanism of the reaction of A with aqueous sodium hydroxide to produce B and C. Indicate clearly in your mechanism how equimolar of B and C is obtained. [12] [Total: 20] 3 (a) A low–cost lithium–copper air fuel cell consisting of a copper electrode immersed in an aqueous alkaline electrolyte and a lithium electrode immersed in an organic electrolyte has recently been developed. Mixing of the two electrolyte solutions is prevented by using a solid electrolyte separator where only lithium ions can pass through the separator. The copper electrode is oxidised by oxygen in the air to copper( I) oxide. During discharge, copper(I) oxide will be reduced to copper solid at the electrode. Similarly, during discharge, lithium will be oxidised to give lithium ions and pass through the separator into the aqueous alkaline electrolyte. (i) State the direction of the flow of electron during discharge in the lithium–copper air fuel cell. (ii) Write ionic half–equations for the reaction that occur at each electrode during discharging. (iii) It is discovered that the voltage of this fuel cell is +2.30 V. Using relevant data from the Data Booklet , predict the electrode potential for the cathode reaction. State an assumption that you have made. (iv) Explain why lithium ions are allowed to pass through the separator. (v) Two different electrolytes are used in this lithium–copper air fuel cell. Suggest a reason why an aqueous alkaline electrolyte cannot be used solely in this fuel cell. [8] (b) The copper(I) oxide is reddish–brown in colour. When concentrated hydrochloric acid is added to it and the mixture warmed, it is observed that the precipitate dissolved to give a colourless solution. The copper complex ion formed is linear in shape. Suggest an identity for the copper complex ion and an equation for the reaction. [2] Solid electrolyte separator Aqueous alkaline electrolyte Organic electrolyte Li metal electrode Cu metal electrode air 527

5 © DHS 2013 9647/03 [Turn Over (c) An experiment is conducted to determine the formula of complex ion formed between copper(II) and ammonia. 100 cm3 of 0.100 mol dm –3 of copper( II) sulfate is mixed with 100 cm 3 of aqueous ammonia. The resulting solution, which contains an excess of ammonia, is then shaken with trichloromethane and allowed to stand for equilibrium to be established. 25.0 cm 3 of trichloromethane layer is fo