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Heat of combustion is a term that describes the amount of thermal energy, or heat, generated when a particular substance is burned completely in an oxygen rich environment. It is expressed as a ratio of energy per unit of the substance and may be written in a number of ways. The standard expression is kilojoules (kJ) per mole (mol) of the substance, but it may also be expressed in terms of energy released per given volume or given mass of the fuel. Depending on the part of the world, this expression may be written in a number of ways, such as kilocalories per gram or British thermal units per pound.
The energy of heat is stored as potential energy in certain types of molecules. In hydrocarbons, burning them releases this stored energy as heat and light. The amount of heat energy released is the heat of combustion, and it does not include the energy released as light. This term can describe the amount of this energy released by the burning of any material, but is generally assumed to apply to hydrocarbons, particularly those used as fuels.
A term used to describe the total amount of energy in a closed system, enthalpy, is sometimes used as part of the definition for heat of combustion. The phrase is sometimes referred to as a negative change in the total enthalpy of a system. This means that the system lost energy through the release of heat due to the combustion of some component of the system. While such a change of enthalpy is referred to as having a negative value, the measurement is always given as a positive value.
When discussing fuels, heat of combustion ratings are often used as a way to compare the relative heat energy contained in different fuels. Different fuels will have distinct figures, which will not change for any pure sample of the same fuel but will vary from one fuel to another. For example, the hydrocarbon octane contains 5,460 kJ/mol while methane contains only 890 kJ/mol. Generally, the more complex and the larger a hydrocarbon molecule is the higher the figure.
A device called a calorimeter is used to test a material to determine its heat of combustion. There are several types of calorimeters, but they all operate on the same principle. A sample is burned in a closed, controlled environment, and the amount of heat released by the combustion of the sample is measured. By relating the amount of heat released to the size of the sample, a figure is attained. Is is not only fuels that are be tested this way, however, and many other substances are tested by chemists, researchers, and students.