VJC H2 CHEM P3 QN Prelim

 VJC 2014 9647/03/PRELIM/14 [Turn over VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Candidates answer on separate paper. 22 September 2014 2 hours Additional Materials: Answer Paper Data Booklet Graph Paper READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear explanation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 11 printed pages and 1 blank page.

2    VJC 2014 9647/03/PRELIM/14   Answer any four questions. 1 (a) Use of the Data Booklet is relevant in this question. The following cell has been used to light up a light bulb. (i) Explain how the cell potential changes when the current has passed through for a while. The electrode potential of the cell is related to the equilibrium constant by the following equation, E o= ln Kc where T is the temperature measured in Kelvin, z is the number of moles of electrons transferred during the redox reaction and F is the Faraday constant. (ii) Determine the equilibrium constant Kc for the reaction between Cu2+ and Mg. (iii) Hence, determine the ratio [Mg 2+]:[Cu2+], when there is no current flowing in this cell. (iv) Suggest the significance of the magnitude of your answer in (a)(iii). [5] (b) Copper minerals often contain suifides of magnesium and silver as impurity. After minerals are reduced with carbon, the solid impure copper is purified by electrolysis. (i) Describe the electrode reactions that take place during the electrolysis and explain in detail how each of the two impurity metals is removed from copper. An impure copper rod is purified by electrolysis using a constant current. After 1 hour, mass of one electrode decreased by 19.0 g while mass of the other electrode increased by 17.8 g. The electrolyte was further analysed and found that the amount of Cu 2+ ions is reduced by 0.020 mol. (ii) Calculate the current used for this process. (iii) Determine the percentage composition by mass of the three elements. (iv) Suggest how magnesium can be recovered as magnesium oxide near the end of the electrolytic process. Write balanced equations for any reactions involving magnesium that might occur. [11] salt bridge Cu electrode Cu2+(aq), 1 mol dm–3 light bulb X Mg electrode Mg2+(aq), 1 mol dm–3 RT zF

3    VJC 2014 9647/03/PRELIM/14 [Turn over  (c) The methods of synthesis shown below are faulty. In each case  Explain why this is so.  Suggest how the synthesis from the initial reactant to the final product could be satisfactorily achieved. (i) (ii)   [4] [Total: 20]

4    VJC 2014 9647/03/PRELIM/14   2 (a) (i) When CH I2CHO was warmed with alkaline aqueous iodine, two organic products are formed. Construct an equation for this reaction. (ii) There are suggestions on using CF 3CH2F to replace CFCs. One problem however is that CF 3CH2F can be converted to CF 3CO2H, which is toxic when ingested. When CF3CH2F eventually reaches the atmosphere, it is thought that CF3CH2F is initially attacked by •OH radicals. A student suggested the following equation to represent the above reaction: CF3CH2F + •OH  CF3CH2 + HOF By considering the  bonds broken and  bonds formed in the above reaction, explain why the above reaction will not occur. (iii) In light of your answers to (a)(ii), suggest a more appropriate equation for the reaction between CF3CH2F and •OH. (iv) Ethanedioic acid, HO2C-CO2H is a dibasic acid, with p Ka1 = 1.3 and p Ka2 = 4.3. Methanoic acid is a monobasic acid with pKa = 3.7. Account for the lower p Ka1 value of ethanedioic acid as compared to the p Ka of methanoic acid. [6] (b) But-2-yne, H 3CC≡CCH3 may undergo the addition of hydrogen at the surface of a metal catalyst to produce an alkene X. Draw relevant diagrams to illustrate how the metal catalyses this reaction. Give the structure of X. [3] (c) (i) Describe briefly how gaseous HC l can be made in the laboratory from NaC l and concentrated H2SO4. (ii) Explain with relevant equations, if HBr and H I can be prepared in a similar method as in (c)(i). (iii) Silver chloride is soluble in aqueous amm onia while silver bromide is soluble in concentrated aqueous ammonia. Explain these observations. [6] •

5    VJC 2014 9647/03/PRELIM/14 [Turn over  (d) A is one of the elements from Na to P in the third period of the Periodic Table. When A is boiled with aqueous sodium hydroxide, a colourless gas, B is formed together with a sodium salt, C. Gas B is weakly basic and reacts with hydrogen iodide to form a solid compound, D, which contains 19.1% of A, 2.5% of hydrogen and 78.4% of iodine by mass. The sodium salt C contains 26.1% of sodium, 2.27% of hydrogen, 35.2% of A and 36.4% of oxygen by mass. A also forms two chlorides which dissolve readily in water to produce strongly acidic solutions of pH 2. Identify the element A and the compounds B, C and D. Write equations for all the reactions involved. [5] [Total: 20]

6    VJC 2014 9647/03/PRELIM/14   3 Alcohols are very useful in organic synthesis and can be used to produce a wide variety of compounds. The structural formula of one such alcohol is shown below. compound A (C13H20O2) (a) When compound A is heated with an excess of concentrated sulfuric acid, a mixture of two isomeric hydrocarbons, P and Q are produced. Draw the structures of P and Q. [2] (b) Compound D can be formed from compound A via a five-step synthesis. (i) State the reagents and conditions required for steps 2 to 4 and the structural formulae of the two intermediates B and C. (ii) Step 1 involves the ‘protection’ of one of the alcohol groups to convert it into an ether, which is inert. Suggest a reason why this step is necessary. (iii) Suggest the type of reaction in step 5. [6] (c) Nitrosyl bromide, NOBr, is a red gas and can be formed by the reversible reaction of nitric oxide with bromine. (i) Explain why nitrosyl bromide has a boiling point of 14 oC whereas nitrosyl chloride, NOCl boils at −5 oC. NOBr decomposes to NO and Br2 as shown below. 2NOBr(g)  2NO(g) + Br2(g) (ii) Given that the bond energy of N −Br is 120 kJ mol−1, use the data from the Data Booklet to calculate the enthalpy change of decomposition of nitrosyl bromide, NOBr.

7    VJC 2014 9647/03/PRELIM/14 [Turn over  (iii) Standard enthalpy changes of formation of nitrosyl bromide and nitrogen monoxide are given below. compound Hf / kJ mol−1 NOBr(g) +82 NO(g) +90 With the aid of an energy cycle, use your answer in (c)(ii) and data above to calculate the enthalpy change of vaporisation of bromine. (iv) Predict and explain the ‘sign’ of entropy change for the decomposition of