d- and f- Block Elements Class 12th Notes

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d-Block Elements (Transition Elements) – The d-block elements lying in the middle of the periodic table belonging to groups 3-12 in which the d-orbitals are progressively filled in each of the four long periods are known as transition elements because their properties are intermediate between those of s– and p– block elements.
There are mainly four series of the transition elements
  • First series/3d- series– This consists 10 elements from 21Sc to 30Zn. All the elements have half filled or incomplete 3d-orbital.
  • Second series/4d-series– This consists 10 elements from 39Y to 48Cd. In this series 4d-orbitals of Cd is completely filled.
  • Third series/5d-series– From 57La, 72Hf to 80Hg there are 10 elements. Hg has completely filled 5d-orbital.
  • Fourth series/6d-orbitals– It consists 89Ac, 104Rf to 112Cn total 10 elements.

Electrical Configuration of d-elements– General electronic configuration of transition elements is (n-1)d1-10 ns1-2. However, zinc (Zn), cadmium (Cd), mercury (Hg) are represented by a general formula (n-1)d10 ns2. These are not regarded as transition elements due to completely filled d-orbitals.

General Properties of Transition Elements

  • They have high melting point, higher than s and p- block elements.
  • They have electrical conductors.
  • They have paramagnetic and catalytic properties .
  • These are hard then s and p block elements.
  • They form coloured salt.

Physical Properties– All the elements (except Zn, Cd, Hg and Mn) have metallic properties like high tensile strength, ductility, malleability, conductivity and shiny nature. In these elements ns-electrons as well as incomplete d-orbitals electrons participate in formation of metallic bonds.

Ionisation Enthalpy– Ionisation enthalpy increases with increase in nuclear charge along each series. However, its value for Cr is lower because of the absence of any change in d-configuration and the value for Zn is higher because it represents an ionisation from the completely filled 4s level.

Density– Density of transition elements from Sc to Cu increases due to high atmoic mass and small atomic volume.

Oxidation States– Due to very less energy difference between ns shell and (n-1)d shell the electrons of both the shell participate in bond formation. Thus, transition elements show variable oxidation states. Transition elements from ionic compounds in low oxidation states (2+ and 3+) but form covalent compounds at high oxidation state Os and Ru show oxidation state upto +8.

Atomisation Enthalpy- Atomisation enthalpy of transition elements is high grater the number of unpaired electrons in an atom of transition element, higher will be the atomisation enthalpy. This is because as the number of unpaired electrons increase there occur a number of strong mutual atomic reactions. As a result their exist strong attraction between their atoms.

  • The electrode potential of first row  of transition elements generally show an increase with increasing atomic numbers. Electrode potential M2+/M of a metal is dependent upon three parameters viz. enthalpy of atomisation, enthalpy of ionisation and enthalpy of hydreation of M2+.

Formation of coloured ions– Colour of transition elements or metallic ions is due to incomplete d-shell adsorb radiation from visible region and get transition from low energy d-shell to high energy d-shell colour  appearing is complementary to the colour adsorbed by the substance. Elements with configuration from d1 to d9 are coloured whereas elements with configuration10 or d0 are colourless.

Magnetic property– Most of the transition metals are paramagnetic due to  the presence of unpaired electrons (paramagnetic character α number of unpaired electrons). The species having all paired electrons are diamagnetic in nature.

The magnetic moment is determined by the formula, \mu =\sqrt { n(n=2) } +{ BM } where, n is the number of unpaired electrons and BM is Bohr Magnetons (unit of magnetic moment).

Formation of alloy – Alloys are formed by atoms with metallic radii that are within about 15 percent of each other. Because of similar radii and other characteristics of transition elements, alloys are readily formed by these metals.

Preparation of Potassium dichromate (K2Cr2O7)

Method of preparation from chromite ore.

