Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

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

Step 1: Identify the Difference in Electrodes

In the given electrolysis setups, we are using two different cathodes - platinum (Pt) and mercury (Hg). The behavior of these electrodes affects the chemical process at the cathode during electrolysis of NaCl solution.

Step 2: Consider Hydrogen Liberation at Pt Electrode

At a more standard cathode such as platinum, which is an inert electrode, hydrogen ions (3H^{+}) from water are reduced to form hydrogen gas (3H_{2}3). This reaction typically involves less overpotential because Pt efficiently supports such reductions.

Step 3: Analyze the Reaction at Hg Electrode

Mercury behaves differently than platinum because it can form an amalgam with sodium. In an electrolytic cell with Hg as the cathode, sodium ions in the solution are reduced to form a sodium amalgam before hydrogen ions can be reduced. This is because mercury has a unique capacity to dissolve alkali metals like sodium, prioritizing their reduction.

Step 4: Assess Overpotential for Hydrogen Formation

The overpotential required for hydrogen reduction at mercury is significantly higher compared to platinum. Therefore, when using Hg, sodium is preferentially reduced over hydrogen ions due to this higher potential requirement.

Step 5: Evaluate Sodium Dissolution in Electrodes

Sodium, once reduced, dissolves in mercury to form an amalgam, whereas it does not dissolve in platinum. This behavior is specific to the interaction between sodium and mercury, further confirming why mercury cathode forms sodium amalgam.

Step 6: Determine Correct Reason Using Analysis

Through the analysis, it's clear that the correct reason behind sodium forming an amalgam with a mercury cathode rather than liberating hydrogen gas (as it does with a platinum cathode) is due to sodium's ability to dissolve in mercury.

Key Concepts

Platinum Electrode

Platinum electrodes are commonly used in electrochemical cells due to their inert nature. This means they do not readily participate in the chemical reactions taking place. When platinum is used in the electrolysis of NaCl, it acts efficiently as a cathode. Because of this inertness, it readily supports the reduction of hydrogen ions (H⁺) in the solution to form hydrogen gas (H₂).

• Platinum doesn't dissolve in the solution, making it a stable choice for various electrolytic processes.
• It requires less energy to facilitate the formation of hydrogen gas, which is a favorable factor in electrolytic reactions.

Thus, when using platinum electrodes, hydrogen liberation is a straightforward result during the electrolysis of NaCl.

Mercury Electrode

Mercury electrodes have a distinct characteristic that sets them apart from platinum electrodes: their ability to form amalgams with certain metals, such as sodium.

• In the electrolysis of NaCl, when mercury is used as the cathode, sodium ions are reduced more readily than hydrogen ions.
• Mercury has a unique ability to dissolve alkali metals, like sodium, creating what is known as sodium amalgam.

This ability means that instead of liberating hydrogen gas, mercury captures the sodium, forming an amalgam due to its affinity for sodium.

Overpotential

Overpotential is an important concept in electrochemistry that affects the efficiency of reactions at electrodes. It refers to the extra potential required to drive a non-spontaneous reaction at an electrode compared to its thermodynamic potential.

• When using mercury electrodes, the overpotential for the reduction of hydrogen ions is significantly higher than that when using platinum.
• This higher overpotential leads mercury to reduce sodium ions first, as a lower value is needed to achieve this reduction compared to overcoming hydrogen overpotential.

Therefore, understanding and managing overpotential is crucial for directing the desired chemical reactions during electrolysis.

Sodium Amalgam

Sodium amalgam is a specific type of alloy formed when sodium is dissolved in mercury. This formation is a highlight of using a mercury cathode in the electrolysis of NaCl, as it captures sodium ions from the solution.

• Sodium amalgam possesses unique chemical properties, distinct from pure sodium or mercury.
• The amalgamation process is driven by mercury's ability to absorb and retain sodium, making it a crucial step in electrochemical applications involving these elements.

The formation of sodium amalgam is a testament to mercury's distinct capability to interact with certain metals.

Hydrogen Liberation

Hydrogen liberation is a common phenomenon in electrolysis, especially when using inert electrodes like platinum. During electrolysis, hydrogen ions are reduced and combine to form hydrogen gas, which is then released as bubbles at the surface of the platinum electrode.

• This process is widely used in hydrogen production and demonstrates the efficiency of platinum in facilitating hydrogen ion reduction.
• With platinum electrodes, there is minimal overpotential for this reaction, making it a cost-effective method for hydrogen generation.

When using a platinum electrode in NaCl electrolysis, hydrogen liberation is expected due to these favorable conditions.