JJC H2 Chemistry Prelim P2 Question

JURONG JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 H2 CHEMISTRY PAPER 2 9746 / 2 Friday 22 nd August 2008 1 hour 30 minutes Name: ______________________ ( No. ________) Class: 07_____ INSTRUCTIONS TO CANDIDATES Do not open this booklet until you are told to do so. Write your name, exam index number and class in the spaces at the top of this page. Write in dark blue or black pen in the spaces provided on the Question Paper. You may use a pencil for any diagrams, graphs, or rough working. Do not use highlighters or correction fluid. Answer all questions. INFORMATION FOR CANDIDATES A Data Booklet is provided. Do not write anything on the Data Booklet. The number of marks is give n in brackets [ ] at the end of each question or part question. You are advised to show all workings in calculations. You may use a calculator. FOR EXAMINER’S USE 1 2 3 4 5 6 TOTAL / 60 This question paper consists of 13 printed pages, including this page. [Turn over

Page 2 of 13 1. Many copper minerals are found in hy drothermal deposits where they were formed by crystallisation from very hot solutions. Deep underground, it is like a giant pressure cooker and water is still in liquid form at high temperature. This means that minerals which are regarded as ‘insoluble’ in the laboratory can dissolve under these cond itions. When the solutions cool, the minerals crystallise out. One such copper mineral is chalcopyrite, CuFeS 2, which is a copper iron sulfide mineral that crystallises out in a tetragonal system. Chalcopyrite is one of the minerals referred to as ‘Fool's Gold’ due to its brassy yellow appearance. (a) Chalcopyrite can be smelted in modern works by heating with air. 3CuFeS2 + 8O2 Æ 3Cu + FeO + Fe2O3 + 6SO2 A rock sample was found to contain 0.5 % by mass of chalcopyrite. (i) Calculate the volume of sulfur dioxide gas released into the atmosphere at r.t.p. if 1 tonne of the rock sample is smelted. (1 tonne = 1000 kg) (ii) Suggest a reason why we should try to limit the emission of sulfur dioxide into the atmosphere. ……………………………………………………………………………… ……………………………………………………………………………… [4]

1. Chalcanthite, known as ‘copper flower’, is a water soluble copper sulfate mineral. When dissolved, aqueous copper(II) sulfate contains [Cu(H2O)6]2+ ions. (b) Water is a simple molecule. The H −O−H bond angle in an isolated water molecule is 104.5 °. The diagram below shows part of the [Cu(H 2O)6]2+ ion and the H −O−H bond angle in the water ligand. H Cu 2+ O H (i) Explain why the H–O–H bond angle in the water ligand is 107 ° while that in an isolated water molecule is 104.5 °. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… (ii) State the hybridisation state of the oxygen atom in the water ligand. ……………………………………………………………………………… [5] (c) Describe and explain the observati ons made when aqueous ammonia is added slowly to a small volume of aqueous copper(II) sulfate solution, until in excess. Identify all of the copper-containing species formed. …………………………………………………………………………………….. …………………………………………………………………………………….. …………………………………………………………………………………….. …………………………………………………………………………………….. …………………………………………………………………………………….. …………………………………………………………………………………….. [3] [Total: 12] 107 ° Page 3 of 13

2. (a) The graph below is an Ellingham diagram, which shows the variation in the standard free energy change of formation, ∆ Gf ,, with temperature, T, for both carbon monoxide and zinc oxide. (i) By considering the relationship between the free energy change, ∆G,, the enthalpy change, ∆H,, and the entropy change ∆S, for a reaction, suggest how you would obtain a value for entropy change from the Ellingham diagram above. ……………………………………………………………………………. ……………………………………………………………………………. (ii) Explain why the line for carbon monoxide in the diagram above has a negative gradient. ……………………………………………………………………………. ……………………………………………………………………………. (iii) Deduce, from the diagram, which oxide, CO or ZnO, is the stronger oxidising agent at 1800 K. ……………………………………………………………………………. [3] (b) Explain why carbon monoxide is toxic, with reference to its interactions with haemoglobin, an important iron −containing complex in red-blood cells that transports oxygen around the body. …………………………………………………………………………………… …………………………………………………………………………………… [1] 0 −200 −400 −600 −800 ∆Gf ,/kJ mol−1 0 500 1000 1500 2000 2500 3000 T / K 2C + O2 Æ 2CO Boiling point of Zn Melting point of Zn 2Zn + O2 Æ 2ZnO Page 4 of 13

2. (c) In heavily polluted air, nitrogen dioxid e is often formed and it appears as a brown haze with a sharp biting odour. Besides causing human respiratory problems and other adverse health effects, nitrogen dioxide also plays a part in destroying the ozone layer through a free radical chain mechanism , similar to that caused by the harmful chlorofluorocarbons (CFCs). Nitrogen dioxide gas is formed by the following reaction between nitrogen oxide and oxygen gas. 2NO(g) + O2(g) 2NO2(g) ΔH < 0 The following mechanism has been proposed for the reaction: Step 1: NO + O 2 NO3 (fast) Step 2: NO 3 + NO 2NO2 (slow) (i) Draw a dot-and-cross diagram to show the arrangement of the valence electrons in NO2. (ii) Based on the proposed kinetics, deduce the rate equation for the reaction. …………………………………………………………………………….. (iii) Sketch an energy profile diagram for the proposed mechanism in the space below. Label your diagram clearly, including the reactants and the products formed. [4] [Total: 8] Energy Reaction Profile Page 5 of 13

Page 6 of 13 3. A cell set-up based on the magnesium-iodi ne system consists of a magnesium electrode dipped in magnesium nitrate electrolyte and a platinum electrode dipped in aqueous iodine-sodium iodide electrolyte. (a) (i) Draw a diagram to illustrate t he set up of the cell, showing the direction of the electron flow in the external circuit. (ii) Write a balanced equation for the cell reaction. (iii) Calculate the cell e.m.f. under standard conditions. (iv) How would you expect the e.m.f. of th is cell to change, if at all, when the concentration of magnesium ions is doubled? Explain your answer. …………………………………………………………………………….. …………………………………………………………………………….. …………………………………………………………………………….. …………………………………………………………………………….. [7]

3. (b) Sodium iodide occurs as colourle ss crystals or as white crystalline powder. The primary use for sodium iodide is in t he treatment of actinobacillosis in cattle, where aqueous s odium iodide solution is injected intravenously into the animals. (i) Draw an energy cycle on the grid below, usi ng the following data, to determine the enthalpy change of solution, ∆Hsoln, for NaI. Enthalpy change of lattice energy of NaI –704 kJ mol −1 Enthalpy change of hydration energy of Na+(g) –390 kJ mol −1 Enthalpy change of hydration energy of I−(g) –247 kJ mol −1 (ii) The enthalpy change of hydration of C l − is − 362 kJ mol −1. Explain why this is more exothermic than that of I−. …………………………………………………………………………….. …………………………………………………………………………….. …………………………………………………………………………….. (iii) Astatine is the element below iodine in Group VII. Identify the halogen-cont aining substances when concentrated sulfuric acid is added to a soli d mixture of sodium chloride and sodium astatide. Write equations for the reactions that occur. [