2021 EJC Prelims Paper 3 (Solutions with comments)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2021 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 0 – INDEX NUMBER CHEMISTRY Paper 3 Free Response 9729/03 22 September 2021 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group, index 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 in the spaces provided on the Question Paper. If addition space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all the questions Section B Answer one question. 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. For Examiner’s Use Paper 3 Section A 1 / 20 2 / 20 3 / 20 Section B 4 / 20 5 / 20 Total / 80 This document consists of 32 printed pages.

2 © EJC 9729/03/J2PE/21 Section A Answer all the questions in this section. 1 Halogens are powerful oxidising agents, and the oxidising power of halogens may be understood via the energy cycle in Fig. 1.1. Fig. 1.1 The corresponding values of some of the energy terms are given in Table 1.1. Table 1.1 energy term I (kJ mol–1) energy term II (kJ mol–1) energy term III (kJ mol–1) F +79 –328 –506 Cl +121 –349 –364 Br +112 –324 –335 I +107 –295 –293 (a) (i) Name the three energy terms, I, II and III. [2] (ii) Explain why energy term I becomes less endothermic moving down the group from chlorine to iodine. [1] (iii) Explain the exceptionally low value for energy term I of fluorine, in contrast to that of the remainder of the halogens. [1] (iv) Explain why energy term II becomes less exothermic moving down the group from chlorine to iodine. [1] (v) Using values from the Data Booklet, account for the difference in the value of energy term III. [2] ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... I II III Hr (i) I: enthalpy change of atomisation II: first electron affinity III: enthalpy change of hydration

3 © EJC 9729/03/J2PE/2021 [Turn Over ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... (ii) As the valence orbital used in bonding is more diffused moving down the group from chlorine to iodine, the overlap of orbitals is less effective (moving down the group), leading to less energy being required to break the bond in the diatomic molecule. (iii) Due to the small atomic size of fluorine (as compared to other halogens), there is high inter -electron repulsion between the non -bonding electrons of the 2p -orbitals in the fluorine molecule. This leads to significantly lower bond dissociation energy. (iv) Since the electron to be gained by the atom is increasingly further away from the nucleus and experiences a decrease in electrostatic forces of attraction between the nucleus and incoming electron , the first electron affinity becomes less exothermic. (v) The ionic radii of F – is significantly smaller at 0.136 nm, while the ionic radii of the other halogens are more comparable, with Cl– being 0.181 nm, Br– being 0.195 nm and I– being 0.216 nm. As the halide ions have the same charge, the F – ion has a significantly higher charge density due to its significantly smaller ionic radius, and hence has stronger ion -dipole interaction with the surrounding water molecules. Therefore, its hydration energy is significantly more exothermic.

4 © EJC 9729/03/J2PE/21 Marker’s Comments: (a)(i) • The naming of the three energy terms was reasonably well-attempted. • Common errors include: – For energy term I: Confusion between enthalpy change of atomisation and that of bond dissociation energy (the value of the two terms are not equivalent, and in fact 1(Bond Energy of X-X)2 atomH= . – For energy term II: Not being specific to the “electron affinity”. The term “first ” was important in mentioning the electron affinity, as the sign and value of different electron affinity can be different. In the case of halogens, first electron affinity was exothermic,