PREFERENTIAL DISCHARGE OF IONS DURING ELECTROLYSIS

PREFERENTIAL DISCHARGE OF IONS DURING ELECTROLYSIS

                                                     

    A molten or acqueos solution contains four (4) ions, two are H+ and OH- these are coming from water and another two ions from elctrolyte

    But only two of this are discharged. The discharge depend on three factors which are;

                  1) Ease of discharge of ions 

                  2) The concentration of ions 

                  3) the nature of electrodes.

1. Ease discharge of ions.

         Metals and hydrogen are anion, the ease of discharge of ions determined by the position of the element in the electrochemical series.

Electro chemical series is an arrangement of elements depending on their ease in gaining electrons.
       
                               

               K+.           least readily
                              discharge           NO3-
               Ca2+.                                 SO4-
               Na+.                                     Cl-
               Mg2+.                                  Br-
               Al3+.                                     I-
               Zn2+.                                  OH-
               Fe2+
               Pb2+
               H+
               Cu2+        most readily discharge
               Hg+ 
               Ag+it
   Anions of the element  high in the electrochemical series are not readily discharged because their ions are very stable.
 For examplecopper sulphate electrolyte
       Ions present in the solution are Cu2+, SO4+, H+ and OH-. Cu2+ and H+ migrate to cathode but copper will be discharged instead of H+ because copper is below hydrogen in electrochemical series.

   And at anode, the hydroxide are discharged instead of sulphate ions because hydroxide is below sulphate in electrochemical series.

2. Concentration of ions.
      The ions with high concentration in electrolyte tend to be favoured in the discharge.

3. Nature of electrode.
   Nature of electrode especially cathode also determine which catode to be discharged first.
          Fo example when sodium chloride solution electrolysed using Mercury cathode, when Mercury cathode used, sodium ions are discharged to form sodium metel which is readily in dissolves in the Mercury to form sodium amalgam.