ELECTROLYSIS

       
          Electrolysis is a process by which electrical energy is used to make a chemical change.

Discovery of Sodium & Potassium by Electrolysis :

       The electrolysis of molten sodium and potassium hydroxides, first carried out in 1808 by Sir Humphrey Davey, led to the discovery of these two metallic elements.

    

Mechanism of Electrolysis :

During electrolysis, ionic substances are broken down (decomposed) into simpler substances by the influence of electricity.Generally, metals & gases may form at the electrodes.

               For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance (i.e., an electrolyte) is dissolved in water or molten (melted). 
A common example of an electrolyte is table salt, sodium chloride. Solid sodium chloride (NaCl) and pure water both are non-conductor of electricity, but a soln. of the salt in water is a good conductor of electricity. A soln. of sugar in water, by contrast, is non-conductor and, therefore, sugar is a non-electrolyte. 

         An ionic solid such as NaCl is composed of charged particles, but these are held so tightly in the crystal lattice that they are unable to move about, so the second requirement mentioned above is not met and solid salt is not a conductor. If the salt is melted or dissolved in water, the ions can move freely and the molten liquid or the solution becomes a conductor. 

        Since positively-charged ions are attracted to a negative electrode that is traditionally known as the cathode, these are often referred to as cations. Similarly, negatively-charged ions, being attracted to the positive electrode, or anode, are called anions.

 The equipment used for electrolysis :

•   A source of   DC i.e., battery;
•  
• Two electrodes (Cathode & anode);
• 
  
•   An electrolyte. 

 










During electrolysis:
   Positively charged ions move to the negative electrode (cathode) during electrolysis. They receive electrons and are reduced.
Negatively charged ions move to the positive electrode (anode) during electrolysis. They lose electrons and are oxidised.


Electrolysis of Dilute aqueous sulphuric Acid 


Pure water is an electrical insulator. Addition of a little bit of H₂SO₄, water behaves as a good conductor. Electrolysis of such dilute acid soln. results in the decomposition of water at both electrodes. 

Reduction at Cathode :

   H⁺(aq) + e  =  ½ H₂(g)    E⁰ = 0.00v                  ........ (1)

H₂O(l) + e  =  ½ H₂(g) + OH^-(aq)    E⁰ = -0.83v     ........ (2)

As the standard reduction potential of reaction-(1) is greater than that of reaction-(2), H⁺(aq) is reduced instead of H₂O(l) & hydrogen gas is evolved at the cathode.

Oxidation at Anode :

H₂O(l)  =  2H⁺(aq) + ½ O₂(g) + 2e    E⁰ = +1.23v     ........ (3)

2SO₄^2-(aq)  =  S₂O₈^2- (aq)  + 2e   E⁰ = +2.00v       ........ (4)

As the standard reduction potential of reaction-(3) is less than that of reaction-(4) i.e., as the standard oxidation potential of reaction-(3) is greater than that of reaction-(4),  H₂O(l) is oxidised instead of SO₄^2-(aq) and oxygen gas is evolved at the anode.

 

When metal ions and hydrogen ions both are present in the soln., during electrolysis the ions which will be reduced at the cathode depends on the position of the metal in the reactivity series.

The reactivity series:

                                          

•    The metal will be produced if it is less reactive than hydrogen.

•    Hydrogen will be produced if the metal is more reactive than H.