Trends in oxidation state of d-block elements Part-3

 Oxidation States (oxidation number)-describes loss/gain of electron by an atom

·      Stability of oxidation state of an element is defined by –

i)      Electronic configuration

ii)      Sum of the ionisation enthalpies

iii)    SEP value

iv)    Hydration enthalpy

·      Sc(+3) and Zn(+2) exhibit only one oxidation state

·      Except scandium, (which has +3 oxidation state) for the elements of first transition series +2 oxidation state is the most common. This state arises due to loss of 4s-electron

·      In 3d series highest oxidation state is +7 (Mn)

·      In d-block series highest oxidation state is +8 (Os, Ru)

·      All transition elements, except first and last member of the series, exhibit a number of oxidation state

·      Generally, within the transition series, the highest oxidation state increases with increase of atomic number, reaching to a maximum in the middle and then starts decreasing.

·      The variable oxidation states of a transition metal is due to the involvement of (n-1)d and, outer ns electrons in bonding as the energies of ns and (n-1)d subshells are nearly equal.

·      In a group of d-block elements, the higher states are more stable for heavier elements.

For example, in group 6, Mo(vi) and W(vi) are more stable than Cr(vi). For example, dichromate having Cr(vi) is a strong. Oxidising agent in acidic medium while MoO3and W03 are stable. oxides.

·      Higher oxidation states are exhibited when ns and (n-1)d-electrons take part in bonding.

·      higher oxidation states are found in compound with fluorine and oxygen because fluorine and oxygen are most electronegative in nature

       Higher oxidation states in oxides are normally more stable than fluorides due to capability of oxygen to form multiple bonds.

·      In p-block lower oxidation states of heavier elements are more stable while in d-block heavier element, higher oxidation state are more stable. Due to inert pair effect found in p-block elements.

            For example, in group 6, Mo(vi) and W(vi) are more stable than Cr(vi). For example, dichromate having Cr(vi) is a strong. Oxidising agent in acidic medium while MoO3and W03 are stable. oxides.

·      Some of the transition metals form compounds in zero oxidation state or lower oxidation state In transition element when a complex compound has ligands capable of π-acceptor character in addition to the σ-bonding or ligand having ability of forming synergic bond/back bonding

Example – Ni(CO)4, Fe(CO)₅

Problems-1. (on the basis of electronic configuration)

a). Ti⁴⁺ (3d^o4s⁰) is more stable than Ti^{3 +} (3d¹4s^o) 

Ans - due to completely filled orbitals in Ti4+ which is most stable than partially filled orbital

b).  Mn²⁺⁽3d⁵4s⁰) is more stable than Mn^{3 +} (3d⁴ 4s⁰)

Ans - due to half-filled orbitals which   is most stable than partially filled orbital

 

 c). Fe³⁺(3d⁵4s⁰) is more stable than Fe^{2 +} (3d⁶ 4s⁰) 

Ans- due to half-filled orbitals which   is most stable than partially filled orbital

Problem-2 (on the basis of sum of ionisation enthalpy)

a). Ni²⁺is more stable than Pt²⁺ while Pt⁴⁺ is more stable than Ni⁴⁺

  Ans- due to sum of IE₁ + IE₂ for Ni²⁺ is lesser than IE₁ + IE₂ for Pt²⁺ while IE₁ + IE₂+ IE₃ + IE₄ for Pt⁴⁺ is smaller than Ni⁴⁺ .

Problem -3 ( on the basis of hydration enthalpy)

a).Cu⁺ is not stable in aqueous solution than Cu²⁺ why?

Ans – since energy is required to remove one electron from Cu⁺.large hydration enery evolve during hydration of Cu⁺ that compensates it .

Problem-4 Name the element which do not show variable oxidation state – Sc

·      Problem -5 which of the 3d- series of transition metal exhibit variable oxidation state and why ?  Ans--Mn due to the involvement of (n-1)d and, outer ns electrons in bonding as the energies of ns and (n-1)d subshells are nearly equal.

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