Heat of combustion is the energy released when a substance is burned in an oxygen-rich environment, expressed as kJ/mol or energy per volume/mass. Hydrocarbons release stored energy as heat and light. Enthalpy is sometimes used in the definition. Heat of combustion values are used to compare fuels, measured using a calorimeter.
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 the ratio of energy per unit of substance and can be written in several ways. The standard expression is kilojoules (kJ) per mole (mol) of the substance, but it can also be expressed in terms of the energy released per given volume or given mass of the fuel. Depending on the part of the world, this expression can be written in different ways, such as kilocalories per gram or British thermal units per pound.
Heat energy is stored as potential energy in certain types of molecules. In hydrocarbons, their combustion releases this stored energy in the form of heat and light. The amount of thermal energy released is the heat of combustion and does not include the energy released in the form of light. This term can describe the amount of this energy released by the combustion of any material, but it is generally assumed to apply to hydrocarbons, especially 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 of 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 has lost energy through the release of heat due to the combustion of some system components. While such an enthalpy change is given as a negative value, the measurement is always given as a positive value.
When discussing fuels, heat of combustion values 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, hydrocarbon octane contains 5.460 kJ/mol while methane contains only 890 kJ/mol. Generally, the more complex and larger a hydrocarbon molecule, 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 work on the same principle. A sample is burned in a closed, controlled environment and the amount of heat released by the burning of the sample is measured. Relating the amount of heat released to the sample size gives a figure. However, not only fuels are tested in this way, and many other substances are tested by chemists, researchers and students.
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