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) \(\mathrm{Na}\) is dissolved in \(\mathrm{Hg}\) while it does not dissolve in \(\mathrm{Pt}\) (d) concentration of \(\mathrm{H}^{+}\)ions is larger when \(\mathrm{Pt}\) electrode is taken.

Short answer

The answer is (b) more voltage is required to reduce \(H^+\) at \(Hg\) than at \(Pt\).

Step-by-step solution

Understand the Electrolysis Process

In the electrolysis of NaCl, electric current is passed through melted or aqueous NaCl, leading to the movement of Na+ ions towards the cathode (negative electrode) and Cl- ions towards the anode (positive electrode). At the cathode, reduction takes place.

Analyze the Role of Electrode Material

Different electrode materials can alter the electrolytic process due to varying reactivity and solubility properties of the metals. The choice between Pt and Hg affects the products produced at the cathode.

Examine Platinum Electrode Reaction

When a platinum electrode is used as the cathode, it does not react with Na, so the reduction of water occurs. Hence, Hydrogen gas (H2) is liberated according to the reaction: \(2H_2O + 2e^- \rightarrow H_2 + 2OH^-\).

Examine Mercury Electrode Reaction

Mercury, as an electrode, interacts differently. Instead of producing hydrogen gas, sodium forms an amalgam with mercury. This happens because sodium dissolves in mercury, creating an amalgam, reducing the free sodium ions that could revert to producing hydrogen gas.

Compare Voltage Requirements for Reduction

The reduction of hydrogen ions requires different voltages at different electrodes. The overpotential for hydrogen reduction is higher on mercury than on platinum. Thus, less energy is available for the reduction of H+ ions on mercury, leading to the preference for forming sodium amalgam.

Identify the Correct Reason

Given these details, option (b) correctly identifies why Hg leads to sodium amalgam: (b) more voltage is required to reduce \(H^+\) at \(Hg\) than at \(Pt\). This is due to the higher overpotential required for hydrogen gas evolution on mercury.

Key concepts

Electrode Material

The material of the electrode plays a crucial role in determining the course of the electrolytic process. In the electrolysis of NaCl, the choice between platinum (Pt) and mercury (Hg) electrodes can lead to significantly different outcomes. This is because each material has unique reactivity and interaction properties with the ions present in the solution.
For instance, platinum is extremely inert, meaning it does not react with other substances easily. This quality makes it a widely used electrode material in electrolysis as it can facilitate the movement of electrons without undergoing any chemical change itself.
On the other hand, mercury is more reactive compared to platinum, allowing it to form compounds with metals like sodium. This property is leveraged in the electrolysis of NaCl to create sodium amalgam, as sodium can readily dissolve in mercury but not in platinum.

Sodium Amalgam

Sodium amalgam is an alloy formed when sodium metal dissolves in mercury. This occurs as a specific outcome when mercury is used as a cathode in the electrolysis of NaCl.
During the process, sodium ions are attracted to the cathode. Unlike platinum, mercury does not allow these sodium ions to remain, but rather captures and dissolves them into its structure, forming a sodium amalgam.
This reaction prevents the formation of hydrogen gas, which would normally occur if the sodium ions were to remain free and subsequently react with water. Sodium amalgam is particularly useful in chemical processes due to its active metallic character and ease of handling.

Hydrogen Gas Evolution

The release of hydrogen gas is a common occurrence during electrolysis, especially when water is involved in the reaction at the cathode. In the electrolysis of NaCl using a platinum electrode, hydrogen gas evolution occurs because the reduction of water is more favorable.
Here, the water molecules accept electrons, leading to the liberation of hydrogen gas (\(2H_2O + 2e^- \rightarrow H_2 + 2OH^-\)). This is due to the inert nature of platinum, which facilitates only the electron transfer process without chemically interacting with the other constituents.
In cases where other electrode materials are used, such as mercury, this reaction may not take place because mercury tends to dissolve metals like sodium, forming amalgams instead.

Overpotential

Overpotential refers to the extra voltage required to drive a non-spontaneous electrochemical reaction. It is particularly significant in reactions involving gas evolution, like hydrogen gas in the electrolysis of NaCl.
The concept explains why different electrodes produce different outcomes in the electrolysis process. With mercury, the overpotential for hydrogen gas evolution is higher than with platinum. This higher overpotential means that more energy is required to reduce hydrogen ions to hydrogen gas on a mercury electrode compared to a platinum one.
This excess energy requirement makes the formation of sodium amalgam more favorable in the presence of mercury because sodium will react with the mercury instead of allowing the reduction of hydrogen ions to occur.

Reduction Reactions

Reduction reactions are a fundamental part of electrolysis, where ions gain electrons to form stable atoms or molecules. In the electrolysis of NaCl, reduction takes place at the cathode.
The ions available for this reduction depend on the electrolyte solution and the electrode material. For example, in the presence of a platinum electrode, water molecules take part in the reduction, resulting in hydrogen gas evolution.\(2H_2O + 2e^- \rightarrow H_2 + 2OH^-\)
However, when a mercury electrode is used, sodium ions undergo reduction to form sodium amalgam rather than hydrogen gas. Thus, the choice of electrode and the conditions of the electrolysis process dictate which species are reduced, showcasing the interdependence of electrode material and the resultant chemical reactions.