Ionisation energy of a species is defined as the energy for removing an electron from the ground state of the species. According to statistical thermodynamics, the ground state of a species is the electronic configuration of that species at absolute zero. If ionisation energy is defined at any other temperatures above absolute zero, there will be a range of ionisation energies for a particular species, as the electron then is removed from various excited states. Hence, ionisation energy is equal to the difference between the sum of enthalpies of formation of the ionised species and the electron at absolute zero, and the enthalpy of formation of the species at absolute zero.
Ionisation energies are presented in data tables using eq73 of the previous article, where is the experimental temperature and
is absolute zero.
Question
Calculate the first ionisation enthalpy of magnesium at 298.15K, given IE1 = 738 kJmol-1 and assuming the heat capacities at constant pressure of all species (including electron) are given by that of a perfect monoatomic gas of .
Answer
Using eq73,
With reference to the above example where the heat capacities at constant pressure of all species (including electron) are given by that of a perfect monoatomic gas of ,
where is at absolute zero.
Similarly,
Therefore, the ionisation enthalpy of is
Electron affinity is also a zero Kelvin process, as it is defined as the energy for the addition of an electron to a species in its ground state. Although an exothermic process, electron affinity (EA) is quoted in data tables as positive values, i.e.
Since the standard enthalpy change of electron gain of a species is the negative of the standard enthalpy change of ionisation of that species with an additional electron attached
, i.e.
the enthalpy change of electron gain, assuming the heat capacities at constant pressure of all species (including electron) are given by that of a perfect monoatomic gas of , is determined by substituting eq83 in eq80 and eq81:
with the electron gain enthalpy of being:
