# Heat of combustion

## Tabulated values of heat of combustion (= energy content) of common substances, together with examples showing how to calculate the heat of combustion

Go to tabulated values

Standard heat of combustion: The energy liberated when a substance X undergoes complete combustion, with excess of oxygen at standard conditions (25°C and 1 bar). In thermodynamical terms it is the negative of the enthalpy change for the combustion reaction.

nX + mO2  →  xCO2 (g) + yH2O (l) + zZ + heat of combustion

where Z is any other products formed during the reaction and n, m, x, y and z are the number of moles of each molecule in the balanced equation.

The heat of combustion is utilised to quantify the performance of a fuel in a combustion system such as furnaces, motors and power generation turbines. It is the same as the gross heating value or energy content.

Generally, the heat of combustion can be expressed as the following:

ΔHc° = -xΔHf°(CO2,g) - yΔHf°(H2O,l) - zΔHf°(Z) + nΔHf°(X) + mΔHf°(O2,g)

where    ΔHc° : heat of combustion at standard conditions (25°C and 1 bar)

ΔHf° : heat (enthalpy) of formation at standard conditions (25°C, 1 bar)

Then, the heat of combustion can be calculated from the standard enthalpy of formation (ΔHf°) of the substances involved in the reaction, given as tabulated values.

For compounds containing carbon, hydrogen and oxygen (as many organic compounds do), a general combustion reaction equation will be:

CaHbOc  +  (a + ¼b - ½c) O2  → aCO2 (g) + ½bH2O (l) + heat of combustion

and the heat of combustion can be calculated from the standard heat of formation of all compounds taking part in the reaction:

ΔHc° = -aΔHf°(CO2,g) - ½b ΔHf°(H2O,l) + ΔHf°(CaHbOc) + (a + ¼b - ½c) ΔHf°(O2,g)  = -a(- 393.51) - ½b(-285.830) + ΔHf°(CaHbOc) + (a + ¼b - ½c)*0

= a(393.51) + b(142.915) + ΔHf°(CaHbOc)

Example:

What is the heat of comustion of liquid ethanol, with the formula C2H5OH (=C2H6O)?

For ethanol, the constants a, b and c are 2, 6 and 1, respectively, and the chemical equation for the combustion of ethanol:

C2H6O(l) + 3O2(g) → 2CO2(g)+ 3H2O(l)

The standard heat of formation of liquid ethanol, ΔHf°(C2H6O, l), is -277.6 kJ/mol.

The heat of combustion of ethanol, ΔHc°(C2H6O, l) = 2*393.51 + 6*142.915 + (-277.6) = 1366.91 kJ/mol. This can be converted to kJ per mass units:

The molweight of ethanol is (2*12.01 + 6*1.01 + 1*16.00) = 46.08 g/mol

The heat of combustion of ethanol, ΔHc°(C2H6O, l) = 1366.91[kJ/mol] *1000[g/kg] / 48.08 [g/mol] = 29664 kJ/kg ethanol  = 29.7 MJ/kg = 12754 BTU/lb = 7086 kcal/kg

The table below shows values of heat of combustion calculated after the above described method. For substances containing nitrogen, it is assumed that the nitrogen atoms ends up as N2 gas with ΔHf°(N2) = 0 kJ/mol. In such cases the general equation applies to also these substances. If it is known that other substances is formed in the cumbustion reactions, the exact products must be known to be able to calculate the heat of combustion. Conversion to other units.

