Question

Explain thermodynamic stability of transition metal compounds on the basis of ionization enthalpy.

Answer 1

i. The thermodynamic stability of transition metal compounds can be predicted on the basis of their ionization enthalpy value. 

ii. When the sum of the ionization enthalpies required to attain a particular oxidation state of transition metal ions is small, the thermodynamic stability of the compounds of the metal in that oxidation state is high.

eg. 

a. Compounds containing Ni (II) are more stable than compounds containing Pt (II). Less amount of energy is required for the ionization of Ni to Ni²⁺, than the energy required for the ionization of Pt to Pt²⁺. This is because the sum of first and second ionization enthalpies (IE₁ + IE₂) for nickel is lesser as compared to that of platinum.

Ni → Ni²⁺ (IE₁ + IE₂ = 2.49 x 10³ kJ mol^-1)

Pt → Pt²⁺ (IE₁ + IE₂ = 2.66 x 10³ kJ mol^-1)

b. Compounds containing Pt (IV) are more stable than compounds containing Ni (IV). Less amount of energy is required for the ionization of Pt to Pt⁴⁺ than the energy required for the ionization of Ni to Ni⁴⁺. This is because the sum of first four ionization enthalpies (IE₁ + IE₂ + IE₃ + IE₄) for platinum is lesser as compared to that of nickel.

Ni → Ni⁴⁺ (IE₁ + IE₂ + IE₃ + IE₄ = 11.29 x 10³ kJ mol^-1)

Pt → Pt⁴⁺ (IE₁ + IE₂ + IE₃ + IE₄ = 9.36 x 10³ kJ mol^-1)

Note: K₂PtCl₆ is a well known compound of Pt (IV). The corresponding compound of nickel is not known