Question

May I know why in electrolysis at cathode if there is a competition between two reactions the reaction which has higher value of Standard Electrode Potential(SEP) is preferred and at anode the reaction which has lower value of SEP is preferred ?

Answer 1

see below

Explanation:

when you make an electrolysis you give energy to a system, till the energy is enough to produce a reaction. So happens that occurs the reaction that need the minimum difference of Electric potential between the two electrods.

Pay attention. in real system you must consider also the ovevoltage of electrode that is the activation energy to make the reaction happen.

For example if you make the electrolysis of NaCl in water you can have at cathode:
`2H^+ + 2e = H_2` E= -0,42 V (pH=7)
or `Na^+ + e = Na` E= -2,7V
while at anode you can have
`4OH^- = 2 H_2O + O_2 + 4e` E= 0,81 V (pH=7)
and `2Cl^- = Cl_2 +2e` E= 1,33 V

Hence, according with theory, you schould have `H_2`at cathode and `O_2` at anode witha a `Delta V`of (0,81-(-0,42))=1,23 V.

In truth dosen't happen anything till you give at least 2,1 V (it depends by the material of electrodes) and you have `H_2` at cathode and `Cl_2` at anode witha a `Delta V`of about (1,5-(-0,6))=2,1 V.