Standard Enthalpy of Formation
The enthalpy change when one mole of a compound forms from its elements in their standard states.
Formula / equation
ΔH_f°
Unit
kJ/mol
In depth
By definition the value is zero for any element in its standard state, which provides the baseline for the entire thermochemical database. Combining tabulated formation enthalpies as products minus reactants gives the enthalpy of any reaction without further measurement.
Examples in the real world
CO₂ has ΔH_f° = −393.5 kJ/mol; a strongly positive value, as for NO at +90 kJ/mol, indicates an unstable compound.