MJC H2 Chem 2013 Prelim P2 Soln

©chemistry@meridian jc 1  (a) (i) 140 × 103 × (0.00412) = (100) (4.18) (T) The temperature change of 1.38C is too small and (experimental) results would be very inaccurate. (ii) Hence, write a detailed plan to ve rify the enthalpy change of reaction between magnesium and sulfuric acid, H1. Your plan should include the following:  Justification of the mass of Mg turnings to be used  Tabulation of raw data  Brief description of how the data can be used to determine the enthalpy change of reaction [6] Justification of mass to be used 140 × 103 × (m/24.3) = (100) (4.18) (7) m = 0.508 g (any suitable mass to achieve a T of 7 C to 10 C) Procedure 1) Using a 100 cm3 measuring cylinder, measure out 100 cm 3 of sulfuric acid into the styrofoam cup supported in a 250 cm3 beaker. 2) Measure and record the initial temperature of the solution. 3) Weigh accurately 0.510 g of Mg turnings in a clean dry weighing bottle using a mass balance. 4) Carefully transfer the Mg turnings into the cup, stir. 5) Measure and record the highest temperature reached. 6) Re-weigh the weighing bottle which may contain some residual Mg turnings; record the actual mass of solid sample used. Tabulation of Data Mass of weighing bottle and Mg turnings / g Mass of weighing bottle + residual solid after transfer / g Mass of Mg turnings used / g M Initial temperature of solution / C Highest temperature of solution / C Change in temperature, T / C Treatment of Data IH1I × M / 24.3 = 100 × 4.18 × T / 1000 H1 = (0.418)(24.3)(T) /M kJ mol-1 2013 MJC H2 Chemistry Paper 2 Suggested Answers 916

2 ©chemistry@meridian jc (b)(i) Is the energy change when 1 mole of MgO (s) is formed from its elements, Mg (s) and O2 (g) under standard conditions (ii) Mg (s) + ½ O2 (g) MgO (s) MgSO 4 (aq) + H2 (g) + ½O2 (g) MgSO4 (aq) + H2O (l) Hf = 602 kJ mol1 2(a) Step I: Nucleophilic Addition ; Step II: Hydrolysis (b)   (c) Acts as a solvent (d)(i) (Strong) covalent bonds in organic mole cules or the strong C-I bond must be broken. (ii) No of moles of pentan–2–one = 6 x 0.814 /86 = 0.05679 No of moles of Mg = 1.50 / 24.3 = 0.0617 Grignard reagent is in excess. + H2SO4 (aq) H1 + H2SO4 (aq) H2 H3   Hf  917

3 ©chemistry@meridian jc (f) (i) w a t e r Mg(OH)I /MgCl/MgI   (ii) saturated sodium hydrogencarbonate HCl (iii) sodium thiosulfat e Iodine (iv) saturated sodium ch loride Water (g) Transfer the diethylether layer into a distilling flask fitted with an air condenser and carry out distillation by boiling over temperature range from 141 oC to 145 oC. 3 (ai) 1 [Cl-] = 31.00 10 10 1000  = 0.100 mol dm-3 [Ag+] = [] spK Cl = 102.02 10 0.100  = 2.02 x