2020 MI Prelim Paper 2 mark scheme

Class Adm No Candidate Name: This question paper consists of 11 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] There are 15 electrons, 15 protons and 16 neutrons. ; (b) State and explain how the atomic radius and anionic radius of phosphorus differ. The number of protons in both the atom and anion are the same, hence nuclear charge remains the same. When it gains electrons the inter-electron repulsion is increased as there are more electrons. Hence the anionic radius is larger than the atomic radius. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………[1] (c) (i) Solid PCl5 can be prepared by the chlorination of liquid PCl3. Write an equation, with state symbols, to represent this chlorination.      3 2 5PC C g PC s  l l l l …………………………………………………………………………………………………[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 2m for diagram 1m for Hess’ law calculations [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 PCl5 (s)+ 4H2O (l)  H3PO4 (aq) + 5HCl (aq) ; pH = 2.0 ; Also accepted: PCl3(s)+ H2O (l)  POCl3 (aq) + HCl (aq) ; …………………………………………………………………………………………………… …………………………………………………………………………………………………[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. P N Cl mass / g 20.5 9.2 70.3 Amount / mol 20.5 / 31 = 0.661 9.2 / 14.0 = 0.657 70.3 / 35.5 = 1.98 simplest mole ratio 1 1 3 Empirical formula: PNCl3 ; Molecular formula: P2N2Cl6 ; [2] (ii) Given that X contains a N–N single bond, suggest the structure of X. or [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] At low temperatures, there is insufficient energy to overcome the strong instantaneous dipole-induced dipole (id-id) interactions between CO2 molecules. ; Hence, there are more CO 2 molecules in a given volume leading to high concentrations. ; 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.

5 [Turn over (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. For Examiner’ s use Amount of CO2 = 0.35 44.0 = 0.00795 𝑚𝑜𝑙 Vol of CO2 = 0.00795 (24) = 190.9 cm3 ; volume percentage of CO2 = (190.9 / 1000) x 100% = 19.1% ; [2] (iii) Hence, comment on the possible danger if people remained in the town. Unconsciousness and death can result. …………………………………………………………………………………………………[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.  Thermal stability decreases from HCl(g) > HBr(g) > HI(g)  Reducing power increases from HCl(g) < HBr(g) < HI(g)  Acid strength increases from HCl(g) < HBr(g) < HI(g) Any two ;; …………………………………………………………………………………………………… ……………………………………………………………………………………….……..…[2] (ii) Explain why HBr(g) deviates more from ideality as compared to HCl(g). …………………………………………………………………………………………………… ………………………………………………………………………………….…………..…[1] The permanent dipole -permanent dipole forces of attraction between HBr molecules are more significant due to the larger electron cloud size in each molecule of HBr. ; (iii) Hydrogen halides can be used in the reaction with alkenes to synthesise halogenoalkanes.

6 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. ; 118ᵒ ; [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. 2SO2 (g) + O2 (g) → 2SO3(g) ; ΔH = [(2x2x500) + 496] – (2x3x500) = - 504 kJ mol-1 ; ; (also accept ½(-504) depending on the equation that students have written) [3] (iii) SO2 can be used to produce H 2SO4 in a multi -step reaction. In one such reaction, 100