AJC Prelims H2 Chem Paper 2 Question paper

NAME:______________________________ PDG:__________ Register No: _____ ANDERSON JUNIOR COLLEGE Preliminary Examination 2008 CHEMISTRY 9746/02 Higher 2 15 September 2008 Paper 2 Structured 1 hour 30 minutes Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, PDG and register number on all the work you hand in. Write in dark blue or black pen. 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 ALL the questions. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Q1 Q2 Q3 Q4 Q5 Total This document consists of 12 printed pages. AJC/H2 Chemistry/Prelims2008 [Turn over

2 AJC/H2 Chemistry/Prelims2008 [Turn over 1 (a) (i) Define, by means of an equation, what is meant by the first ionisation energy of an element, M. (ii) Phosphorus and iodine are two elements found in Period 3 and Period 5 of the Periodic Table respectively. Suggest a reason why they have similar first ionisation energies. [3] (b) When iodine is added to a solution of white phosphorus, P 4, in carbon disulphide at room temperature, phosphorus triiodide is formed. (i) Write an equation for the formation of phosphorus triiodide from white phosphorus. (ii) Draw a ‘dot-and-cross’ diagram of the phosphorus triiodide molecule and name its shape. Shape of molecule (iii) Unlike phosphorus triiodide, phosphorus pentaiodide, PI5, is highly unstable and cannot be prepared under room temperature. Suggest a reason for this. [4]

3 (c) Phosphorus triiodide is widely used in organic chemistry for converting alcohols to alkyl iodides. Phosphorus acid, H3PO3, is formed as a by-product. PI3 + 3ROH → 3RI + H3PO3 Phosphorus acid has the following structure. P O H O O H H It is a dibasic acid and the two corresponding acid dissociation values are: Ka (1) = 1.0 × 10−2 mol dm−3 Ka (2) = 2.5 × 10−7 mol dm−3 (i) Circle the acidic proton(s) present in the structure of phosphorus acid shown above. (ii) Write equations to illustrate the dissociation corresponding to each of the Ka values. Ka (1) Ka (2) (iii) Why is the second acid dissociation value smaller than the first? (iv) Briefly explain why H3PO3 is not tribasic. [4] [Total: 11] AJC/H2 Chemistry/Prelims2008 [Turn over

4 2 (a) (i) Hydrogen chloride can be prepared by heating sodium chloride with concentrated sulphuric acid. Write an equation for the formation of hydrogen chloride in this reaction. (ii) Hydrogen iodide, however, cannot be prepared in a similar manner described in (a)(i). Explain why this is so. [2] (b) Industrially, 1,2-dichloroethane is manufactured through the oxychlorination of ethene. A mixture of dry HC l and pure oxygen react with ethene in a reaction chamber maintained at 350 °C to form 1,2-dichloroethane and water. C2H4 + 2HCl + O2 C2H4Cl2 + H2Ocatalyst1 2 To measure the percentage conversion of ethene to 1,2-dichloroethane, the amount of unreacted HCl that emerges from the chamber is bubbled through 1 dm 3 of 0.100 mol dm −3 NaOH for one minute. The resultant mixture is then analysed by a titrimetric method. In one investigation, the feed gas entering the reaction chamber contains C 2H4(g), HCl (g) and O 2(g) in molar ratio 1 : 2 : 2 1 . The flow rate of HC l into the reaction chamber is 3.65 g min−1. (i) Calculate the flow rate of ethene, in mol min−1, into the reaction chamber? (ii) In the titrimetric analysis, a 10 cm 3 sample of the resultant mixture requires exactly 2.50 cm3 of a 0.100 mol dm−3 HNO3 solution for complete neutralization. Calculate the number of moles of unreacted NaOH present in the 1 dm 3 solution. AJC/H2 Chemistry/Prelims2008 [Turn over

5 AJC/H2 Chemistry/Prelims2008 [Turn over (iii) Calculate the number of moles of unreacted HCl that emerged from the reaction chamber in one minute. (iv) Hence, calculate the molar percentage conversion per minute of ethene to 1,2-dichloroethane in the oxychlorination process. (v) State one assumption made in your calculations. [6] (c) 1,2-dichloroethane can be prepared from ethene in the laboratory via a different reaction. (i) State the reagent(s) and condition(s) for the laboratory preparation. reagent(s) condition(s) (ii) Name and outline the reaction mechanism for the laboratory preparation of 1,2-dichloroethane from ethene. Type of reaction mechanism

6 AJC/H2 Chemistry/Prelims2008 [Turn over (iii) Sketch and label a reaction pathway diagram for the proposed mechanism in (c)(ii), assuming that the reaction is exothermic. (iv) Both the industrial and laboratory preparations of 1,2-dichloroethane are carried out in an anhydrous environment to reduce the formation of by-products. Suggest a possible by-product that can be produced in a non-anhydrous environment and explain why it is formed. [8] [Total: 16]

7 AJC/H2 Chemistry/Prelims2008 [Turn over 3 (a) Magnesium and barium are two alkaline earth metals which react differently with water. However, their nitrates decompose similarly on heating. (i) How and under what conditions do magnesium and barium react with water? State what you would observe in each reaction. (ii) Write an equation for the decomposition of either of the metal nitrate. (iii) There is a noticeable difference in the decomposition temperatures of these two nitrates. State and explain which of these nitrates decomposes at a lower temperature? [6] (b) The numerical value for Ksp of magnesium fluoride is 3.70 × 10–8. (i) Write an expression for the solubility product of magnesium fluoride. (ii) Calculate the solubility, in mol dm−3, of magnesium fluoride.

8 AJC/H2 Chemistry/Prelims2008 [Turn over (iii) In the treatment of drinking water, fluoride ions are added to natural water as a means to fight tooth decay. Fluorinated drinking water contains about 1 ppm fluoride ions. With reference to your answers in (b)(ii), deduce whether magnesium fluoride is a good source of fluoride ions for water treatment. (Given: Density of solution = 1 g cm −3, 1 ppm fluoride ions = 1 g of fluoride per 106 g of solution) (iv) Equal volumes of 0.02 mol dm -3 aqueous sodium fluoride and an unknown concentration of aqueous magnesium nitrate were mixed in a beaker. Calculate the maximum concentration of magnesium nitrate such that no precipitate of magnesium fluoride is formed upon mixing the two solutions. [6] [Total: 12]

9 4 Photochemical smog consists of a mixture of pollutants which includes oxides of nitrogen, namely nitric oxide (NO) and nitrogen dioxide (NO 2). The smog often has a brown haze due to the presence of NO2. In most major cities, such as Mexico and Los Angeles, the exhaust fumes from cars are the main sources of these oxides of nitrogen. In car engines, nitric oxide is formed when atmospheric nitrogen reacts with oxygen: N2(g) + O2(g) 2NO(g) -------- (I) When oxygen is present in excess, nitric oxide will be further converted to nitrogen dioxide: 2NO(g) +