Standard Enthalpy of Formation

Standard enthalpy of formation (ΔH0f)

Definition: One mole of the compound is formed from its elements in their standard states. For example, sodium chloride (NaCl) is formed from Na and Cl2 as follows:

Na(s) + ½ Cl2 (g) → NaCl(s) ΔH0f = -411 Kj mol-1

Standard enthalpy of atomisation (ΔH0atom)

Definition: One mole of isolated atoms in the gas phase is made from an element in its standard state. For example, Na is a solid at room temperature and requires 109kj to become a gas as shown below:

Na(s) → Na(g) ΔH0atom  = +109 Kj mol-1

From example, Cl2 is already a gas; however chlorine is present as molecules, not as atoms as shown below:

½ Cl2 (g) → Cl (g) ΔH0atom = +121 Kj mol-1