TJC Prelim P2 Qn & Ans

1 9729/02/TJC Prelims/2018 [Turn over s CANDIDATE NAME CIVICS GROUP / CENTER NUMBER S INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 23 August 2018 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group, centre number, index number and name on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 18 printed pages For Examiner’s Use 1 / 21 2 / 13 3 / 9 4 / 8 5 / 14 6 / 10 Total / 75 PRELIMINARY EXAMINATIONS HIGHER 2

2 9729/02/TJC Prelims/2018 [Turn over SECTION A Answer all questions in the spaces provided. 1 (a) For many compounds, the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy change of combustion can be used. The enthalpy change of combustion of butan-1-ol can be found by a calorimetry experiment in which the heat given off during combustion is used to heat a known mass of water and the temperature change recorded. temperature of water before heating = 25.0 °C temperature of water after heating = 66.1 °C mass of spirit burner and butan-1-ol before heating = 80.44 g mass of spirit burner and butan-1-ol after heating = 79.70 g (i) Explain the meaning of the term standard enthalpy change of combustion. • The standard enthalpy change of combustion of a substance , ΔHco, is the heat energy evolved when one mole of the substance is completely burnt in oxygen at 298 K and 1 bar. [1] (ii) The heat transfer from the spirit burner to water is known to be only 70% efficient. From the data provided above, determine the enthalpy change of combustion of butan-1-ol. mass of butan-1-ol reacted = 80.44 – 79.70 = 0.74 g No. of moles of butan-1-ol reacted = 0.74 / 74 = 0.0100 mol amount of heat taken in by water = mcΔT = 100 x 4.18 x (66.1 – 25.0) • = 17.2 kJ Heat provided by butan-1-ol = 100 x 17.2 / 70 = 24.6 kJ beaker 100 cm3 of water spirit burner butan-1-ol wick thermometer clamp

3 9729/02/TJC Prelims/2018 [Turn over enthalpy change of combustion of butan-1-ol = - 24.6/ 0.0100 • = - 2460 kJ mol-1 [2] (iii) The entropy change of combustion of butan-1-ol is -252 J K -1 mol-1. Using your answer in (a)(ii), calculate the Gibb’s free energy for combustion of butan-1-ol at 298 K. ΔG = ΔH – TΔS = -2460 – (298)(-252 x 10-3) • = - 2380 kJ mol-1 (allow ecf from (a)(ii)) [1] (b) The table below gives some data on four fuel sources: methanol, ethanol, hydrogen and octane. Octane can serve as a rough approximation of petrol. name formula molar mass/ g mol-1 density/ g cm-3 ΔHcӨ (298K)/ kJ mol-1 ΔHfӨ (298K)/ kJ mol-1 methanol CH3OH 32 0.793a -726.0 -239.1 ethanol CH3CH2OH 46 0.789a -1367.3 liquid hydrogen H2 2 0.0711b -285.8# octane CH3(CH2)6CH3 114 0.703a -5470.2 -250.0 a At 298K and 1 bar pressure b At 20K and 1 bar pressure # standard enthalpy change of combustion of hydrogen gas (i) State the value of the standard enthalpy change of formation of hydrogen gas, H2. • 0 kJ mol -1 (by definition, elements in standard state are reference zero) [1] (ii) Calculate the density of gaseous hydrogen at 298 K and 1 bar pressure. Give your answers in g cm-3. PV = nRT PV = (m/Mr) RT ρ = P×Mr RT = 105×2 8.31×298 = 80.8 g m-3 • = 8.08 x 10-5 g cm-3 [1]

4 9729/02/TJC Prelims/2018 [Turn over (iii) Write the chemical equation that represents the standard enthalpy change of combustion of ethanol, including the state symbols. • CH3CH2OH (l) + 3O2(g) → 2CO2(g) + 3H2O(l) [1] (iv) The standard enthalpy change of combustion of carbon is -393.5 kJ mol -1. Using this value and the standard enthalpy change of combustion data in the table, calculate the standard enthalpy change of formation of ethanol. Show your working clearly in the form of an energy cycle diagram. 2 C(s) + 3 H2(g) + 7/2 O2(g) CH3CH2OH(l) + 3 O2(g) 2 CO2(g) + 3 H2O(l) • Energy cycle, balanced equations, state symbols By Hess’ law, • ΔHfӨ -1367.3 = 2(-393.5) + 3(-285.8) • ΔHfӨ = -277.1 kJ mol-1 [3] (v) An important property of a fuel, especially when the fuel has to be lifted such as in aviation, is the energy released on combustion per gram of fuel. Calculate the enthalpy change of combustion per gram of fuel at 1 bar pressure and 298K for methanol. Enthalpy change of combustion per gram of fuel = -726/32 = -22.7 kJ g-1 [1] (vi) Another important characteristic of a fuel, especially when there is a fuel tank of limited size, is the energy released on combustion per cm3 of fuel. Calculate the enthalpy change of combustion per cm3 of fuel for octane. Enthalpy change of combustion per cm3 of fuel = -5470.2/ (114/0.703) = -33.7 kJ cm-3 [1] (vii) Explain why, given the data in the question, it is not strictly possible to make a fair comparison of the energy released per cm3 of liquid hydrogen with other fuels. Enthalpy change of combustion value for hydrogen is for standard conditions, and so relates to gaseous hydrogen, not to liquid hydrogen. Latent heat of vaporisation of hydrogen is not accounted for. Allow comment about how the value of density of liquid hydrogen is unsuitable for the calculation of energy per unit volume for gaseous hydrogen. [1] ΔHfӨ 2(-393.5) + 3(-285.8) -1367.3

5 9729/02/TJC Prelims/2018 [Turn over (c) Apart from alkanes and alcohols, alkenes are commonly used as fuel. Alkenes can be synthesised through the Wittig reaction, using halogenoalkane and carbonyl compound as the reactants in the presence of triphenyl phosphine. The equation below shows the synthesis of propene using the Wittig reaction. triphenyl phosphine propene (i) Draw the structures of the two cis-tran s isomers formed from the reaction of the following compounds. + [2] • • (ii) Draw the structural formula of the organic reactant used to generate 1- methylcyclohexene through the Wittig reaction. 1-methylcyclohexene • OR [1] O Cl O Cl

6 9729/02/TJC Prelims/2018 [Turn over Alkenes react readily with interhalogen compound ICl to give halogenalkane. ICl reacts faster w