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Applications of Electrolysis
The phenomenon of electrolysis has wide application. The important ones are:
(1) Determination of equivalent masses of elements:
According to second law of electrolysis when the same quantity of electronic current is passed through solutions of salts of two different cells, the amounts of the metals deposited on the cathodes of the two cells are proportional to their equivalent masses of the respective metals. If the amounts of the metals deposited on the cathodes be WA and WB respectively, then
WA/WB = (Equivalent mass of A)/(Equivalent mass of B)
Knowing the equivalent mass of one metal, the equivalent mass of the other metal can be calculated from the above relationship. The equivalent masses of those non-metals which are evolved at anodes can also be determined by this method.
The metals like sodium, potassium, magnesium, calcium aluminum, etc., are obtained by electrolytes of fused electrolytes.
Fused electrolyte Metal isolated
NaCl + CaCl2 + KF Na
CaCl2 + CaF2 Ca
Al2O3 + cryolite Al
MgCi2 (35%) + NaCl (50%) + CaCl2 (15%) Mg
KCl + CaCl2 K
(3) Manufacture of non-metals:
Non-metals like hydrogen, fluorine, chlorine are obtained by electrolysis.
(4) Electro-refining of metals:
The metals like copper, silver, gold, aluminum, tin, etc., are refined by electrolysis.
(5) Manufacture of compounds:
Compounds like NaOH, KOH, Na2CO3 KCIO3, white lead, KMnO4, etc., are manufactured by electrolysis.
The process of coating an inferior metal with a superior metal by electrolysis is known as electroplating.
The aims of electroplating are:
(i) To prevent the inferior metal from corrosion.
(ii) To make it more attractive in appearance.
The object to be electroplated is made the cathode and block of the metal to be deposited is made the anode in an electrolytic bath containing a solution of a salt of the anodic metal. On passing electric current in the cell, the metal of the anode dissolves out and is deposited on the cathode-article in the form of a thin film. The following are the requirements for fine coating:
(i) The surface of the article should be free from greasy matter and its oxide layer. The surface is cleaned with chromic acid or detergents.
(ii) The surface of the article should be rough so that the metal deposited sticks permanently.
(iii) The concentration of the electrolyte should be so adjusted as to get smooth coating.
(iv) Current density must be the same throughout.
CuSo4 + dilute H2So4
Nickel ammonium sulphate
Thickness of coated layer
Let the dimensions of metal sheet to be coated be (a cm × b cm).
Thickness of coated layer = c cm
Volume of coated layer = (a × b × c) cm3
Mass of the deposited substance = Volume × density
= (a × b × c) × d g
=> (a × b × c) × d = (I×t×E)/96500
Using above relation we may calculate the thickness of coated layer.
Note: Sometimes radius of atom of deposited metal is given instead of density, e.g.,
Radius of silver atom = 10-8 cm
Atomic mass of Ag = 108
Mass of single silver atom = 108/(6.023× 1023 ) g
Volume of single atom = 4/3× πR3
= 4/3×3.14×(10-8)3 cm3
Density of Ag = (Mass of single atom )/(Volume of single atom)
= (108/6.023 × 1023)/(4/3×3.14×10-8 )3
Sometimes the ammeter shows false current due to mechanical fault. In this case,
% Current efficiency = (Actual current)/(Ammeter current)×100