2020 MI Prelim Paper 2

Class Adm No Candidate Name: This question paper consists of 15 printed pages and 1 blank page. 2020 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 14 Sep 2020 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. 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. Question 1 2 3 4 5 Total Marks 11 23 15 15 11 75

2 Answer all questions in the spaces provided. 1 (a) Describe the structure of a 31P atom, in terms of the number and type of subatomic particles. For Examiner’s use ………………………………………………………………………………………………………[1] (b) State and explain how the atomic radius and anionic radius of phosphorus differ. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………[1] (c) (i) Solid PCl5 can be prepared by the chlorination of liquid PCl3. Write an equation, with state symbols, to represent this chlorination. …………………………………………………………………………………………………[1] (ii) By means of an energy cycle and the information given below, calculate the standard enthalpy change of chlorination of PCl5.      23 3P s C g PC2  l l l ΔH = -339 kJ mol-1      252P s 5C g 2PC s  ll ΔH = -926 kJ mol-1 [3]

3 [Turn over (iii) Write an equation, including state symbols to illustrate the reaction of PCl5 with water. Suggest the approximate pH of the solution formed. For Examiner’s use …………………………………………………………………………………………………… …………………………………………………………………………………………………[2] (d) PCl5 reacts with hydrazine, N2H4, to give a molecular compound X with composition by mass shown below and a molecular formula of 303.0. P, 20.5 % ; N, 9.2%; Cl, 70.3% (i) Calculate the empirical formula of X and hence, state its molecular formula. [2] (ii) Given that X contains a N–N single bond, suggest the structure of X. [1] [Total: 11]

4 2 Volcanoes can contribute to air pollution through the release of toxic gases when volcanoes erupt. The most abundant volcanic gas is water vapour. However, significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen halides are also emitted. Table 1: Volcanic gas composition in town A Gas Volume Percentage Water vapour, H2O 87.1 Carbon dioxide, CO2 unknown Sulfur dioxide, SO2 0.5 Hydrogen, H2 0.7 Carbon monoxide, CO 0.01 Hydrogen sulfide, H2S 0.23 Hydrogen halides unknown The composition of the volcanic gases are expressed in terms of volume percentage, wh ich can be calculated using the formula below. volume of gasvolume percentage = total volume × 100% For Examiner’s use (a) When carbon dioxide is emitted from volcanoes, it become s diluted to low concentrations quickly and is not a harmful gas. However, the carbon dioxide gas when cooled, can flow into low-lying areas where it can reach higher concentrations. (i) Suggest why at low temperature s, carbon dioxide would accumulate to high concentrations. Explain your answer. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[2]

5 [Turn over People living in town A had to be evacuated as the levels of carbon dioxide gas was increasing rapidly. Breathing air with more than 3% CO2 can lead to headaches, increased heart rate and difficulty breathing. At about 15%, unconsciousness and death can result. For Examiner’s use (ii) Given that 0.35 g of CO 2 was present in 1 dm3 of gas mixture at r.t.p, determine the volume percentage of carbon dioxide present. [2] (iii) Hence, comment on the possible danger if people remained in the town. …………………………………………………………………………………………………[1] (b) Further analysis of the hydrogen halides composition of the volcanic gases revealed that the hydrogen halides were mostly made up of HCl(g) and HBr(g). (i) Table 2: Bond energies of hydrogen halides Bond Bond energy / kJ mol-1 H–Cl 432 H–Br 366 State the trends of two chemical properties of hydrogen halides that can be explained by the data given in Table 2. …………………………………………………………………………………………………… ……………………………………………………………………………………….……..…[2] (ii) Explain why HBr(g) deviates more from ideality as compared to HCl(g). …………………………………………………………………………………………………… ………………………………………………………………………………….…………..…[1]

6 (iii) Hydrogen halides can be used in the reaction with alkenes to synthesise halogenoalkanes. Draw the displayed formula of both the major and minor products of the reaction of HCl(g) with 2-methylbut-2-ene. Label the major and minor products clearly. For Examiner’s use [2] (c) Another volcanic gas, sulfur dioxide, also has harmful effects on human health. (i) Draw the ‘dot-and-cross’ diagram of one molecule of SO2 and state its bond angle. [2] (ii) SO2 can be converted SO3 in the presence of atmospheric oxygen. By using appropriate values from the data booklet, write an equation for the conversion of SO 2 to SO3 and calculate the enthalpy change of this reaction. [3]

7 [Turn over (iii) SO2 can be used to produce H 2SO4 in a multi -step reaction. In one such reaction, 100 cm3 of H2SO4 was produced. The H2SO4 produced was tested for its concentration where 25.0 cm3 of the resultant solution required 23.50 cm3 of 2.00 mol dm-3 NaOH for complete titration. Calculate the amount of H2SO4 produced in the method above. For Examiner’s use [2] (d) Student A suggested that one possible method to remove harmful SO2 from volcanic gases is to react the gaseous SO2 with magnesium oxide. (i) Explain the Chemistry behind why this method is possible. …………………………………………………………………………………………………[1] Student B suggested that besides magnesium oxide, other metal oxides like sodium oxide and calcium oxide can also be used to remove sulfur dioxide. (ii) The pH of the resulting solutions when one mole of MgO and Na 2O are added to 1 dm3 of water separately are shown in Table 3 below. Table 3: pH of metal oxides Metal Oxide pH of resulting solution MgO 9 Na2O 13 CaO ?

8 Predict and suggest a n explanation for the pH of a solution containing 1 mol dm-3 of CaO. For Examiner’s use …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[3] Student C suggested that seawater can also be used to absorb and remove SO2. The main substance in seawater that is responsible for removing SO2 is the HCO3- ions. When SO2 is absorbed in water, the following two equilibria reactions take place: SO2(g) + H2O(l) + ½O2(g) ⇌ SO42−(aq) + 2H+(aq)  (1) HCO3−(aq) + H+(aq) ⇌ H2O(l) + CO2(g)  (2) (from seawater) (iii) Explain how the use of seawater would allow the removal o