Shaillee Kaushal is teaching live on Unacademy Plus
Preferential discharge theory: If an electrolytic solution consists of more than two ions and electrolysis is done, it is observed that all the ions are not discharged at the electrodes simultaneously but certain ions are liberated at the electrodes in preference to others. This is explained by preferential discharge theory It states that more than one type of ions are attracted towards a particular electrode, then the one discharged is the ion which requires least energy The potential at which the ion is discharged or deposited on the appropriate electrode is termed the discharge or deposition potential. The values for discharge potential are different for different ions
Electrolvsis of NaCl solution The solution of sodium chloride besides Nat and Cl ions possess H and OH ions due to ionisation of water. When potential difference is applied across the two electrodes, Na and H ions move towards cathode and Cl and OH ions move towards anode. At cathode H ions are discharged in preference to Nat ions as the discharge potential of H+ ions is lower than Na* ions. Similarly at anode Cl ions are discharged in preference to OH ions NaCl Na+ + Cl- At cathode H++e- H At anode Cl- Cl + e- Thus Na and OH ions remain in solution and the solution when evaporated yields crystals of NaOH.
Electrolysis of CuSO using platinum electrodes At cathode At anode: 2Cu2+ + 2e- Cu 20H- H2O + 0 + 2e- Copper is discharged at cathode as Cu2+ ions have lower discharge potential than H+ ions OH ions are discharged at anode due to lower discharge potential than SO ions