In electrolysis of NaCl when Pt electrode is taken then H2 is liberated at cathode while with Hg cathode it forms sodium amalgam. The reason for this is
When sodium chloride is dissolved in water, it ionises as NaCl⇌Na++Cl−. Water also dissociates as : H2O⇌H++OH− During passing of electric current through this solution using platinum electrode, Na+ and H+ ions move towards cathode. However, only H+ ions are discharged more readily than Na+ ion because of their low discharge potential (In the electromotive series hydrogen is lower than sodium). These H+ ions gain electrons and change into neutral atoms. At cathode H++e−→H,H+H→H2 Cl− and OH− ions move towards anode. Cl− ions lose electrons and change into neutral atom. At anode, Cl−−e−→Cl, Cl+Cl→Cl2 If mercury is used as cathode, H+ ions are not discharged at mercury cathode because mercury has a high hydrogen overvoltage. Na+ ions are discharged at the cathode in perference to H+ ions, yielding sodium, which dissolves in mercury to form sodium amalgam. At cathode : Na++e−=Na