 Name Formula State* ΔHc°kJ/mol ΔHc° kJ/g or MJ/kg ΔHc° kJ/kg ΔHc° BTU/lb ΔHc° kcal/kg Acetaldehyde C2H4O liq 1167 26.49 26487 11387 6326 Acetamide C2H5NO cry 1185 20.06 20058 8623 4791 Acetic acid C2H4O2 liq 874 14.55 14552 6256 3476 Acetone C3H6O liq 1790 30.81 30814 13248 7360 Acetonitrile C2H3N liq 1256 30.59 30589 13151 7306 Acetylene (ethyne) C2H2 gas 1300 49.92 49923 21463 11924 l-Alanine C3H7NO2 cry 1577 17.70 17697 7608 4227 Ammonia NH3 gas 383 22.48 22477 9663 5369 Aniline C6H7N liq 3393 36.43 36429 15662 8701 Anthracene C14H10 cry 7068 39.65 39654 17048 9471 Benzene C6H6 liq 3268 41.83 41833 17985 9992 Benzoic acid C7H6O2 cry 3228.2 26.43 26432 11364 6313 1,3-Butadiene C4H6 gas 2542 46.99 46987 20201 11223 Butane C4H10 gas 2878 49.50 49501 21282 11823 1-Butanol C4H10 liq 2676 36.09 36092 15517 8621 2-Butanone C4H8O liq 2444 33.89 33888 14569 8094 1-Butene C4H8 gas 2718 48.43 48432 20822 11568 cis-2 Butene C4H8 gas 2710 48.29 48289 20761 11534 trans-2-Butene C4H8 gas 2706 48.22 48218 20730 11517 Butanoic acid C4H8O2 liq 2183.6 38.91 38909 16728 9293 Butylbenzene C10H14 liq 5872.7 104.65 104646 44990 24994 Carbon (graphite) C cry 394 32.81 32806 14104 7836 Carbon monoxide CO gas 283 10.10 10104 4344 2413 Cyclobutane C4H8 gas 2745.1 48.91 48914 21029 11683 Cyclobutene C4H6 gas 2588 47.84 47837 20566 11426 Cyclohexane C6H12 liq 3920 46.57 46567 20020 11122 Cyclopentane C5H10 liq 3291.6 46.92 46922 20173 11207 Cyclopropane C3H6 gas 2091 49.68 49679 21358 11866 Decane C10H22 liq 6778 47.63 47625 20475 11375 Diethyl ether (ethoxyethane) C4H10O liq 2724 36.74 36741 15796 8775 Dimethyl ether (methoxymethane) C2H6O gas 1460 31.68 31684 13622 7568 Ethane C2H6 gas 1561 51.89 51895 22311 12395 1,2-Ethanediol C2H6O2 liq 1185 19.09 19088 8206 4559 Ethanol C2H6O liq 1367 29.67 29666 12754 7086 Ethyl acetate C4H8O2 liq 2238 25.40 25397 10919 6066 Ethylene (ethene) C2H4 gas 1411 50.29 50285 21619 12010 Formaldehyde CH2O gas 571 19.01 19014 8175 4541 Formic acid CH2O2 liq 254 5.52 5518 2372 1318 Glycerol C3H8O3 liq 1654 17.96 17957 7720 4289 Heptane C7H16 liq 4817 48.06 48059 20662 11479 Heptanoic acid C7H14O2 liq 4145.2 31.83 31835 13687 7604 Hexane C6H14 liq 163 1.89 1891 813 452 Hexadecanoic acid C16H32O2 liq 10031.3 39.11 39112 16815 9342 Hexanoic acid C6H12O2 liq 3492.2 30.06 30059 12923 7179 Hydrazine N2H4 liq 622 19.40 19401 8341 4634 Hydrogen H2 gas 286 141.58 141584 60870 33817 Hydrogen cyanide CHN gas 672 24.86 24861 10688 5938 Ketene C2H2O gas 1025 24.38 24382 10482 5824 Methane CH4 gas 891 55.51 55514 23867 13259 Methanol CH4O liq 726 22.65 22652 9739 5410 Methyl acetate C3H6O2 liq 1592 21.49 21487 9238 5132 Methylamine CH5N gas 1086 34.95 34953 15027 8348 Methylcyclohexane C7H14 liq 4565.3 46.48 46485 19985 11103 Methylcyclopentane C6H12 liq 3938.1 46.78 46782 20113 11174 Methyl formate C2H4O2 liq 973 16.20 16200 6965 3869 Methyl tert-butyl ether C5H12O liq 3369 38.21 38209 16427 9126 Naphthalene C10H8 cry 5157 40.23 40232 17294 9609 Nitric oxide NO gas 91 3.03 3032 1304 724 Nitrobenzene C6H5NO2 liq 3088 25.08 25081 10783 5990 Nitromethane CH3NO2 liq 710 11.63 11630 5000 2778 Nitrous oxide N2O gas 82 1.86 1863 801 445 Nonane C9H20 liq 6125 47.74 47743 20526 11403 Octane C8H18 liq 5470 47.87 47873 20582 11434 1-Octanol C8H18O liq 5294 40.64 40642 17473 9707 Pentane C5H12 liq 3509 48.62 48621 20903 11613 Pentanoic acid C5H10O2 liq 2837.3 27.78 27776 11942 6634 1-Pentanol C5H12O liq 3331 37.78 37779 16242 9023 Phenanthrene C14H10 cry 7055 39.58 39581 17017 9454 Phenol C6H6O cry 3054 32.45 32448 13950 7750 Propanal C3H6O liq 1522 26.20 26201 11264 6258 Propane C3H8 gas 2220 50.33 50329 21638 12021 Propanenitrile C3H5N liq 1911 34.69 34689 14914 8285 Propanoic acid C3H6O2 liq 1527.3 20.61 20614 8862 4924 1-Propanol C3H8O liq 2021 33.62 33622 14455 8030 2-Propanol C3H8O liq 2006 33.37 33372 14347 7971 Propene C3H6 gas 2058 48.90 48895 21021 11678 Propyne C3H4 gas 185.0 4.62 4617 1985 1103 Pyridine C5H5N liq 2782 35.17 35166 15119 8399 Toluene (methylbenzene) C7H8 liq 3910 42.43 42431 18242 10134 Trimethylamine C3H9N gas 2443 41.32 41316 17763 9868 2,4,6-Trinitrotoluene C7H5N3O6 cry 3406 14.99 14994 6446 3581 Undecane C11H24 liq 7431.4 47.53 47530 20434 11352 Urea CH4N2O cry 632.7 10.53 10533 4528 2516 *  gas = gas,  liq = liquid,  cry = crystalline (solid)

Unit conversion

• 1 kJ/kg = 1 J/g = 10-3 GJ/tonne = 0.000278 kWh/kg = 0.4299 Btu/ lbm = 0.23884 kcal/kg
• 1 Btu/lbm = 2.326 kJ/kg = 0.55 kcal/kg
• 1 kcal/kg = 4.1868 kJ/kg = 1.8 Btu/lbm

## Related Topics

• Combustion - Boiler house topics - fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency
• Thermodynamics - Effects of work, heat and energy on systems
• Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more

## Search the Engineering ToolBox

- "the most efficient way to navigate!"

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!