Question

The following reactions occur at the anode during the electrolysis of aqueous sodium chloride solution: \[ Cl^-_{(aq)} \rightarrow \frac{1}{2}Cl_2(g)+e^- \qquad E^\circ_{\text{cell}}=1.36\,V \] \[ 2H_2O(l) \rightarrow O_2(g)+4H^+(aq)+4e^- \qquad E^\circ_{\text{cell}}=1.23\,V \] Which reaction is feasible at the anode and why?

Hint:

In electrolysis of aqueous NaCl (brine): \[ \boxed{\text{Cathode: } H_2 \text{ gas is produced}} \] \[ \boxed{\text{Anode: } Cl_2 \text{ gas is produced}} \] Although water can give oxygen, chloride ions are preferentially oxidised due to: \[ \boxed{\text{High } Cl^- \text{ concentration + oxygen overvoltage}} \]

Answer 1

Concept: During electrolysis, oxidation occurs at the anode. The species that loses electrons most easily is preferentially oxidised. In an aqueous sodium chloride solution, the ions present are: \[ Na^+, \ Cl^-, \ H^+, \ OH^- \] At the anode, possible oxidation reactions are: \[ Cl^- \rightarrow \frac{1}{2}Cl_2+e^- \] and \[ 2H_2O \rightarrow O_2+4H^++4e^- \]

Comparison of electrode potentials: The given values are: \[ E^\circ_{\text{oxidation}}(Cl^-/Cl_2)=1.36V \] \[ E^\circ_{\text{oxidation}}(H_2O/O_2)=1.23V \] From the values alone, oxidation of water appears easier because it has a lower oxidation potential. However, in practice, chlorine is evolved instead of oxygen.

Reason: The concentration of chloride ions in aqueous NaCl solution is very high compared to hydroxide ions produced from water. Moreover, oxygen evolution at the anode requires additional overvoltage, which makes the oxidation of water more difficult. Therefore, chloride ions are preferentially oxidised. The anode reaction is: \[ \boxed{ 2Cl^- \rightarrow Cl_2+2e^- } \] Hence, chlorine gas is obtained.

Final Answer: The feasible reaction at the anode is: \[ \boxed{ Cl^-_{(aq)} \rightarrow \frac{1}{2}Cl_2(g)+e^- } \] because chloride ions are present in high concentration and oxygen evolution from water requires higher overvoltage. Therefore, chlorine gas is liberated at the anode during electrolysis of aqueous NaCl solution.