2019 EJC Prelims H2 Chemistry Paper 2 with solution and comments

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2019 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 8 – INDEX NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 17 September 2019 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. 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 2 1 / 15 2 / 20 3 / 20 4 / 20 Total / 75 This document consists of 20 printed pages.

2 © EJC 9729/02/J2PE/19 For Examiner’s Use 1 The compounds of magnesium have important applications. Magnesium oxide is often used as a refractory material to line furnaces while magnesium chloride is used to de -ice roads and pavements in winter. (a) (i) Explain why magnesium oxide has the formula MgO. ................................................................................................................................ ................................................................................................................................ ............................................................................................................................ [1] (ii) State and explain the property that allows magnesium oxide to be used as a refractory material. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ............................................................................................................................ [1] (iii) Describe the action of water on magnesium oxide. Write an equation for any reaction that occurs, and suggest the pH of the solution formed. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ............................................................................................................................ [2] Comments:  Many candidates simply wrote “because it is formed from Mg2+ and O 2-“, without explaining why the atoms form those ions in the first place (to achieve stable octet).  “To form stable ions” is also not accepted as the ions are not stable! Comments:  The term “ionic bonds” are not accepted.  Some students did not know what a refractory material is. Comments:  This straightforward recall type question was surprisingly not well-answered. Each Mg atom has 2 valence electrons. One Mg atom loses 2 valence electrons to one O atom to achieve stable octet configuration. MgO has a high melting point . A large amount of energy is required to overcome the strong electrostatic forces of attraction between oppositely charged ions. MgO does not react very vigorously and it dissolves sparingly in water to form a weakly alkaline solution of pH = 9. MgO + H2O  Mg(OH)2

3 © EJC 9729/02/J2PE/19 [Turn Over For Examiner’s Use (iv) State and explain whether magnesium or calcium has a higher tendency to form oxides. ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ................................................................................................................................ ............................................................................................................................ [2] Comments:  This straightforward recall type question was surprisingly not well-answered.  Two common misconceptions involve arguments along the lines of stability of CaO (either involving charge density or lattice energy). However, candidates fail to appreciate that charge density explains why CaO does not undergo thermal decomposition readily, while lattice energy is about forming CaO from its gaseous ions (rather than atoms). Down the group, the atomic radius increases. Weaker electrostatic force of attraction between the nucleus and valence electrons. Metal atoms lose their valence electrons to form M2+ cations more easily. Hence, Ca has a greater tendency to be oxidised and form oxide than Mg.

4 © EJC 9729/02/J2PE/19 For Examiner’s Use (b) In an experiment, 40.0 cm 3 of aqueous magnesium chloride was titrated with 1.00 mol dm–3 sodium hydroxide. The reaction involves the precipitation of magnesium hydroxide. MgCl2(aq) + 2NaOH(aq)  Mg(OH)2(s)+ 2NaCl(aq) The pH change of the solution is given in Fig. 1.1. Fig. 1.1 5 6 7 8 9 10 11 12 13 14 0 5 10 15 20 25 30 pH volume of 1.00 mol dm–3 NaOH added / cm3

5 © EJC 9729/02/J2PE/19 [Turn Over For Examiner’s Use (i) Write an equation to illustrate the hydrolysis of the hydrated magnesium ion, [Mg(H2O)6]2+. ............................................................................................................................ [1] (ii) Using your answer to (b)(i), write an expression for the acid dissociation constant, Ka, of the hydrated magnesium ion, [Mg(H2O)6]2+. [1] (iii) Using Fig. 1.1, calculate the initial concentration of H+. [1] (iv) Calculate the concentration, in mol dm–3, of the aqueous MgCl2. [1] Comments:  This straightforward recall type question was surprisingly not well -answered. As the ion undergoes partial hydrolysis, the reversible arrow is required. Comments:  The most common mistake wa s to put the charge outside of the square bracket, which represents concentration in the Kc expression) Comments:  This was generally well-attempted. Comments:  Some candidates did not notice the mole ratio. [Mg(H2O)6]2+(aq)  [Mg(H2O)5(OH)]+(aq) + H+(aq) From the graph, initial pH = 6 Initial [H+] = 1.00  10–6 mol dm–3 2MgC NaOH 1 2nn l   2 1 20.01.002 1000MgC 40.0 1000 30.250 mol dml  25 a 2 26 H Mg(H O) (OH) Mg(H O)         K

6 © EJC 9729/02/J2PE/19 For Examiner’s Use (v) Using your answer to (b)(iii) and (b)(iv), determine the Ka for the hydrated magnesium ion, [Mg(H2O)6]2+. [1] (vi) When 10 cm3 of alkali have been added, calculate: I. the hydroxide ion concentration, II. the hydrated magnesium ion concentration. [2] (vii) Hence, calculate a value for the solubility product, Ksp, of magnesiu