The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of the compound from its elements, with all substances in their standard states. Its symbol is ΔHf
O or ΔfH O. The superscript theta (zero) on this symbol indicates that the process has been carried out under standard conditions. Standard States are as follows:
- For a gas: standard state is a pressure of exactly 1 bar
- For a substance present in a solution: a concentration of exactly 1 M at a pressure of 1 bar
- For a pure substance in a condensed state (a liquid or a solid): the pure liquid or solid under a pressure of 1 bar
- For an element: the form in which the element is most stable under 1 bar of pressure and the specified temperature. (Usually 25 degrees Celsius or 298.15 K) One exception is phosphorus: most stable under 1 bar is black phosphorus, but white phosphorus is used as the reference for zero enthalpy of formation
For example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the following reaction under the conditions above:
- C(s,graphite) + O2(g) → CO2(g)
Note that all elements are written in their standard states, and one mole of product is formed. This is true for all enthalpies of formation.
The standard enthalpy of formation is measured in units of energy per amount of substance. Most are defined in kilojoules per mole (kJ mol−1), but can also be measured in calories per mole, joules per mole or kilocalories per gram (any combination of these units conforming to the energy per mass or amount guideline). In physics the energy per particle is often expressed in electronvolts which corresponds to about 100 kJ mol−1.
All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.
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