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CHEMICAL ENGINEERING
3.12. HEATS OF COMBUSTION
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The heat of combustion of a compound H is the standard heat of reaction for complete
c
combustion of the compound with oxygen. Heats of combustion are relatively easy to
determine experimentally. The heats of other reactions can be easily calculated from the
heats of combustion of the reactants and products.
The general expression for the calculation of heats of reaction from heats of
combustion is
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H D H , reactants H , products 3.27
r c c
Note: the product and reactant terms are the opposite way round to that in the expression
for the calculation from heats of formation (equation 3.26).
For compounds containing nitrogen, the nitrogen will not be oxidised to any significant
extent in combustion and is taken to be unchanged in determining the heat of combustion.
Caution. Heats of combustion are large compared with heats of reaction. Do not round
off the numbers before subtraction; round off the difference.
Two methods of calculating heats of reaction from heats of combustion are illustrated
in Example 3.9.
Example 3.9

Calculate the standard heat of reaction for the following reaction: the hydrogenation of
benzene to cyclohexane.
(1) C 6 H 6 (g) C 3H 2 (g) ! C 6 H 12 (g)
1
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(2) C 6 H 6 (g) C 7 O 2 (g) ! 6CO 2 (g) C 3H 2 O(l) H D 3287.4kJ
2 c
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(3) C 6 H 12 (g) C 9O 2 ! 6CO 2 (g) C 6H 2 O(l) H D 3949.2kJ
c
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(4) C(s) C O 2 (g) ! CO 2 (g) H D 393.12 kJ
c
1
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(5) H 2 (g) C O 2 (g) ! H 2 O(l) H D 285.58 kJ
2 c
Note: unlike heats of formation, the standard state of water for heats of combustion is
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liquid. Standard pressure and temperature are the same 25 C, 1 atm.
Solution
Method 1
Using the more general equation 3.26
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H D H , products H reactants
r f f
the enthalpy of formation of C 6 H 6 and C 6 H 12 can be calculated, and from these values
the heat of reaction (1).
From reaction (2)
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H C 6 H 6 D 6 ð H CO 2 C 3 ð H H 2 O H C 6 H 6
c c c f
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3287.4 D 6 393.12 C 3 285.58 H C 6 H 6
f
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H C 6 H 6 D 3287.4 3215.52 D 71.88 kJ/mol
f