Define ionisation enthalpy and electron affinity.

Explanation

lonisation Enthalpy : The enthalpy change observed by the removal of an electron from the neutral isolated gaseous atom is called ionisation enthalpy.
`X_(g) overset(I.E.)to xx overset(+)(g) +e^(-)`
Electron affinity: The enthalpy evolved by the addition of electron to neutral gaseous atom is called electron affinity.
`X_(g)+e^(-) to X_(g)^(-)+EA`
Observe the following spontaneous reactions
(1) `H_(2(g)) +1/2 O_(2(g)) H_(2)O" "triangleH^(0)="-285.8kJ l mole"`
(2) `C_("graphite") +2S_(g) to CS_(2(g)) " "triangleH^(0)="91.91 Kj l mole"`
In reaction (1) `triangleH = -ve,` in reaction (2) `triangleH=+ve`
The decrease in Enthalpy may be a condition but not a necessary and sufficient condition for spontaneity of reaction.
lonization Energy and Electron Affinity : lonization energy and electron affinity are defined at absolute zero. At any other temperature, heat capacities for the reactants and the products have to be taken into account. Enthalpies of reactions for
`M(g) +M^(+) (g) +e^(-)` (for ionization)
`M(g)+e^(-) to M^(-) (g)` (for electron gain) at temperature, T is
`triangle_(r) H^(theta) (T)=triangle_(r) H^(theta) (0)+underset(0)overset(T) triangle_(r) C_(p)^(theta) dt`
The value of `C_p` for each species in the above reaction is 5/2 R `(C_v= 3//2R)`
So, `triangler C_(p)^(theta)= 5//2 R ("for ionization")`
`triangler C_(p)^(theta)= -5//2 R` (for electron gain)
Therefore,
`triangle_(r) H^(theta)` = (lonization enthalpy)
`=E_0` (ionization energy)+5/2 RT
`triangler H^(theta)=("electron gain enthalpy")`
=-A (electron affinity) =5//2 RT