d & f Block elements Part - 3 | Class - 12th

 

Enthalpy of atomisation, ΔaH0, is the change in enthalpy when one mole of bonds are completely broken to obtain atoms in the gas phase.

 

·    Enthalpy of Atomization: Transition elements (except Zn, Cd and Hg) are much harder and less volatile. They exhibit high enthalpies of atomization. The maximum value lies at about middle of each series indicating the interatomic interaction increases with the number of unpaired d-electrons. The metals of 4d and 5d transition series have greater enthalpies of atomization than the corresponding elements of the first (3d) series. This is due to much more frequent metal-metal bonding in their compounds.

·      Formation of Coloured Compounds:  They form coloured ions due to presence of partially filled d – orbitals and unpaired electrons, they can undergo d – d transition. When an electron from a lower energy d orbital is exited to higher d level, it absorbs energy and this is equal to the energy of certain colours in visible region. So, the colour observed is the complementary colour of the light absorbed.

·       Formation of Complexes: Transition metals form complexes due to

(i)         Presence of vacant d – orbitals of suitable energy

(ii)       Smaller size

(iii)      Higher charge on cations.

(iv)      Ability to show variable oxidation state.

Eg: K₄[Fe(CN)₆], K₃[Fe(CN)₆], [Ni(CO)₄] etc.

·       Catalytic Properties:  Most of transition metals are used as catalyst because of

(i)      presence of incomplete or empty d – orbitals

(ii)    Large surface area

(iii)  Variable oxidation state

(iv)  Ability to form complexes

E.g., Fe, Ni, V₂O₃, Pt, Mo, Co and used as catalyst.

·       Interstitial Compounds:  Transition metals have empty spaces or interstitial sites in which non-metals C, H, N, B etc. can fit into resulting in formation of interstitial compounds. They are non – stoichiometric, i.e., their composition is not fixed, e.g., steel. They are harder and less malleable and ductile.

E.g.: Fe₃H, Mn₄N, TiC, VH_0.56, TiH_1.7 etc.

Some the properties of these compounds are:

(i)      They have high melting point.

(ii)    They are very hard.

(iii)  They retain metallic conductivity.

(iv)  They are chemically inert.

 

·       Alloy Formation: Alloys are homogeneous solid solutions of elements in which at least one element is a metal. They are formed by atoms with metallic radii within about 15% of each other. Because of similar radii and other characteristics of Transition metals, they readily form alloys. The alloys formed are hard and have high m.p. e.g.: Bronze (Cu, Zn), Stainless steel (Fe, C, Ni, Mn and Cr).

 

ü  General Properties of First Row Transition Metal Compounds Oxides and oxometal ions:

·       Oxides of metals in low oxidation states + 2 and + 3 (MO, M₃O₄ and M₂O₃) are generally basic except Cr₂O₃ which is amphoteric in character.

·       Oxides of metals in higher oxidation states + 5 (M₂O₅, MO₃, M₂O₇) are generally acidic in character.

·       Oxides of metals in their intermediate oxidation states + 4 (MO₂) are generally amphoteric in nature. Besides the oxides, the oxocations, which stabilise V (V) species is VO₂⁺, V (IV) species is VO²⁺ and Ti(IV) species is TiO²⁺.

·       As the oxidation number of the metal in the oxide increases, ionic character decreases and acidic character increases.

Thus,        +2                    +8/3                 +3                    +4                    +7     

     MnO                Mn₃O₄           Mn₂O₃               MnO₂             Mn₂O₇

→ Ionic Character Decreases

→ Acidic Character Increases