Intermediate Topic 349 of 1102 Thermodynamics & Energetics
Enthalpy of Combustion
Heat released when one mole of a substance burns completely in oxygen.
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In-depth explanation of Enthalpy of Combustion
The enthalpy of combustion is the heat released when one mole of a substance is burned completely in excess oxygen, with carbon forming carbon dioxide and hydrogen forming liquid water. It is always negative because combustion is exothermic. Combustion enthalpies are measured with bomb or flame calorimeters and are used to compare the energy content of fuels, since a larger magnitude per mole or per gram means more useful heat. They also provide an indirect route to formation enthalpies, because formation values can be derived through Hess's law when the combustion values of the elements and compound are known. Such values underpin much of practical energy planning.
Going deeper: Enthalpy of Combustion
Key ideas: Write the balanced combustion with one mole of fuel and products CO2 gas and liquid water. The value is negative, and heat per gram equals the molar value divided by molar mass. Compare fuels per gram for mass-limited uses and per litre for volume-limited uses.
Common mistakes and connections: Do not forget that water vapor instead of liquid gives a smaller magnitude, the difference between higher and lower heating values. Incomplete combustion to CO or soot releases less heat and is not the standard reaction. Combustion enthalpies connect to calorific values, energy density and, through Hess's law, to formation enthalpies.
Visualization of Enthalpy of Combustion
Standard enthalpy of combustion per mole. Larger hydrocarbons release more energy per mole as more C-H and C-C bonds are converted to CO2 and H2O. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The standard enthalpy of combustion of octane, a major component of gasoline, is about -5470 kJ per mole.
Frequently asked questions about Enthalpy of Combustion
What is Enthalpy of Combustion?
Heat released when one mole of a substance burns completely in oxygen.
Is Enthalpy of Combustion a beginner, intermediate or advanced topic?
Enthalpy of Combustion is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Enthalpy of Combustion?
Study these earlier topics in Thermodynamics & Energetics first: Thermochemical Equation, Standard Enthalpy of Formation, Standard State.
What comes after Enthalpy of Combustion?
The next topics in Thermodynamics & Energetics are Enthalpy of Neutralization and Enthalpy of Solution.
Can I practise Enthalpy of Combustion with quizzes?
Yes. The interactive lesson includes a self-check question on Enthalpy of Combustion, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Enthalpy of Combustion fits in Thermodynamics & Energetics
Enthalpy of Combustion is topic 21 of 91 in Thermodynamics & Energetics.