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f-Block: Lanthanoids - General Characteristics And Uses

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f-Block: Lanthanoids - General Characteristics And Uses - Lesson Summary

Physical properties of lanthanoids:

All lanthanoids are soft and silvery white metals.

  • They are electropositive.
  • They rapidly tarnish in air due to the formation of an oxide coating on their surface.

           4Ln           +        3O2          →           2Ln2O3
       Lanthanoid             oxygen                    lanthanoid oxide

  •  They are good conductors of heat and electricity.
  • Their M.P and density change gradually, except in the case of europium, ytterbium, samarium and thulium.
Element Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Melting Point(K) 1068 1208 1297 1315 1345 1099 1585 1629 1680 1734 1802 1818 1097 1925 Density (g cm -3) 6.77 6.77 7.01 7.26 7.52 5.26 7.90 8.23 8.54 8.79 9.07 9.32 6.90 9.84
  • Many of the trivalent lanthanoid ions are coloured, both in their crystalline state and in aqueous solutions.
  • Lanthanoid ions that have empty f orbitals, like La (III), half-filled f-orbitals, like Gd (III), or completely filled 4f orbitals, like Lu (III) ions, are colourless.

  • Due to the presence of unpaired electrons in the 4f orbitals, all lanthanoid ions show paramagnetic behaviour.


Exceptions:

La (III), Ce (IV) ions with fO configuration, & Yb (II) & Lu ( III) ions with f14 configuration show diamagnetic nature.

Paramagnetism is the maximum in Dysprosium.

The value of the first ionisation energy of the lanthanides is about

600 K J/mole and of the second ionisation energy is about 1200 K /mole


Chemical reactivity:

The first few members of the lanthanoids are quite reactive and show chemical behaviour similar to that of calcium. The standard electrode potentials of the Ln+3/Ln couple indicate that all the lanthanides are more reactive than aluminium.

These metals combine with hydrogen when heated gently in the gas, and form hydrides of the type LnH2 and LnH3.

lanthanoids (Ln)
Ln             +             H2           →               LnH2
lanthanoid             Hydrogen                  Lanthanoid hydride

Ln             +             O2           →               LnO2
lanthanoid             Oxygen                  Lanthanoid Oxide

Calcium (Ca)
Ca             +           H2           →              CaH2
Calcium                 Hydrogen               Calcium hydrogen

Ca             +           O2           →              CaO2
Calcium                 oxygen               Calcium peroxide

On being heated, these elements combine directly with non-metals, and form carbides with carbon, nitrides with nitrogen, sulphides with sulphur, and halides with halogens.

                                  2773K
Ln        +             2C      →          2LnC2
lanthanoid            carbon                 Carbide

                                             Δ
2Ln        +             N2            →          2LnN
lanthanoid            Nitrogen                 Nitride

                                             Δ
2Ln        +             3S             →          2Ln2S3
lanthanoid            Sulphur                    Sulphide

                                             Δ
  2Ln        +             3X2         →          2LnX3
lanthanoid             Halogen                 Halide

They liberate hydrogen from dilute acids.

                                             Δ
     2Ln        +           6HX        →          2LnX3    +    3H2
lanthanoid            Nitrogen                    Halide        hydrogen

They burn in oxygen to form oxides of the type Ln2O3.

They react slowly with cold water, but more rapidly with hot water, and liberate hydrogen gas.

The lanthanoid oxides and hydroxides are basic in nature.

Uses:

  • Lanthanoids are widely used for the production of alloy steels for pipes and plates.
  • A well-known alloy, mischmetal, which is used in a magnesium-based alloy to produce bullets, shells and lighter flint.
  • Mixed oxides of lanthanoids are used as catalysts in petroleum cracking.
  • EuO2 and YbO 2 are used to produce the red colours on television screens.
  • The oxides of praseodymium and neodymium are used in glass, such as in television screens.

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