What is the product obtained at the anode in the electrolysis of concentrated sodium chloride using graphite electrode?

Analysis

The question asks for the product formed at the anode during the electrolysis of concentrated sodium chloride (brine) using graphite electrodes.

Concept Explanation

In the electrolysis of concentrated sodium chloride solution (brine), the electrolyte contains the following ions:

• From NaCl: $Na^+$ and $Cl^-$
• From $H_2O$: $H^+$ and $OH^-$

During electrolysis, ions migrate to the electrodes based on their discharge potential:

1. At the Cathode (Negative Electrode): Both $Na^+$ and $H^+$ ions migrate here. However, $H^+$ ions have a lower discharge potential than $Na^+$ ions. Therefore, $H^+$ ions are preferentially discharged to form Hydrogen gas ($H_2$).
2. At the Anode (Positive Electrode): Both $Cl^-$ and $OH^-$ ions migrate here. In a concentrated solution, the high concentration of chloride ions ($Cl^-$) overcomes the discharge potential of the hydroxide ions ($OH^-$). Consequently, $Cl^-$ ions are preferentially discharged to form Chlorine gas ($Cl_2$).

Step-by-Step Solution

1. Identify the ions present: $Na^+$, $Cl^-$, $H^+$, and $OH^-$.
2. Determine the migration: The negatively charged ions ($Cl^-$ and $OH^-$) move toward the positive anode.
3. Apply the concentration rule: Because the solution is concentrated, the chloride ions ($Cl^-$) are in excess.
4. Write the half-equation at the anode: $2Cl^-_{(aq)} \rightarrow Cl_{2(g)} + 2e^-$
5. Conclusion: The gas evolved at the anode is Chlorine gas.

Exam Tip

Remember the "Concentration Effect": In dilute NaCl, Oxygen gas is produced at the anode because $OH^-$ is preferentially discharged. But in concentrated NaCl (brine), Chlorine gas is always the product at the anode. Think of it as "Concentrated = Chlorine".