Q6
1

Bond Energy is the average energy required
to break 1 mole of a covalent bond in the
gaseous state.
2
2C8H18 + 25O2  16CO2 + 18H20(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
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
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 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
9
2C8H18 + 25O2  16CO2 + 18H20(g)

ΔHcø 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
1
0


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
11
Alkane
Formula
ΔHc / kJ mol-1
Heptane
C7H16
-4817
Octane
C8H18
-5470
Nonane
C9H20
-6125
Suggest a reason for the discrepancy
between the ΔHc for octane you
calculated in (b)(ii) and that given in the
table
12

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

Alkane
Formula
ΔHc / kJ mol-1
Heptane
C7H16
-4817
Octane
C8H18
-5470
Nonane
C9H20
-6125
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

Heptane C7H16

Octane C8H18

Nonane C9H20
15
16
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
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
19


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