2020 EJC JC2 Prelim H2 Chemistry Paper 3 Suggested Solutions

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2020 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 9 – INDEX NUMBER CHEMISTRY Paper 3 Free Response 9729/03 17 September 2020 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 1 / 22 2 / 19 3 / 19 4 / 20 5 / 20 Total / 80 This document consists of 32 printed pages.

2 © EJC 9729/03/J2PE/20 Section A Answer all the questions in this section. 1 (a) Describe and explain the trend observed in the thermal stabilities of the carbonates of Group 2 elements. [3] (b) (i) Elements in period 3 can react with chlorine gas to form different chlorides. State the types of bonding in the chlorides of sodium, aluminium and silicon. [1] (ii) Write appropriate equations to illustrate the reaction s, if any, when separate samples of sodium chloride, aluminium chloride and silicon chloride were added to excess water. [3] ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. (i) Size of cations increases down the group, leading to a decrease in charge density and polarising power of the cations . The ability of cations to distort the electron cloud of 2 3CO  decreases, weakening the C–O bond to a lesser extent. Hence, thermal stability of Group 2 carbonates increases down the group. (ii) Sodium chloride: ionic bonding; Aluminium chloride and silicon chloride: covalent bonding (accept id-id) (iii) Sodium chloride does not react with water but merely dissolves in it. NaCl (s) + aq  Na+ (aq) + Cl– (aq) Aluminium chloride dissolves in water to give cations of high charge densities. AlCl3 (s) + 6H2O  [Al(H2O)6]3+ (aq) + 3Cl– (aq) The aqua complexes of these ions hydrolyse in water to give an acidic solution. [Al(H2O)6]3+ (aq) + H2O (l) ⇌ [Al(H2O)5(OH)]2+ (aq) + H3O+ (aq) Silicon tetrachloride hydrolyses in water to give a strong acid: SiCl4 (aq) + 2H2O (l) → SiO2 (s) + 4HCl (aq)

3 © EJC 9729/03/J2PE/20 [Turn Over (c) Acids, bases and buffers play important roles in balancing biological systems. (i) Explain what is meant by the term buffer solution. Write equations to explain how an aqueous solution of ammonia and an ammonium salt maintains the pH when a little acid or base is added into it. [3] (ii) Calculate the pH of the solution formed when 48 cm 3 of gaseous ammonia (Kb of ammonia = 1.8 × 10−5 mol dm−3) is channeled into 20 cm3 of 0.05 mol dm−3 of sulfuric acid at room temperature and pressure. Hence, predict the colour observed when a few drops of thymolphthalein is added to the resulting solution. [3] ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ .............