Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

In electrolysis of \(\mathrm{NaCl}\) when \(\mathrm{Pt}\) electrode is used, then \(\mathrm{H}_{2}\) is liberated at cathode while with Hg cathode it forms sodium amalgam. The reason for this is (a) more voltage is required to reduce \(\mathrm{H}^{+}\)at \(\mathrm{Hg}\) than at \(\mathrm{Pt}\) (b) concentration of \(\mathrm{H}^{+}\)ions is larger when \(\mathrm{Pt}\) electrode is taken. (c) Na is dissolved in Hg while it does not dissolve in Pt (d) \(\mathrm{Hg}\) is more inert than \(\mathrm{Pt}\)

Step-by-Step Solution

Answer

(c) Na is dissolved in Hg while it does not dissolve in Pt.

Step 1: Understanding Electrolysis

Electrolysis involves the decomposition of an electrolyte using an external electric current. In the case of sodium chloride (NaCl), the ions are Na⁺ and Cl⁻. During electrolysis, these ions move to the electrodes, where they can undergo reduction or oxidation reactions.

Step 2: Electrode Reactions at Cathode

At the cathode, the reduction reactions occur. When using a platinum (Pt) electrode, hydrogen ions (H⁺) in the solution are reduced to hydrogen gas (H₂): \[ 2 ext{H}^+ + 2 ext{e}^- ightarrow ext{H}_2 \, (g) \] However, with a mercury (Hg) cathode, sodium ions (Na⁺) are reduced to sodium metal, which forms a sodium amalgam with mercury: \[ ext{Na}^+ + ext{e}^- ightarrow ext{Na} \ ext{Na} + ext{Hg} ightarrow ext{Na (amalgam)} \]

Step 3: Examining Given Options

We need to examine why hydrogen is liberated at the Pt cathode and sodium amalgam is formed at the Hg cathode: - Option (a) talks about voltage requirements, but voltage differences are not the primary reason for the formation of amalgam. - Option (b) involves concentration differences, but similar ion concentrations are present in the solution for both electrodes. - Option (c) focuses on solubility of sodium in the electrode material. - Option (d) considers the inertness of electrodes but does not directly affect the formation of amalgam.

Step 4: Conclusion on the Correct Option

Option (c) is correct because it correctly identifies the role of the solubility factor; Na dissolves in Hg to form an amalgam but does not dissolve in Pt, leading to different observed products during electrolysis.

Key Concepts

NaCl electrolysis

NaCl electrolysis is a fascinating process where an electric current is passed through an aqueous solution of sodium chloride (NaCl). This setup splits the compound into its constituent ions: sodium (Na⁺) and chloride (Cl⁻). By applying this electric current, these ions migrate towards the electrodes; sodium ions move to the cathode, while chloride ions head to the anode.

• At the anode, chloride ions are oxidized to form chlorine gas (Cl₂).
• At the cathode, depending on the material used, different reactions can occur.

The choice of electrode material at the cathode (platinum or mercury) is crucial, as it determines the type of reaction that will take place. This factor can significantly alter the resulting products of electrolysis.

sodium amalgam formation

Sodium amalgam formation is a unique reaction taking place during the electrolysis of NaCl when a mercury (Hg) cathode is used. Let's break down what happens:

• Sodium ions (Na⁺) from the solution are attracted to the mercury cathode.
• These ions gain electrons (are reduced) to form metallic sodium (Na).
• The sodium does not exist freely in the solution; instead, it dissolves into the mercury, creating what's known as sodium amalgam.

This reaction is characterized by the affinity of sodium for mercury, which allows for its solubility and the formation of a stable amalgam. This does not occur with a platinum cathode, as sodium doesn't dissolve into platinum, leading to different products.

cathode reactions in electrolysis

Cathode reactions in electrolysis involve reduction, where ions in the solution gain electrons. The material of the cathode plays a pivotal role in determining which ions are reduced.
When platinum (Pt) electrodes are used, hydrogen ions (H⁺) are reduced at the cathode, leading to the release of hydrogen gas (H₂). On the other hand, a mercury (Hg) cathode facilitates a different reaction. The sodium ions (Na⁺) are reduced to metallic sodium, which then forms a sodium amalgam.

• Platinum cathode:
  • Reaction: \[2 ext{H}^+ + 2 ext{e}^- \rightarrow ext{H}_2 (g) \]
  • Product: Hydrogen gas
• Mercury cathode:
  • Reaction: \[ ext{Na}^+ + ext{e}^- \rightarrow ext{Na} \ ext{Na} + ext{Hg} \rightarrow ext{Na (amalgam)} \]
  • Product: Sodium amalgam

This difference highlights how slight variations in the cathode material can impact the reactions and the substances produced during electrolysis.