RVHS H2 CHEM P2 Ans Prelim

River Valley High School 9647/02/PRELIM II/16 [Turn over 2016 Preliminary Examination II RIVER VALLEY HIGH SCHOOL YEAR 6 PRELIMINARY EXAMINATION II CANDIDATE NAME CLASS 6 CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9647/02 Paper 2 Structured Questions 13 September 2016 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class, centre number and index number on all the work you hand in. Write in dark blue or black pen on both sides of 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 all questions in the space provided. A Data Booklet is provided. Do NOT write anything on it. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Paper 2 Question Number 1 2 3 4 5 6 Total (Paper 2) Marks 12 13 8 9 16 14 72 Paper 1 40 Paper 3 80 Total 192

2 River Valley High School 9647/02/PRELIM II/16 2016 Preliminary Examination II 1 (a) (i) [1] (ii) Amount of salicylic acid in 10 g = 138 10 = 0.0725 mol Theoretical amount of salicylic acid = 60 0725.0 × 100 = 0.121 mol Amount of methyl salicylate required = 0.121 mol Mass of methyl salicylate required = 0.121 × 152 = 18.4 g Volume of methyl salicylate required = 174.1 4.18 = 15.7 cm3 Amount of sodium hydroxide required = 0.121 × 2 = 0.242 mol Volume of sodium hydroxide required = 0.6 242.0 ×1000 = 40.3 cm3 [2] (b) Water out Liebig condenser Liebig condenser Liebig condenser (Liebig) condenser Water in (100 cm3) round bottom flask containing reaction mixture Heating mantle

3 River Valley High School 9647/02/PRELIM II/16 [Turn over 2016 Preliminary Examination II 1. Using a 25 cm3 measuring cylinder, measure 16 cm3 (accept up to 20 cm 3) of methyl salicylate and place it in a 100 cm3 round bottom flask. 2. Using a 50 cm3 measuring cylinder, measure 41 cm3 (accept up to 50 cm3) of aqueous sodium hydroxide and place it into the 100 cm 3 round bottom flask. Add some boiling chips to the mixture. 3. Set up the reflux set -up shown in the above diagram and heat the mixture for 30 minutes. 4. Allow the reaction mixture to cool down to room temperature and remove the boiling chips. 5. Place the round bottom flask in an ice bath. 6. Add concentrated hydrochloric acid to the reaction mixture slowly, with stirring. 7. Test the acidity of the mixture with a blue litmus paper. 8. Filter the mixture and wash the residue (crude salicylic acid) with a small amount of cold water. 9. Transfer the crude salicylic acid to a clean 100 cm 3 conical flask. Add a small volume of water and heat the mixture until all the crude product dissolves. 10. Filter the hot solution, using a fluted filter paper , into a pre- warmed clean and dry conical flask. Allow the solution to cool down slowly for crystals to form. 11. When the solution is cooled to room temperature, submerge the conical flask into an ice bath to allow more crystals to form. 12. Filter the mixture to obtain the pure salicylic a cid crystals and pat dry the crystals using filter paper. [9] [Total: 12] 2 (a) pV = nRT (150 × 103)(60 × 103) = n(8.31)(300 + 273) Amount of nitrogen gas = 1.89 mol Amount of sodium azide = 2/3 89.1 = 1.26 mol Mass of sodium azide = 1.26 × 65.0 = 81.9 g [2] (b) (i) S = (51.3 + 2 3 × 191.6)  70.5 = +268.2 J mol1 K1 [1] (ii) H is negative, S is positive, TS is negative. Since G = H  TS, G is always negative regardless of temperature. [1]

4 River Valley High School 9647/02/PRELIM II/16 2016 Preliminary Examination II (iii) The activation energy of the reaction is very high. [1] (c) (i) Lattice energy of sodium oxide is the enthalpy change when one mole of solid sodium oxide is formed from its constituent gaseous ions Na+ and O2. [1] (ii) LE (Na2O) = 416  2(+107)  ½(+496)  2(+494)  (+702) = 2568 kJ mol1 [3] (iii) 2nd electron affinity = +702  (142) = +844 kJ mol1 The positive sign of 2 nd EA means that energy is required to overcome the repulsion between the anion and the electron to be added since both are negatively charged. [2] (d) Safe depressurisation rate = 04.02 )101150(   = 25 kPa s1 Since the airbag being tested depressurises more slowly than the safe depressurisation rate, it is not safe for use. [2] [Total: 13] Energy / kJ mol1 0 2Na(s) + ½O2(g) Na2O(s) 2Na(g) + O(g) 2Na+(g) + 2e + O(g) 2Na+(g) + O2(g) 416 2(+107) + ½(+496) 2(+494) +702 LE (Na2O)

5 River Valley High School 9647/02/PRELIM II/16 [Turn over 2016 Preliminary Examination II 3 (a) What do you understand by the term standard electrode potential? Standard electrode potential, E ⦵, of an electrode is the relative potential of this electrode under standard conditions compared with the standard hydrogen electrode whose electrode potential is assigned as 0 V. The standard hydrogen electrode consists of H2(g) at 1 atm bubbling over pla tinum electrode coated with finely divided platinum which is dipped into 1 mol dm–3 H+(aq) at 298 K. [2] (b) The following cell was set up between a copper electrode and an unknown metal electrode M 2+(aq)/M(s). The standard cell potential was found to be 0.76 V, and the copper foil was connected to the positive end of the voltmeter. (i) Use the Data Booklet to calculate the standard electrode potential of the M2+(aq)/M(s) system. Since copper is the positive electrode of the galvanic cell, it is the cathode. Standard electrode potential of the M2+(aq)/M(s) = 0.34 – 0.76= –0.42 V [1] (ii) Draw an arrow in the box above to show the direction of electron flow through the voltmeter. [1] (iii) Predict the outcomes of the following situations. Describe what you will see and write ionic equations , with state symbols, for any reactions that occur. salt bridge Direction of electron flow 1 mol dm3 CuSO4 Copper foil V Unknown metal wire, M 1 mol dm3 M2+

6 River Valley High School 9647/02/PRELIM II/16 2016 Preliminary Examination II I A rod of metal M is dipped into a solution of 1 mol dm−3 CuSO4. Metal M dissolves in (blue) solution and pink solid of Cu is formed. (Blue solution lightens in colour.) M(s) + Cu2+(aq) Cu(s) + M2+(aq) [2] II Dilute sulfuric acid is added into a beaker containing a powdered sample of metal M. Efferversence is observed; colourless odourless gas evolved that extinguishes a lighted splint with a pop sound. Metal M dissolves in (colourless) solution. 2H+(aq) + M(s) M2+(aq) + H2(g) [2] [Total: 8] 4 The four most abundant salts in sea-water are as follows. Salt kg per m3 Sodium chloride 27.5 Magnesium chloride 6.75 Magnesium sulfate 5.625 Calcium sulfate 1.80 Magnesium oxide is obtained from sea-water by the following steps. The relevant numerical values of the solubility products are given below. sea water filtrate Mg(OH)2 MgO Step 1: controlled addition of CO32− Step 2: addition of OH− Step 3: heat

7 River Valley High School 9647/02/PRELIM II/16 [Turn over 2016 Preliminary Examination II Salt Ksp Sodium carbonate - Calcium carbonate 5.0 × 10−9 Magnesium carbonate 1.0 × 10−5 Magnesium hydroxide 1.5 × 10−11 Calcium hydroxide 7.9 × 10−6 (a) Explain why the addition of carbonate ions in Step 1 is necessary and has to be controlled. Both calcium carbonate and magnesium carbonate are sparingly soluble salts . Carbonate i