Heat of Formation
The heat of formation is the heat that is absorbed when a substance is formed from its elements. In common usage, the heat of formation is used in place of the more precise term the enthalpy of formation, which has the symbol ()Hf. The enthalpy of formation of a substance is the heat of reaction for the combination of elements in their standard (normal) physical states to form one mole of the substance, but only under three specific conditions: (1) the pressure remains constant (2) the only possible work that occurs is expansion against the atmosphere (so- called P()V work) and (3) the temperature remains constant during the process.
In practice, a formation reaction is likely to result in an increase or decrease in temperature (reactions that occur with an evolution of heat are called exothermic and reactions that require heat are termed endothermic). In addition, it is generally easier experimentally to use a sealed container of constant volume to carry out formation reactions than to attempt to construct an apparatus that would maintain constant pressure. Therefore, the heat of formation is not generally measured directly. Instead, it is calculated from other quantities that are directly measured.
Most reactions by which compounds are formed directly from their constituent elements are very difficult or impossible to carry out directly under conditions that allow careful measurements to be taken. In most cases, however, the heats of formation can be obtained indirectly from measurements of other reactions. Because the heat of formation 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 formation 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 formation are often specifically designated to show the conditions under which they apply. The temperature for which the heat of formation applies should always be designated. A subscript indicating the temperature on the Kelvin or absolute scale is generally used, for example, ()Hf, 300 for a reaction at 300 K. If no temperature is indicated, it is assumed that the temperature is 298 K or 25°C. If the heat of formation 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 atmosphere pressure, it is called a standard heat of formation, designated by a superscript "o", for example ()Hof, 300. The units typically used for heats of solution are kilojoules per mole or kilocalories per mole.
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