SRJC_H2_CHEM_P3 ANS

SRJC 2009 9746/03/Prelim/2009 [Turn over SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/03 Preliminary Examination Paper 3 Free Response 20th August 2009 MARK SCHEME 2 hours CONFIDENTIAL DOCUMENT Teachers to keep this document under lock and key until 20 August 2009 12pm. This document consists of 19 printed pages and 1 blank page

2 SRJC 2009 9746/03/Prelim/2009 [Turn over 1 (a) A student dissolved 8.4 g of sodium fluoride in 250 g of water. Given the following data, calculate the initial temperature of water if the final temperature of the solution is 20.0 °C. Assume that the specific heat capacity of sodium fluoride solution is 4.2 J g–1 K–1. Lattice energy of NaF –918 kJ mol–1 Enthalpy change of hydration of F- –457 kJ mol–1 Enthalpy change of hydration of Na+ –390 kJ mol–1 [3] ½ m ½ m ½m ½m ½m ½m ∆H soln NaF (s) Na+ (aq) + F- (aq) ∆H latt ∆Hhyd ∆Hhyd N a + (g) + F- (g) ∆Hsoln (NaF) = -∆Hlatt (NaF) + ∆Hhyd (Na + ) + ∆Hhyd (F - ) = -(-918) + (-457) + (-390) {give method marks, if answer is wrong} = +71.0 kJ mol-1 (since ∆Hsoln > 0, temp of surroundings decreases) Amt of NaF dissolved = 8.4 / (23+19) = 0.2000 mol Q = ∆H soln x Amt of NaF = 71 x 0.2000 = 14.2 kJ {give method marks, if answer is wrong} Q = mc ∆T 14.2 x 1000 = 250 x 4.2 x (T i - 20) Note: since temp surr decreases, Ti > Tf {give method marks, if answer is wrong} 14200 = 1050 (T i – 20) 14200 = 1050T i - 21000 1050T i = 35200 Ti = 33.5°C (b) In the manufacture of fuel for industries, liquid MTBE is added as a solvent to petroleum to reduce pollution. C CH3 CH3 CH3 O CH3 Methyl t-butyl ether (MTBE) (i) What is meant by the term standard enthalpy change of formation of MTBE? Support your answer with the aid of an appropriate equation with state symbols. 1m 1m Standard enthalpy change of formation of MTBE is the energy evolve/change when 1 mole of MTBE is formed from its elements under standard conditions. 5C(s) + 6H 2 (g) + ½O2 (g) → C5H12O (l) {½ for state symbol, ½ for equations} M T B E

3 SRJC 2009 9746/03/Prelim/2009 [Turn over (ii) Use the following data and by means of an appropriate energy cycle, prove that the bond energy for the C–H bond in MTBE is +418 kJ mol-1. Enthalpy change of atomisation of C(s) +715 kJ mol–1 Enthalpy change of formation of MTBE –383 kJ mol–1 Enthalpy change of vapourisation of MTBE +30.4 kJ mol–1 Bond Energy of H-H +436 kJ mol-1 Bond Energy of O=O +496 kJ mol-1 Bond Energy of C-O +360 kJ mol-1 Bond Energy of C-C +350 kJ mol-1 2m