Download Heat of Combustion

HEAT OF COMBUSTION
The heat of combustion of a substance at 25°C can be calculated from the enthalpy of formation (∆fH°) data in the table “Standard Thermodynamic
Properties of Chemical Substances” in this Section. We can write the general combustion reaction as
X + O 2 → CO 2 (g) + H 2 O(l) + other products
For a compound containing only carbon, hydrogen, and oxygen, the reaction is simply
1
1 
1

C a H b O c + a + b – c O 2 → a CO 2 (g) + b H 2 O(l)

4
2 
2
and the standard heat of combustion ∆cH°, which is defined as the negative of the enthalpy change for the reaction (i.e., the heat released in the
combustion process), is given by
1
∆ c H ° = – a ∆ f H °(CO 2 , g) – b ∆ f H °(H 2 O, l) + ∆ f H °(C a H b O c )
2
393.51 a + 142.915 b + ∆ f H °(C a H b O c )
This equation applies if the reactants start in their standard states (25°C and one atmosphere pressure) and the products return to the same conditions.
The same equation applies to a compound containing another element if that element ends in its standard reference state (e.g., nitrogen, if the product
is N2); in general, however, the exact products containing the other elements must be known in order to calculate the heat of combustion.
The following table gives the standard heat of combustion calculated in this manner for a few representative substances.
Molecular
formula
Name
∆cH°/kJ mol–1
Inorganic substances
C
CO
H2
H3N
H4N2
N2O
Carbon (graphite)
Carbon monoxide (g)
Hydrogen (g)
Ammonia (g)
Hydrazine (g)
Nitrous oxide (g)
393.5
283.0
285.8
382.8
667.1
82.1
Hydrocarbons
CH4
C2H2
C2H4
C2H6
C3H6
C3H6
C3H8
C4H6
C4H10
C5H12
C6H6
C6H12
C6H14
C7H8
C7H16
C10H8
Methane (g)
Acetylene (g)
Ethylene (g)
Ethane (g)
Propylene (g)
Cyclopropane (g)
Propane (g)
1,3-Butadiene (g)
Butane (g)
Pentane (l)
Benzene (l)
Cyclohexane (l)
Hexane (l)
Toluene (l)
Heptane (l)
Naphthalene (s)
890.8
1301.1
1411.2
1560.7
2058.0
2091.3
2219.2
2541.5
2877.6
3509.0
3267.6
3919.6
4163.2
3910.3
4817.0
5156.3
Alcohols and ethers
CH4O
C2H6O
C2H6O
C2H6O2
Methanol (l)
Ethanol (l)
Dimethyl ether (g)
Ethylene glycol (l)
726.1
1366.8
1460.4
1189.2
Molecular
formula
C3H8O
C3H8O3
C4H10O
C5H12O
C6H6O
Name
1-Propanol (l)
Glycerol (l)
Diethyl ether (l)
1-Pentanol (l)
Phenol (s)
∆cH°/kJ mol–1
2021.3
1655.4
2723.9
3330.9
3053.5
Carbonyl compounds
CH2O
C2H2O
C2H4O
C3H6O
C3H6O
C4H8O
Formaldehyde (g)
Ketene (g)
Acetaldehyde (l)
Acetone (l)
Propanal (l)
2-Butanone (l)
570.7
1025.4
1166.9
1789.9
1822.7
2444.1
Acids and esters
CH2O2
C2H4O2
C2H4O2
C3H6O2
C4H8O2
C7H6O2
Formic acid (l)
Acetic acid (l)
Methyl formate (l)
Methyl acetate (l)
Ethyl acetate (l)
Benzoic acid (s)
254.6
874.2
972.6
1592.2
2238.1
3226.9
Nitrogen compounds
CHN
CH3NO2
CH5N
C2H3N
C2H5NO
C3H9N
C5H5N
C6H7N
Hydrogen cyanide (g)
Nitromethane (l)
Methylamine (g)
Acetonitrile (l)
Acetamide (s)
Trimethylamine (g)
Pyridine (l)
Aniline (l)
671.5
709.2
1085.6
1247.2
1184.6
2443.1
2782.3
3392.8