Standard Enthalpy and Enthalpy of Formation

[Kyleigh Follis 2H]

What is the difference in units between the standard enthalpy and the enthalpy of formation?

[Luc Lorain 1L]

In short, yes.

The standard (molar) enthalpy of formation, _f^o, is defined as the change in enthalpy as we form one mol of a substance in its standard state from elements in their standard state. (Aside: standard state just means that the element is without impurities and at 1 atm of pressure and 298.15 K). Every substance has exactly one standard enthalpy of formation for a given phase of matter, making them useful constants when calculating overall standard enthalpy of a reaction. To do this, simply subtract follow this formula: , where n= the amount of moles of each substance. The standard enthalpy of formation will usually be provided in J or kJ per mol.

Since standard enthalpy change of a reaction is derived from mols * J/mol units, it will usually be provided in just J (or kJ, depending on the units used with standard enthalpy of formation.