JJC H2 Chem 2013 Prelim P2 Soln

© Jurong Junior College Page 1 of 5 2013 Prelim Paper 2 Answers Suggested Mark Scheme for 2013 JJC Prelim Exam Paper 2 (9647/02) 1. (a) KNO3(s)  K+(aq) + NO3 –(aq) H > 0 ---(*) When the temperature is increased, the equilibrium position of (*) shifts right to favour endothermic reaction so as to absorb some heat. Hence, the solubility of KNO3 increases with increasing temperature. (b) The method does not apply to solid that decomposes on heating as it will result in greater mass loss. (c) 1. Using a 50 cm3 burette/measuring cylinder, add 50 cm3 of water into a small beaker. 2. Place the small beaker containing water into water–bath with thermostat set at 30 °C. 3. Using a spatula, add KNO3(s) into the water. Stir to dissolve all solid. 4. Repeat step 3 until some solid remains undissolved. 5. Stir the mixture until temperature of solution reaches 30 °C. Let the mixture stand in the water–bath at 30 °C for some time. 6. Using an electronic weighing balance, measure and record the mass of an empty , dry crucible. 7. Using a dry filter funnel and filter paper , filter the mixture and collect the filtrate in the crucible. 8. Using a Bunsen Burner, heat the filtrate to dryness. 9. Using an electronic weighing balance, measure and record the mass of crucible with solid residue. 10. Repeat step 1 to 9 at 40°C, 50°C, 60°C and 70°C. (d)   3 3mass of KNO dissolved in 50 cm of water gyx   3solubility of KNO diss olved in 100 g of water g /100g    100 250 yx yx (e) Use oven/heat resistant gloves or tongs to handle the hot beaker/crucible. OR Cool hot crucible before handling. 2. (a) (i) It is to quench the reaction by removing H 2SO4/H+ in the reaction mixture via acid– carbonate reaction. (ii) I2(aq) + 2S2O3 2–(aq)  2I–(aq) + S4O6 2–(aq) (iii) Since H2SO4 is a catalyst , it will be regenerated such that [H2SO4] will remain constant throughout the reaction. Hence, it is not necessary to use H2SO4 in large excess in order to make [H2SO4] constant. solubility / (g/100g) temperature / °C (or K) 614

© Jurong Junior College Page 2 of 5 2013 Prelim Paper 2 Answers 2. (b) (i) (ii) Order with respect to [iodine] = 0 (iii) Order with respect to [propanone] = 1 (iv) Rate = k [H+] [propanone] (c) (i)   r bonds broken bonds formedHE E             410 151 240 299  122.0 kJ mol (ii) The bond energy values quoted from the Data Booklet are only average value derived form the full range of molecules that contains the particular bonds. OR The reactants are in aqueous states while the bond energies from the Data Booklet are for gaseous species. (iii) (d) (i)      pVn2 .03 molRT     33101 10 50 10 28.31 300 time /min volume of Na2S2O3 /cm3 Reaction coordinate Energy/ kJ mol1 (CH3)2C