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) \(\mathrm{Na}\) is dissolved in \(\mathrm{Hg}\) 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) \(\mathrm{Na}\) is dissolved in \(\mathrm{Hg}\) while it does not dissolve in \(\mathrm{Pt}\).

Step 1: Identify the Context of the Problem

In this exercise, we're analyzing the behavior of the electrolysis of \(\mathrm{NaCl}\) using different electrodes, which affects the products formed at the cathode. The exercise explores why hydrogen is liberated when a Pt electrode is used, and why sodium forms an amalgam with a mercury (Hg) electrode.

Step 2: Understanding Electrolysis with Pt Electrode

When using a platinum (Pt) electrode, hydrogen gas (\(\mathrm{H}_2\)) is liberated at the cathode. This is because Pt does not have an affinity to form alloys or amalgams with sodium, allowing the reduction of \(\mathrm{H}^+\) ions to occur more readily.

Step 3: Understanding Electrolysis with Hg Electrode

With a mercury (Hg) electrode, sodium forms an amalgam. This occurs because Hg has the ability to dissolve sodium, forming a sodium amalgam, which is more stable than forming hydrogen gas from \(\mathrm{H}^+\) ions. Essentially, sodium prefers to get reduced and dissolved in mercury.

Step 4: Identifying the Correct Explanation

The key to this problem is understanding that the formation of sodium amalgam at the Hg cathode happens because sodium dissolves in mercury, whereas it does not dissolve in platinum. This aligns with the given option (c) which states that \(\mathrm{Na}\) is dissolved in \(\mathrm{Hg}\) while it does not dissolve in \(\mathrm{Pt}\).

Key Concepts

Electrode Reaction

In the process of electrolysis, different reactions can take place at the electrodes depending on the materials used. For the electrolysis of sodium chloride \(\mathrm{NaCl}\), the electrolyte breaks down in solution, causing ions to move towards the electrodes. The electrode reaction is key, as it determines which ions will be reduced or oxidized.

• At the cathode, reduction takes place.
• At the anode, oxidation occurs.

When using platinum \(\mathrm{Pt}\) electrodes, \(\mathrm{H}^+\) ions from the electrolyte are reduced to form hydrogen gas \(\mathrm{H}_2\). This is because platinum does not react with sodium, so the hydrogen ions are preferentially reduced. This preference for \(\mathrm{H}^+\) reduction over sodium ions is crucial for understanding why \(\mathrm{H}_2\) gas is liberated.

Na Amalgam Formation

Amalgam formation is an interesting characteristic of metals like mercury \(\mathrm{Hg}\). An amalgam is essentially an alloy where one of the components is mercury. When \(\mathrm{NaCl}\) undergoes electrolysis with a mercury cathode, sodium ions become involved in forming a sodium amalgam instead of producing hydrogen gas. The reaction at the cathode involves sodium ions being reduced and dissolving into the mercury to form this alloy.

• Sodium dissolves readily in mercury but not in platinum.
• The formed sodium amalgam is stable compared to the liberation of hydrogen gas.

The interaction between sodium ions and mercury leads to a stable amalgam, a notable feature because it prevents the evolution of hydrogen gas at the cathode when \(\mathrm{Hg}\) is used.

Pt Electrode

Platinum \(\mathrm{Pt}\) is a noble metal, known for its inertness. In electrochemical reactions, the inertness of an electrode affects the type of reactions that occur. Using a \(\mathrm{Pt}\) electrode in the electrolysis of \(\mathrm{NaCl}\) leads to hydrogen gas production, not due to any interaction with sodium but rather its reluctance to react structurally. The properties of platinum include:

• Resistance to corrosion and chemical reactions.
• No tendency to form amalgams or alloys with sodium.

This makes Pt ideal in processes where clear and specific chemical reactions are required without interference from the electrode itself, allowing \(\mathrm{H}^+\) ions to reduce and form \(\mathrm{H}_2\) gas easily.

Hg Cathode Behavior

Mercury \(\mathrm{Hg}\) behaves differently from other metals in its capacity to form amalgams. Using mercury as a cathode in the electrolysis of \(\mathrm{NaCl}\) leads to a distinct reaction pathway. Sodium ions are reduced and dissolve into the mercury surface rather than releasing hydrogen gas. The Hg cathode behavior is significant because:

• It alters the cathodic reaction pathway entirely.
• Creates a sodium amalgam which is stable and non-gaseous.

This behavior demonstrates how choosing different electrode materials impacts the electrolytic process's outcome, offering a practical example of how material properties influence electrochemical reactions.