Standard enthalpy of formation ($\Delta H_{\mathrm{f}}^{\circ }$)

The standard enthalpy of formation is the enthalpy change for forming one mole of a specified chemical substance from its chemical elements in their reference states at the stated temperature. For sodium chloride at $298.15\mathrm{K}$, those reactant states are solid sodium and diatomic gaseous chlorine. Standard state specifies conditions; it does not by itself select a temperature.