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Thursday, 23 February 2017

The D and F-Block Elements - II


Preparation of Potassium dichromate (K2Cr2O 7):

It is prepared by fusion of chromate ore (FeCr2O4) with sodium carbonate in excess of air.

Effect of pH on chromate and dichromate ions:

The chromates and dichromates are inter-convertible in aqueous solution depending upon pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.

Potassium dichromate acts as a strong oxidizing agent in acidic medium:


Preparation of Potassium permanganate (KMnO4):
  • Potassium permanganate is prepared by fusion of MnO4 with alkali metal hydroxide (KOH) in presence of O2 or oxidising agent like KNO3. It produces dark green K2MnO4 which undergoes oxidation as well as reduction in neutral or acidic solution to give permanganate.

  • Commercially, it is prepared by the alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate (Vl).
  • In laboratory, Mn²+ salt can be oxidized by peroxodisulphate ion to permanganate ion.
  • In acidic medium: 
  • In neutral or faintly basic medium: 
Properties of Lanthanoids:
  • +3 oxidation state is most common along with +2 and +4.
  • Except Promethium, they are non – radioactive.
  • The magnetic properties of lanthanoids are less complex than actinoids.
Properties of Actinoids:
  • Actinoids also show higher oxidation states such as +4, +5, +6 and +7.
  • They are radioactive.
  • The magnetic properties of the actinoids are more complex than those of the lanthanoids.
  • They are more reactive.
Mischmetall
  • It is a well-known alloy which consists of a lanthanoid metal (~ 95%) and iron (~ 5%) and traces of S, C, Ca and Al.
  • A good deal of mischmetall is used in Mg-based alloy to produce bullets, shell and lighter flint.
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