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Chemical Energetics and Thermodynamics

Q6

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Bond Energy is the average energy required

to break 1 mole of a covalent bond in the

gaseous state.

a) What do you understand by the term bond energy?

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2C8H18 + 25O2 16CO2 + 18H20(g)

b)(i) Write an equation for the complete combustion of C8H18 to form CO2(g) and H20(g)

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Octane:7 C-C Single Bonds18 C-H Single Bonds

Oxygen1 O=O Double Bond

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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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

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Energy required to break bonds in 2 mole of C8H18 = = 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 O2 = 25 x 496 kJ mol-1 = +12400 kJ mol-1

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Carbon Dioxide:2 C=O Double Bonds

Water:2 O-H Single Bonds

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Bond Energies:

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

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Energy released in forming bonds in 16 mole of CO2 = 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 H2O = 18 [2 x 460] kJ mol-1 = 18 [920] kJ mol-1 = - 16560 kJ mol-1

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

2C8H18 + 25O2 16CO2 + 18H20(g) ΔHc

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-110

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Standard Enthalpy change of combustion is the heat evolved when 1 mole of a substance is completely burnt in excess oxygen.

ΔHcø for 1 mole of octane

= ½ x -8180 kJ mol-1

= -4090 kJ mol-1

b)(ii) Use the bond energy data given in the Data Booklet to calculate a value for the enthalpy change of combustion of octane

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Alkane Formula ΔHc / kJ mol-1

Heptane C7H16 -4817

Octane C8H18 -5470

Nonane C9H20 -6125

c) The accurate experimental enthalpy changes of combustion of three hydrocarbons are given in the table below:

Suggest a reason for the discrepancy between the ΔHc for octane you calculated in (b)(ii) and that given in the table

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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

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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

c)(ii) Suggest what the regular increase in the values of ΔHc given in the table represents

Alkane Formula ΔHc / kJ mol-1

Heptane C7H16 -4817

Octane C8H18 -5470

Nonane C9H20 -6125

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Heptane C7H16

Octane C8H18

Nonane C9H20

16

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C7H16 + 11O2 7CO2 + 8H20

C8H18 + 25/2O2 8CO2 + 9H20

C9H20 + 14O2 9CO2 + 10H20

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

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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

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d) When 10 cm3 of ethanol was burned under a container of water, it was found that 1kg of water was heated from 25oC to 65oC. The density of ethanol is 0.79 g cm-3

The process was known to be only 70% efficient.

Use these data and values from the Data Booklet to calculate the enthalpy change of combustion of ethanol.

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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

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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

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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