Electro Chemistry

             Making rolled gold ornaments, electroplating, extraction of metals, purification of metals, alloys, car battery, dry cells, preparation of chemical substances like NaOH, redox reactions, electrolysis and production of electrical energy by electrochemical cells are some of the applications of electrochemistry. The branch of chemistry that explains the relation between electrical energy and chemical energy. Flow of electrons is considered as electric current. If the flow of current takes place through metals it is called electronic conduction, if takes place through fused or aqueous, acidic, alkaline, salt solutions is called electrolytic conduction. Electrolytic conductors are electrolytes. Strong acids, bases, salt solutions are strong electrolytes whereas weak acids, bases are weak electrolytes. If the substances are unable to pass electricity either in molten state or in aqueous state are called non electrolytes.
              Electrolytic solutions offer resistance (R) to the flow of current, which is directly proportional to 'l' (distance between the two parallel electrodes of the cell) and inversely proportional to a (area of cross-section of the electrode).
         
 

Specific conductance (K): The conductance of an electrolytic solution enclosed between two parallel electrodes of Unit area of cross-section (a) and separated by a Unit distance (l).
            K = C.                             Units of K: ohm^-1 m^-1
 

Equivalent conductance (Λ): The conductance of a solution containing 1 gram equivalent of electrolyte present between two parallel electrodes, separated by Unit distance (1 m or 1 cm).
                          Λ = K ×   ohm^-1 m² equivalent^-1
 

Molar conductance (µ): The conductance of a solution containing 1 gram mole of electrolyte present between two parallel electrodes, separated by Unit distance.
                             ohm^-1 m² mole^-1
 

Conductance ratio (α): The ratio of equivalent conductance at any concentration (Λ_c) to that at infinite dilution (Λ₀).
                           
       For weak electrolytes 'α' is called degree of ionization. Debye - Huckel - Onsagar equation explains the relation between Λ_c and Λ_∞ as below.
      
 D = Dielectric constant of water                C = Concentration
 T = Temperature in Kelvin scale               η = Viscosity of water.
 

Kohlrausch's law: The equivalent conductance of an electrolyte at infinite dilution (Λ_∞ ) is equal to the algebraic sum of equivalent conductance of cation (λ⁺_∞ ) and anion (λ^-_∞ ).
                          (Λ_∞^Electrolyte )  =  λ⁺_∞  + λ^-_∞
 

Applications: 1) In the calculation of ''Λ∞'' of weak electrolyte.
                       2) To calculate "α" of weak electrolytes.
    3) To calculate solubilities of sparingly soluble salts like BaSO₄.
 

Degree of dissociation (α): α =  
            λ⁺_∞, λ^-_∞ are ionic conductances at infinite dilution. These are directly proportional to ionic mobilities (u⁺, U ^-).
                λ⁺_∞ =  K. u⁺_∞ , λ^-_∞ = K. U^-_∞
                       K =  Proportionality constant. 
              
          The process of decomposition of a chemical compound (either in molten state or in aqueous state) into its constituent elements under the influence of EMF is called "electrolysis". Anions move towards anode and oxidised and cations move towards cathode and reduced. Two Faradays electricity is required to electrolyse 1 mole of water (acidified/ alkaline). Mg, Na (Castner, Down methods) can be extracted by electrolysis. NaOH (Nelson cell) or Cl2 also obtained during electrolysis. To carry out electrolysis, inert electrodes (Pt) or metallic rods of same salt solution (active electrodes) are used.