MJC H2 CHEM P3 QP

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. (ii) Calculate the solubility product of calcium carbonate, stating its units. [3] (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 clear ly whether the temperature increases or decreases. (ii) The enthalpy change of solution assumes that an infinitely dilute solution is formed when one mole of ionic compound dissolves in water . Based on this assumption, suggest a reason to account for the maximum temperature change n ot being achieved in practice other than the system is not 100% efficient. [3]

3 [Turn Over (c) The standard entropy change of solution, ∆S o sol, of sodium chloride is +42.6 J mol–1 K–1. (i) Calculate the standard Gibbs free energy change of solution, ∆G o sol, of sodium chloride, explaining why sodium chloride is soluble. (ii) The standard Gibbs free energy change of solution, ∆G o sol, of magnesium chloride is –126.3 kJ mol –1. Some standard enthalpy change of formation, ∆Ho f, related to magnesium chloride are provided below. Substances MgCl2(s) Mg2+(aq) Cl –(aq) ∆Ho f / kJ mol–1 –641.6 –462.0 –167.5 Calculate the ∆S o sol of magnesium chloride. Hence, explain with reasoning, whether the sign of ∆So sol obtained is as expected. [5] (d) The melting point of CaCO3 is 825 oC and its thermal decomposition temperature is 837 oC. (i) The melting point of BeCO 3 is 54 oC. Briefly explain why the melting point of BeCO3 is much lower than that of CaCO3. (ii) By quoting data from the Data Booklet , predict whether the thermal decomposition temperature of SrCO 3 would be higher or lower than that of CaCO3. [5] (e) Using relevant data from the Data Booklet and information given below, construct an energy cycle to calculate the lattice energy of MgCO3. enthalpy change of formation of magnesium carbonate = –1096 kJ mol–1 enthalpy change of formation of CO2 (g) = –393 kJ mol–1 enthalpy change of atomisation of magnesium = +148 kJ mol–1 sum of first and second electron affinities of oxygen = +657 kJ mol–1 CO2 (g) + O2– (g) CO3 2– (g) ; ∆H1 = –778 kJ mol–1 [4] [Total: 20]

4 [Turn Over 2 This question is about phosphoric acid. (a) Phosphoric acid, H3PO4, is a triprotic acid and its structure is given below. phosphoric acid The p Ka values of the two successive dissociations of H 3PO4 at 25 oC are as shown. pKa1 pKa2 Phosphoric acid 2.1 7.2 Small quantities of H 3PO4 are used to impart the sour taste to many soft drinks. A typical can of soft drink with density of 1.00 g cm –3 contains 0.05 % by mass of H3PO4. (i) Briefly explain why the pKa2 of phosphoric acid is higher than its pKa1. (ii) Calculate the concentration, in mol dm–3, of phosphoric acid in the soft drink. (iii) Hence, by considering only the first dissociation of H 3PO4, determine the pH of this soft drink. Assume that the acidity of the soft drink arises only from H3PO4. (iv) Suggest, with reasoning, a suitable indicator that can be used to detect the first end point of the titration between H3PO4 and sodium hydroxide. [5] (b) A buffer solution was prepared by mixing 50 cm 3 of 1.00 mol dm -3 Na2HPO4 and 50 cm 3 NaH2PO4 of unknown concentration. When 5.00 x 10 –3 moles of sodium hydroxide was added to the buffer solution, the pH increased to 7.38. Determine the unknown concentration of NaH2PO4 used to prepare the buffer. [2] (c) When pure H 3PO4 undergoes dehydration, another phosphorus -containing acid Y is produced. The addition of calcium hydroxide to Y produces a white precipitate, Ca2P2O7. Draw the structure, clearly indicating the shape of Y. [1]

5 [Turn Over (d) Porous ceramics containing calcium phosphate, Ca3(PO4)2, are commonly used in the manufacture of artificial teeth. Given: Ksp of Ca3(PO4)2 = 1.21 x 10–26 mol5 dm–15. (i) Calculate the solubility of calcium phosphate in water. (ii) Some calcium phosphate is dissolved into a solution of 0.50 g dm –3 calcium hydroxide, Ca(OH) 2, to produce a saturated solution. Calculate the solubility of Ca3(PO4)2 in the saturated solution. (iii) Comment on the difference in the solubility values of calcium phosphate in (d)(i) and (d)(ii). [6] (e) When c oncentrated phosphoric acid and concentrated sulfuric acid are reacted separately with solid potassium bromide , different bromine-containing substances A and B are formed respectively. When concentrated sulfuric acid is reacted with solid potassium chloride , the product formed reacted with manganese(II) oxide to give a chlorine-containing substance C. Substances C and B are structurally similar. For the following questions, you need to use the actual identity of A, B and C in your answers. (i) Bolandiol is a banned synthetic anabolic steroid. bolandiol Draw the structural formulae of the organic products formed when bolandiol is heated with: 1. A. 2. B. (ii) Write the equation when C is reacted with hot concentrated NaOH. (iii) By considering the reactions of C and I2 with thiosulfate, S 2O3 2–, explain the difference in the oxidising strength of C and I2 in terms of their oxidation numbers. Include any relevant equations in your answers. [6] [Total: 20]

6 [Turn Over 3 In mammals including humans, nitrogen monoxide NO is an important cellular signaling molecule. It acts as a powerful vasodilator. (a) Although elements in the same group can be said to have similar chemical properties, there are variations, e.g. in reactivity or products formed in a reaction. For example