Standard enthalpy of formation

In <a href="/facts/Chemistry/SrNqkGVm">chemistry</a> and <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, the standard enthalpy of formation or standard heat of formation of a <a href="/facts/Chemical_compound/37FXqqtv">compound</a> is the change of <a href="/facts/Enthalpy/bBou8lCE">enthalpy</a> during the formation of 1 <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a> of the substance from its constituent <a href="/facts/Chemical_element/0jducgWD">elements in their reference state</a>, with all substances in their <a href="/facts/Standard_state/adrN3C0a">standard states</a>. The standard <a href="/facts/Pressure/eyc37GRG">pressure</a> value p⦵ = 105 <a href="/facts/Pascal_(unit)/Am1To2Qm">Pa</a> (= 100 kPa = 1 <a href="/facts/Bar_(unit)/wwFWTTkT">bar</a>) is recommended by <a href="/facts/IUPAC/2AV6myCK">IUPAC</a>, although prior to 1982 the value 1.00 <a href="/facts/Atmosphere_(unit)/aTbYALII">atm</a> (101.325 kPa) was used. There is no standard temperature. Its symbol is ΔfH⦵. The superscript <a href="/facts/Standard_state/adrN3C0a">Plimsoll</a> on this symbol indicates that the process has occurred under standard conditions at the specified temperature (usually 25 °C or 298.15 K).
Standard states are defined for various types of substances. For a gas, it is the hypothetical state the gas would assume if it obeyed the <a href="/facts/Ideal_gas_equation/Pphokwhf">ideal gas equation</a> at a pressure of 1 bar. For a gaseous or solid <a href="/facts/Solute/MGchhMQV">solute</a> present in a diluted <a href="/facts/Ideal_solution/Ho6rPlcs">ideal solution</a>, the standard state is the hypothetical state of concentration of the solute of exactly one mole per liter (1 <a href="/facts/Molar_concentration/P2WL5b9q">M</a>) at a pressure of 1 bar extrapolated from infinite dilution. For a pure substance or a <a href="/facts/Solvent/LoMPMz8S">solvent</a> in a condensed state (a liquid or a solid) the standard state is the pure liquid or solid under a pressure of 1 bar.
For elements that have multiple <a href="/facts/Allotropy/2WUdHWxg">allotropes</a>, the reference state usually is chosen to be the form in which the element is most stable under 1 bar of pressure. One exception is <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a>, for which the most stable form at 1 bar is <a href="/facts/Black_phosphorus/GarX3byj">black phosphorus</a>, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation.
For example, the standard enthalpy of formation of <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> is the enthalpy of the following reaction under the above conditions:

C
          
            (
            s
            ,
            graphite
            )
          
          +
          
            O
            
              2
            
            
              
            
          
          
            (
            g
            )
          
          ⟶
          
            CO
            
              2
            
            
              
            
          
          
            (
            g
            )
          
        
      
    
    {\displaystyle {\ce {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 <a href="/facts/Joule_per_mole/MCWSGtRG">kilojoule per mole</a> (kJ mol−1), but also in <a href="/facts/Kilocalorie_per_mole/hBcdPkEY">kilocalorie per mole</a>, <a href="/facts/Joule/9ndq9jD2">joule</a> per mole or kilocalorie per <a href="/facts/Gram_(unit)/sRSW5RJb">gram</a> (any combination of these units conforming to the energy per mass or amount guideline).
All elements in their reference states (<a href="/facts/Oxygen/acyn6o8r">oxygen</a> gas, solid <a href="/facts/Carbon/ukrgQexo">carbon</a> in the form of <a href="/facts/Graphite/sSf3KMXr">graphite</a>, 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 bar, 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 ΔfH⦵298 K.

Standard enthalpy of formation open-in-new

Standard enthalpy of formation