Various Types of Enthalpy Changes.

Standard Enthalpy Change of Atomisation, ∆H^θat.

∆H^θat of an element is the energy absorbed when 1 of gaseous atoms are formed from its elements under standard conditions.

1/2Cl2(g) → Cl(g) ∆H^θat = +121 kJ mol^-1.

Na(s) → Na(g) ∆H^θat = +108 kJ mol^-1.

For diatomic gases, H2, Cl2 etc; Cl2(g) → 2Cl(g) ∆H = +242 kJ mol^-1.

Therefore: ∆H^θat = ½ x bond energy

= ½ x +242 = +121 kJ mol^-1.

Standard Enthalpy Change of Electron Affinty. ∆H^θEA.

The first electron affinity (EA) of an element is the enthalpy change when 1 mole of gaseous atoms accept a mole of electrons to form a mole of uninegatively charged ions.

Cl(g) + e → Cl^̄ (g) ∆H^θea = ˗ 364 kJ mol^-1.

The first electron affinity (EA) is always negative.

The second and successive electron affinity of any element is always positive because energy absorbed to overcome repulsive forces between the negatively charged anions and the added electron.

O(g) + e → O^¬(g) ∆H^θea1 = ˗ 142 kJ mol^-1.

O^¬(g) + e → O^2¬(g) ∆H^θea2 = + 844 kJ mol^-1.