Ncert Exercises & Solutions

Value of standard electrode potential for the oxidation of Cl^- ions is more positive than that of water, even then in the electrolysis of aqueous sodium chloride, why is Cl^- oxidised at anode instead of water?

Under the condition of electrolysis of aqueous sodium chloride, oxidation of water at anode requires over potential. So, Cl^- is oxidized at anode instead of water. Possible oxidation half cell reactions occurring at anode are

{Cl}^{-} (aq) ⟶ \frac{1}{2} {Cl}_{2} (g) + e^{-}; E_{c e ll}^{°} = 1.36 V

2 H_{2} O (l) ⟶ O_{2} (g) + 4 H^{+} (a q) + 4 e^{-}; E_{c e l l} ° = 1.23 V

Species having lower

E_{cell}^{°}

cell undergo oxidation first than the higher value but oxidation of H₂O to O₂ is kinetically so slow that it needs some overvoltage.

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