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]