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- Q721706 subject Q7031259.
- Q721706 subject Q7213730.
- Q721706 abstract "The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the compound from its constituent elements, with all substances in their standard states at 1 atmosphere (1 atm or 101.3 kPa). Its symbol is ΔHfO or ΔfHO. The superscript theta (zero) on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 degrees Celsius or 298.15 K). Standard states are as follows:For a gas: the standard state is a pressure of exactly 1 atmFor a solute present in an ideal solution: a concentration of exactly one mole/liter (M) at a pressure of 1 atmFor a pure substance or a solvent in a condensed state (a liquid or a solid): the standard state is the pure liquid or solid under a pressure of 1 atmFor an element: the form in which the element is most stable under 1 atm of pressure. One exception is phosphorus, for which the most stable form at 1 atm is black phosphorus, but white phosphorus is chosen as the standard reference state 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)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, usually stated in kilojoule per mole (kJ mol−1), but also in calorie per mole, joule per mole or kilocalorie 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.The formation reaction is a constant pressure and constant temperature process. Since the pressure of the standard formation reaction is fixed at 1 atm, the standard formation enthalpy or reaction heat is a function of temperature. For tabulation purposes, standard formation enthalpies are all given at a single temperature: 298 K, represented by the symbol ΔHf298O.".
- Q721706 thumbnail Born-haber_cycle_LiF.svg?width=300.
- Q721706 wikiPageExternalLink chemistry.
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- Q721706 comment "The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the compound from its constituent elements, with all substances in their standard states at 1 atmosphere (1 atm or 101.3 kPa). Its symbol is ΔHfO or ΔfHO. The superscript theta (zero) on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 degrees Celsius or 298.15 K).".
- Q721706 label "Standard enthalpy of formation".
- Q721706 depiction Born-haber_cycle_LiF.svg.