# Q6 1.  Bond Energy is the average energy required to break 1 mole of a covalent bond in the gaseous state. 2.

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Q6 1

 Bond Energy is the average energy required to break 1 mole of a covalent bond in the gaseous state. 2

2C 8 H 18 + 25O2  16CO 2 + 18H 2 0(g) 3

 Octane: 7 C-C Single Bonds 18 C-H Single Bonds  Oxygen 1 O=O Double Bond 4

 Bond Energies:  C – C Single Bond: 350 kJ mol -1  C – H Single Bond: 410 kJ mol -1  O=O Double Bond: 496 kJ mol -1 5

 Energy required to break bonds in 2 mole of C 8 H 18 = = 2 [(7 x 350) + (18 x 410)] kJ mol -1 = 2 [2450 + 7380] kJ mol -1 = + 19660 kJ mol -1  Energy required to break bonds in 25 moles of O 2 = 25 x 496 kJ mol -1 = +12400 kJ mol -1 6

 Carbon Dioxide: 2 C=O Double Bonds  Water: 2 O-H Single Bonds 7

 Bond Energies:  C=O Double Bond: 740 kJ mol -1  O–H Single Bond: 460 kJ mol -1 8

 Energy released in forming bonds in 16 mole of CO 2 = 16 [2 x 740] kJ mol -1 = 16 [1480] kJ mol -1 = - 23680 kJ mol -1  Energy required to break bonds in 18 moles of H 2 O = 18 [2 x 460] kJ mol -1 = 18 [920] kJ mol -1 = - 16560 kJ mol -1 9

2C 8 H 18 + 25O2  16CO 2 + 18H 2 0(g)  ΔH c ø for 2 moles of octane = Energy required + Energy Released = (+19660 + 12400) + (-16560 + (-23680)) kJ mol -1 = (32060) + (-40240) kJ mol -1 = -8180 kJ mol -1 10

 Standard Enthalpy change of combustion is the heat evolved when 1 mole of a substance is completely burnt in excess oxygen.  ΔH c ø for 1 mole of octane = ½ x -8180 kJ mol -1 = -4090 kJ mol -1 11

AlkaneFormulaΔH c / kJ mol -1 HeptaneC 7 H 16 -4817 OctaneC 8 H 18 -5470 NonaneC 9 H 20 -6125 12 Suggest a reason for the discrepancy between the ΔH c for octane you calculated in (b)(ii) and that given in the table

 Calculated Value = - 4090 kJ mol -1  Theoretical Value = - 6125 kJ mol -1  Calculated Value is less than theoretical Value Given that the values are calculated based on bond energies provided in the Data Booklet, which are the average values from empirical data from a large range of molecules, thus there may be discrepancies in using those values to represent the value of enthalpy change of combustion 13

 Increase in magnitude of values:  Reaction is more exothermic  More energy is released from the reaction  The greater the number of carbon atoms, the more exothermic the reaction 14 AlkaneFormulaΔH c / kJ mol -1 HeptaneC 7 H 16 -4817 OctaneC 8 H 18 -5470 NonaneC 9 H 20 -6125

 Heptane C 7 H 16  Octane C 8 H 18  Nonane C 9 H 20 15

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C 7 H 16 + 11O 2  7CO 2 + 8H 2 0 C 8 H 18 + 25/2O 2  8CO 2 + 9H 2 0 C 9 H 20 + 14O 2  9CO 2 + 10H 2 0  For each equation, there is  an increase in 1 –CH2- unit in the alkane and 1.5 moles of oxygen  an increase in 1 mole of carbon dioxide and water formed 17

 For Each equation, change in enthalpy = additional Energy required to break Bonds + additional Energy released from forming bonds = [(2 x 350)+(2 x 410) + 3/2(496)] – [(2 x 740) + (2x460)] = +2264 – 2400 kJ mol -1 = -136 kJ mol -1 therefore, there is a regular increase in magnitude of enthalpy change of combustion from heptane to octane to nonane as more energy is released each time 18

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 q = m x c x Δt  q = 1000 x 4.18 x (65-25) = 1000 x 4.18 x 40 = 167200 20

 Mass of ethanol = 0.79 g cm -3 x 10 cm -3  = 7.9g  Amount of Ethanol =_ 7.9 _ 2x12.0 + 6x1.0 + 1x16.0 = 7.9 / 46 mol = 0.172 mol 21

 ΔH = - q/n = -167200/0.172 J mol -1 = - 972093 J mol -1  Since the system is only 70% efficient,  Actual ΔH = 972093 x 70% = 1388704 J mol -1  = 1388.704 kJ mol -1  = 1390 kJ mol -1 22

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