What is the product of the electrolysis of aqueous sodium chloride (NaCl) using inert electrodes?

Analysis of the Question

This question tests your understanding of electrolysis of aqueous solutions. When we electrolyze a solution, we must consider the competition between ions moving to the electrodes.

Concept Explanation

In an aqueous solution of sodium chloride ($\text{NaCl}$), the following ions are present:

• From $\text{NaCl}$: $\text{Na}^+$ and $\text{Cl}^-$
• From water ($\text{H}_2\text{O}$): $\text{H}^+$ and $\text{OH}^-$

At the Cathode (Negative Electrode): Both $\text{Na}^+$ and $\text{H}^+$ ions are attracted to the cathode. According to the electrochemical series, hydrogen is lower than sodium. Therefore, $\text{H}^+$ ions are preferentially discharged over $\text{Na}^+$ ions.

• Reaction: $2\text{H}^+ + 2e^- \rightarrow \text{H}_2(g)$

At the Anode (Positive Electrode): Both $\text{Cl}^-$ and $\text{OH}^-$ ions are attracted to the anode. In a concentrated aqueous solution of $\text{NaCl}$ (brine), the concentration of $\text{Cl}^-$ is very high, which allows the chloride ions to be discharged instead of the hydroxide ions.

• Reaction: $2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^-$

Step-by-Step Solution

1. Identify the ions: The solution contains $\text{Na}^+$, $\text{H}^+$, $\text{Cl}^-$, and $\text{OH}^-$.
2. Determine the cathode product: Since $\text{H}^+$ is lower in the electrochemical series than $\text{Na}^+$, hydrogen gas ($\text{H}_2$) is produced at the cathode.
3. Determine the anode product: Because the solution is aqueous sodium chloride, the high concentration of chloride ions leads to the discharge of chlorine gas ($\text{Cl}_2$) at the anode.
4. Conclusion: The electrolysis results in hydrogen gas at the cathode and chlorine gas at the anode.

Identifying the Answer

Comparing our findings with the provided options:

• Option A matches our conclusion perfectly.

Exam Tip

Remember the "Preferential Discharge" rule:

• Cathode: The ion lower in the electrochemical series is discharged (usually Hydrogen over Group 1 or 2 metals).
• Anode: If the solution is concentrated, the halide (like $\text{Cl}^-$) is discharged. If the solution is very dilute, oxygen gas ($\text{O}_2$) is usually produced from the $\text{OH}^-$ ions.