MJC_H2_CHEM_P3_answers

Name _________________________ Class: 13S________ Reg Number: _____ MERIDIAN JUNIOR COLLEGE JC 2 Preliminary Examination Higher 2 ___________________________________________________________________________ Chemistry 9647/03 Paper 3 Free Response 17 September 2014 2 hours Additional Materials: Data Booklet Writing Paper _____________________________________________________________________ INSTRUCTIONS TO CANDIDATES Write your name, class and register number in the spaces provided at the top of this page. Answer 4 out of 5 questions in this paper. Begin each question on a fresh page of writing paper. Fasten the writing papers behind the given Cover Page for Questions 1, 2 & 3 and Cover Page for Questions 4 & 5 respectively. Hand in Questions 1, 2 & 3 and 4 & 5 separately. You are advised to spend about 30 minutes per question only. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. You are reminded of the need for good English and clear presentation in your answers. This document consists of 12 printed pages (excluding the cover pages for answer scripts).

2 [Turn Over Answer any 4 out of 5 questions in this paper. Begin each question on a fresh sheet of writing paper. 1 The following tables show some standard enthalpies change of solution, ∆H o sol, regarding some Group I chlorides and Group II carbonates, as well as their corresponding solubility in water under standard conditions. Group I chlorides NaCl KCl RbCl ∆Ho sol / kJ mol–1 +3.9 +17.2 +16.7 Solubility / mol per 100 cm3 of water 0.615 0.481 0.781 Group II carbonates MgCO3 CaCO3 SrCO3 ∆Ho sol / kJ mol–1 –25.3 –12.3 –3.4 Solubility / mol per 100 cm3 of water 1.26 x 10–4 1.30 x 10–5 7.45 x 10–6 Use relevant data from the tables above, if applicable, to answer the following questions. (a) It is a well -known fact that the solubility of Group II carbonates decreases down the group. (i) Using the ∆Ho sol data from both tables, comment on whether ∆H o sol is a good predictor for the solubilities of salts in water. Group II carbonates have negative / exothermic ∆H o soln even though they are insoluble, On the other hand, ∆H o soln of soluble Group I chlorides are positive / endothermic. Hence, ∆Ho soln is not a good predictor of solubility. (ii) Calculate the solubility product of calcium carbonate, stating its units. [3] Solubility of CaCO3 = 1.30 x 10–5 x 10 = 1.30 x 10–4 mol dm–3 Ksp of CaCO3 = [Ca2+] [CO3 2–] = (1.30 x 10–4)2 = 1.69 x 10–8 mol2 dm–6 (b) (i) Assuming a 100% efficient system, calculate the maximum temperature change that could take place when potassium chloride is dissolved in 100 cm 3 of water. State clearly whether the temperature increases or decreases. 0.481 mol of KCl dissolves in 100 cm3 water to give a s