Heat of Combustion | Research & Encyclopedia Articles

Heat of Combustion

The heat of combustion is more precisely termed the enthalpy of combustion and is given the symbol H_comb. The term heat of combustion is typically used interchangeably with the more precise term the enthalpy of combustion. The enthalpy of combustion of the heat of reaction for the combination of one mole of a substance with oxygen to form oxidized forms of the elements of the substance, such as carbon dioxide from carbon atoms, water from hydrogen atoms and sulfur dioxide from sulfur atoms. As is the case for other types of enthalpy, the heat of combustion is the heat absorbed for one mole of the particular substance that undergoes the reaction (combustion) under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called PV work) and (3) the temperature remains constant during the process.

In practice, combustion reactions cause an increase in temperature. In addition, it is experimentally easier to use a sealed container of constant volume to carry out combustion reactions than to attempt to construct an apparatus that would maintain constant pressure. Therefore, the heat of combustion is not generally measured directly. Instead, it is calculated from other quantities that are directly measured.

In a typical experiment to determine the heat of combustion, a known amount of substance would be combusted in a sealed container (a bomb calorimeter) submerged in a well-insulated water bath. As the reaction occurs, the heat released from the combustion would increase the temperature of the water and the bomb calorimeter itself. At the same time, the pressure within the bomb calorimeter would change due to the temperature change and a change in the number of moles of gas contained within it, according to the stoichiometry of the combustion reaction. The energy change for the reaction is calculated from the temperature change and heat capacities of the water bath and calorimeter minus whatever energy was used to start the combustion reaction. The heat of combustion is then calculated by adding the equivalent of the PV work, in this process actually nRT from the Ideal Gas Law.

The heat of combustion experimentally determined for an actual combustion reaction is the same as the enthalpy for the reaction of the substance with oxygen under any conditions for which the same chemical equation pertains. This means that the enthalpy for the metabolism of foods, such as fats and sugars, is the same as the heat of combustion measured experimentally.

Many combustion reactions are very difficult or impossible to directly carry out under conditions that allow careful measurements to be taken. Often, however, the heats of combustion can be obtained indirectly from measurements of other reactions. Because the heat of combustion is an enthalpy of reaction, Hess's Law applies: when the equation for an reaction can be obtained by algebraically combining a series of other reactions, the enthalpy for the reaction can be obtained by algebraically combining the other equations in the same manner. For example, if reaction 1 and reaction 2 can be added to obtain the equation for the desired reaction, a combustion reaction that cannot be measured directly, the enthalpy of the desired reaction is the sum of the enthalpies of reactions 1 and 2.

Heats of combustion are often specifically designated to show the conditions under which they apply. The temperature for which the heat of combustion applies should always be designated. A subscript indicating the temperature on the Kelvin or absolute scale is generally used, for example, H_{comb, 400} for a reaction at 400 K. If no temperature is indicated, it is assumed that the temperature is 298 K or 25 C. If the heat of combustion corresponds to a reaction in which all of the reactants and products are in their standard physical states, solid, liquid, or gas, at the stated temperature and they are present in standard concentrations, one molar for solutions, pure solids and gases at one bar pressure, it is called a standard heat of combustion, designated by a superscript "o", for example H^o_{comb, 400}. The units typically used for heats of solution are kilojoules per mole or kilocalories per mole.