Sometimes the value for the bond dissociation enthalpy is instead given. Note that this bond dissociation enthalpy refers to one bond, so is double the value of the atomisation enthalpy as shown below:
Cl2 (g) → 2 Cl (g) ΔH0B.E. = +242 Kj mol-1
First ionisation enthalpy (ΔH0ion)
Definition: The first ionisation enthalpy (AHt.) is the energy required to remove one electron from each atom to form an ion. The standard refers to one mole of atoms in the gas phase.
Na(g) → Na+ (g) + e– ΔH0ion = +492 kj.mol-1
Second ionisation enthalpy
If the metal M concerned forms M2+ ions, then the calculation of ionisation enthalpy needs to be done in two stages. The second ionisation enthalpy (ΔHoion) is the energy required to remove a second electron from the M+ ion to form an M2+ ion.
Mg+ (g) Mg2+ (g) + e– ΔH0ion = +4560 kJ mol-1
