SRJC H2 CHEM P2 ANS

1 SRJC 9647/02/Prelim/2014 Turn Over] SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CLASS CHEMISTRY 9647/02 Preliminary Examination 22 August 2014 Paper 2 Structured Questions (SPA) 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet. READ THESE INSTRUCTIONS FIRST Write your name and class 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 work. Answer all questions in the space provided. You are advised to spend 30 minutes on question 1. At the end of the examination, fasten all your work securely together. The number of marks is given in the brackets [ ] at the end of each question or part questions. For Examiner’s Use 1(P) /12 2 /20 3 /13 4 /12 5 /15 TOTAL P2 /72 For Examiner’s Use P1 /40 P2 /72 P3 /80 GRAND TOTAL /192 % GRADE

2 SRJC 9647/02/Prelim/2014 Turn Over] Serangoon Junior College Preliminary Examination 2014 H2 Paper 2 Solutions 1 Planning (P) Borax, also known as sodium tetraborate decahydrate, Na 2B4O5(OH)4.10H2O, is a controlled poison in Singapore and cannot be bought off the shelf. Borax dissolves slightly in water to form an alkaline solution: Na2B4O5(OH)4(s) ⇌ 2Na+(aq) + [B4O5(OH)4]2-(aq) [B4O5(OH)4]2-(aq) + 5H2O(l) ⇌ 4H3BO3(aq) + 2OH-(aq) The solubility product of borax at 30°C can be determined via volumetric analysis using hydrochloric acid solution. (a) Assuming that a cm3 of 0.500 mol dm -3 HCl is required for titrating 25.0 cm 3 of borax solution at 30°C, calculate the solubility product of borax (excluding units) in terms of a. nHCl = a/1000 × 0.500 = 5.000 x 10-4 a mol 2HCl ≡ 2OH- ≡ [B4O5(OH)4]2- ≡ Na2[B4O5(OH)4] Amt of Na2[B4O5(OH)4] = 5.000 x 10-4 / 2 = 2.500 x 10-4 a mol [borax] = 3 0 0002500 25 10 a    = 0.01000a mol dm-3 [Na+] = 0.02000a mol dm-3 [tetraborate anion] = 0.01000a mol dm-3 Ksp =[Na+]2 [tetraborate anion] =(0.02000a)2(0.01000a) = 4.00 x 10-6 a3 [3] (b) Deduce the sign for ∆S for the dissolution of borax . Hence, predict whether this process will be more or less feasible at a higher temperature, given that ∆H is endothermic. Since ∆G = ∆H - T∆S Since ∆H and ∆S are both positive, as there is a change in state from solid to aqueous in the dissolution process which results in an increase of disorderliness. When T increases from 30°C to 60°C, ∆G becomes more negative. Dissolution process is more feasible. [2]

3 SRJC 9647/02/Prelim/2014 Turn Over] (c) The solubility of borax can also be defined as 30 g of solid that will dissolve in 100 g of water at 60°C. Other than volumetric analysis, write a plan to validate the above information. Your plan should also include  steps to prepare a saturated solution  steps to maintain the temperature throughout the experiment 1. Weigh approximately x g of solid borax using a weighing balance. 2. Using a 100 cm 3 measuring cylinder , place 100 cm 3 of distilled water into a 250 cm3 pre-weighed dry beaker. 3. Weigh again with the distilled water inside to obtain the mass of the distilled water. 4. Maintain the temperature of the mixture at 60°C using a water bath. 5. Add the solid to water and stir with a glass rod to dissolve the solid, add more solid to the water until more solid dissolves. 6. Allow the mixture to stand for a period of time to establish equilibrium. 7. Filter the mixture to obtain the residue and transfer the residue to a pre-weighed dry evaporating dish. 8. Heat the residue to dryness by placing the evaporating dish on top of a bunsen burner with tripod. 9. Cool and reweigh the evaporating dish to obtain mass of the residue. 10. Repeat the heating, cooling and weighing until a constant mass is obtained. 11. Calculate using the formula: 100Total mass of solute added mass of residue mass of water  Solubility [6] (d) Identify one potential safety hazard in either experiment (a) or (c) and state how you would minimise this risk. Borax is poisonous / hydrochloric acid is corrosive. Hence, gloves / goggles must be worn OR conduct experiment in fume hood. [1] [Total: 12]

4 SRJC 9647/02/Prelim/2014 Turn Over] 2 Sodium and chlorine are two common elements on Earth. Elemental chlorine is commercially produced from brine, also known as concentrated sodium chloride, by electrolysis. The electrolysis of brine solution containing neutral litmus solution is carried out in the following apparatus using graphite electrodes. The colour changes of the litmus solutions around regions A and B were observed during the process. (a) (i) With the aid of the Data Booklet, account for the colours changes observed around regions A and B. Include balanced equations where appropriate. Region Colour of litmus solution Reason X Litmus solution first turned red, and then turned colourless At the anode, Cl- is preferentially oxidised due to its high concentration 2Cl-  Cl2 + 2e Next, Cl2 undergoes disproportionation in water OR Cl2 (g) + H 2O ( l)  HCl (aq) + HClO (aq) red, due to formation of HCl /HClO/H+ and colourless due to bleaching action of HClO Y Litmus solution turned blue At the cathode, H2O is preferentially reduced: 2H2O + 2e  H2 + 2OH– OR As such, blue is caused by OH– formed. asbestos diaphragm brine solution with neutral purple litmus solution graphite electrodes A B

5 SRJC 9647/02/Prelim/2014 Turn Over] (ii) The asbestos diaphragm is a porous material that prevents the products from electrolysis from mixing. Write an equation to illustrate the reaction that will occur at room temperature when the diaphragm was removed after the electrolysis. Cl2 + 2NaOH  NaClO + NaCl + H2O (ii) Useful chlorine-containing products such as sodium chlorate( V) and chlorine dioxide may be obtained from further processing of the products in (a)(ii). By considering the numbers of bonding and non–bonding electron pairs, draw diagrams in the boxes below to show the likely shape of ClO3 – and ClO2. In your diagrams, state the shape and clearly indicate the respective bond angle values. [6] chlorate (V) ion ClO O O - Bond angle 107o Shape : trigonal pyramidal chlorine dioxide ClO O Bond angle  any value 104.5o <  < 120o Shape : bent / V–shaped (b) Chlorine dioxide is an unstable gas which decomposes into chlorine gas and oxygen gas readily. (i) Write a balanced equation for this decomposition. 2ClO2 Cl2 + 2O2 (ii) At a pressure of 1 atm and decomposition temperature of 40C, calculate the volume of c hlorine gas formed when 150.0 g of chlorine dioxide is decomposed. Amount of ClO2 = )]0.16(25.35[ 150  = 2.22 mol Amount of Cl2 = )22.2(2 1 = 1.11 mol Using PV = nRT