EJC Prelim P2 Answer

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2018 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 7 – INDEX NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 13 September 2018 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided on the Question paper. 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 1 2 3 4 5 6 Total This document consists of 21 printed pages and 1 blank page.

2 © EJC 9729/02/J2PE/18 For Examiner’s Use Answer all the questions in the spaces provided. 1 The table below shows the melting points of some oxides of Period 3 elements. compound chemical formula melting point / ºC sodium oxide Na 2O 1132 aluminium oxide Al2O3 2980 sulfur trioxide SO 3 16 (a) (i) Briefly relate the melting points of these oxides to their structure and bonding. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ........................................................................................................................... [2] (ii) Describe the reactions, if any, of these oxides with water, stating the approximate pH of any solution formed. Write equations for all the reactions that take place. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ........................................................................................................................... [4] Na 2O is an ionic oxide that reacts with water to form strongly alkaline solutions. Na2O(s) + H2O(l) → 2NaOH(aq) [1m] (pH ≈ 13–14). Al2O3 does not dissolve in water because its lattice energy is highly exothermic [1m]. (pH = 7) SO3 is an acidic oxide that react readily with water to form strongly acidic solutions. SO3(l) + H2O(l) → H2SO4(aq) [1m] (pH ≈ 1–2) [all three pH’s correct – 1m] Both Al2O3 and Na2O are of giant ionic lattice structure. During melting, a lot of energy is required to break strong electrostatic forces of attraction between oppositely charged ions [both, 1m] in these two compounds, resulting in their high melting points. Al2O3 has higher melting point than Na2O as Al2O3 has a larger magnitude of lattice energy (since Al3+ ion has higher charge and is smaller in size compared to Na+ ion). SO3 has covalent simple molecular structure that has weak intermolecular instantaneous dipole-induced dipole (id-id) interactions [both, 1m] between SO3 molecules, requiring little amount of energy to overcome during melting. Thus, SO3 has the lowest melting point.

3 © EJC 9729/02/J2PE/18 [Turn over For Examiner’s Use (b) Suggest a reason for each of the following observations: (i) Effervescence is observed when aqueous sodium carbonate is added to an aqueous solution of aluminium chloride. Write equations for the reactions involved. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ........................................................................................................................... [2] (ii) Silicon tetrachloride, SiC l 4, hydrolyses in water while carbon tetrachloride, CC l4, remains insoluble in water. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ........................................................................................................................... [1] (c) Gaseous N 2O5 dissociates to form NO 2 and O 2 as shown in the following reaction equilibrium: () () ()25 2 22 N Og 4 N Og Og +ˆˆˆ †‡ˆˆ ˆ The value of the equilibrium constant, Kp, is 1.80×104 atm3 at 750 K and 3.23×103 atm3 at 700 K. (i) Write an expression for Kp for the above dissociation. [1] Carbon in CCl4, unlike silicon in SiCl4, does not have low-lying vacant orbitals to accommodate the lone pair from water during hydrolysis. In addition, steric hindrance from the 4 bulky chlorine atoms prevents approach/attack of water nucleophiles at the carbon atom in CCl4. [both, 1m] Al3+