\underset { Chromite }{ { 4FeCr }_{ 2 }{ O }_{ 4 } } +{ 8Na }_{ 2 }{ CO }_{ 3 }+{ 7O }_{ 2 }\overset { Diffusion }{ \longrightarrow } \underset { Sodium\quad chromate\\ (Yellow) }{ { 8Na }_{ 2 }{ CrO }_{ 4 } } +{ 2Fe }_{ 2 }{ O }_{ 3 }+{ 8CO }_{ 2 }

\underset { Sodium\quad chromate }{ { 2Na }_{ 2 }{ CrO }_{ 4 } } +{ H }_{ 2 }{ SO }_{ 4 }\longrightarrow { Na }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 }+{ Na }_{ 2 }{ SO }_{ 4 }+{ H }_{ 2 }{ O }

\underset { Sodium\quad \\ dichromate }{ { Na }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 } } +{ 2KCl }\longrightarrow \underset { Potassium  dichromate }{ { K }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 } } +{ 2NaCl }

In aqueous solution transition of chromate to dichromate take place, on changing pH they get interchanged.

\underset { Yellow\\ (chromate  ion) }{ { 2CrO }_{ 4 }^{ 2- } } +{ 2H }^{ + }\rightleftharpoons \underset { Orange\\ (dichromate) }{ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- } } +{ H }_{ 2 }{ O }

Properties-

{ Cr }_{ 2 }{ O }_{ 7 }^{ -2 }+{ 14H }^{ + }+{ 6I }^{ - }\longrightarrow { 2Cr }^{ 3+ }+{ 7H }_{ 2 }{ O }+{ 3I }_{ 2 }

{ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+{ 14H }^{ + }+{ 3Sn }^{ 2+ }\longrightarrow { 3Sn }^{ 4+ }+{ 2Cr }^{ 3+ }+{ 7H }_{ 2 }{ O }

{ Cr }_{ 2 }{ O }_{ 7 }^{ - }+{ 14H }^{ + }+{ 6Fe }^{ 2+ }\longrightarrow { 2Cr }^{ 3 }+{ 6Fe }^{ 3+ }+{ 7H }_{ 2 }{ O }

{ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+{ 3H }_{ 2 }{ S }+{ 8H }^{ + }\longrightarrow { 2Cr }^{ 3+ }+{ 3S }+{ 7H }_{ 2 }{ O }

{ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+{ 2H }^{ + }+{ 3SO }_{ 2 }\longrightarrow { 2Cr }^{ 3+ }+{ 3SO }_{ 4 }^{ 2- }+{ H }_{ 2 }{ O }

Preparation of Potassium Permanganate

Method of preparation form pyrolusite ore.

{2MnO}_{2}+{4KOH}+{O}_{2} \longrightarrow \underset {(Green)}{{2K}_{2}{MnO}_{4}}+{{2H}_{2}O}

{ 3MnO }_{ 4 }^{ 2- }+{ 4H }^{ + }\longrightarrow { 2MnO }_{ 4 }^{ - }+{ MnO }_{ 2 }+{ 2H }_{ 2 }{ O }

Industrial Method

{MnO}_{2}\overset { Fusion  with  KOH }{ \longrightarrow }   \underset { Manganate\quad ion\\ (Green) }{ { MnO }_{ 4 }^{ 2- } }

{ { MnO }_{ 4 }^{ 2- } }\xrightarrow [ Electrical ]{ oxidation } \underset { Permanganate  ion  (violet) }{ { { MnO }_{ 4 }^{ - } } }

Important oxidising reactions of KMnO4

In acidic solutions

  • { 10I }^{ - }+{ { 2MnO }_{ 4 }^{ - } }+{ 16H }^{ + }\longrightarrow { 2Mn }^{ 2+ }+{ 8H }_{ 2 }O+{ 5I }_{ 2 }
  • { 5Fe }^{ 2+ }+{ MnO }_{ 4 }^{ - }+{ 8H }^{ + }\longrightarrow { 2Mn }^{ 2+ }+{ 8H }_{ 2 }O+{ 5Fe }^{ 3+ }
  • { 5SO }_{ 3 }^{ 2- }+{ 2MnO }_{ 4 }^{ - }+{ 6H }^{ + }\longrightarrow { 2Mn }^{ 2+ }+{ 3H }_{ 2 }{ O }+{ 5SO }_{ 4 }^{ 2- }
  • { 5NO }_{ 2 }^{ - }+{ 2MnO }_{ 4 }^{ - }+{ 6H }^{ + }\longrightarrow { 2Mn }^{ 2+ }+{ 5NO }_{ 3 }^{ - }+{ 3H }_{ 2 }{ O }

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