Question

In electrolysis 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. The reason for this is: (a) more voltage is required to reduce \(\mathrm{H}^{+}\) at \(\mathrm{Hg}\) than at Pt. (b) concentration of \(\mathrm{H}^{+}\) ions is larger when \(\mathrm{Pt}\) ele trode is taken. (c) Na is dissolved in \(\mathrm{Hg}\) while it does not dissolve in Pt. (d) \(\mathrm{Hg}\) is more inert than \(\mathrm{Pt}\).

Short answer

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

Step-by-step solution

Understanding Electrolysis

During electrolysis of NaCl, different reactions occur depending on the type of electrode used. At the cathode, reduction reactions can lead to the liberation of hydrogen gas or the formation of sodium metal. Understanding which reaction occurs involves studying electrode materials and their properties.

Analyzing Pt Electrode

Using a platinum (Pt) electrode, the reduction of hydrogen ions (H⁺) is favored, leading to the liberation of hydrogen gas at the cathode. This happens because Pt is a noble metal and provides a suitable surface for hydrogen evolution.

Analyzing Hg Electrode

When a mercury (Hg) electrode is used at the cathode, sodium ions are reduced to form sodium amalgam (a mixture of sodium metal dissolved in mercury) rather than liberating hydrogen gas. Mercury forms an amalgam with sodium, a factor that significantly impacts the reaction.

Evaluating the Options

Option (a) suggests more voltage is required to reduce H⁺ at Hg, which would indicate differences in reduction potentials. Option (b) refers to H⁺ concentration, which isn't a primary difference between electrode types. Option (c) mentions Na's solubility in Hg and not in Pt, aligning with the formation of amalgam. Option (d) suggests inertness, but Hg's reaction with Na is reactive.

Choosing the Correct Option

The correct answer is option (c), 'Na is dissolved in Hg while it does not dissolve in Pt'. Sodium's ability to form an amalgam with mercury is why sodium amalgam forms when Hg is used.

Key concepts

Sodium Amalgam

Sodium amalgam is a fascinating chemical substance formed when sodium metal dissolves in mercury. This combination creates a liquid metal alloy, blending the properties of both elements. One practical application of sodium amalgam is in chemical processes where controlled sodium reactions are necessary.

During electrolysis of \(\mathrm{NaCl}\), if a mercury electrode is used at the cathode, sodium ions are reduced to form sodium metal, which then amalgamates with mercury. This amalgamation occurs because mercury acts as a solvent for sodium.

• Electrolysis introduces electrons to sodium ions (\(\mathrm{Na}^+\)), reducing them to sodium metal (\(\mathrm{Na}\)).
• Due to the high affinity between sodium and mercury, the sodium does not stay as separate metal but dissolves in mercury.

The result is a safer and more manageable source of reactive sodium than pure sodium metal, which is why sodium amalgam is useful in industrial and laboratory settings.

Mercury Electrode

Mercury electrodes play a pivotal role in electrochemical processes, specifically in the electrolysis of sodium chloride (NaCl). Unlike more common electrodes like platinum (Pt), mercury has the unique ability to form amalgams with certain metals, such as sodium.

When mercury is used as the cathode during the electrolysis of NaCl, it encourages the reduction of sodium ions over other possible reductions, like the reduction of hydrogen ions to produce hydrogen gas. This preference is due to several factors:

• The affinity of mercury for sodium, which leads to the formation of sodium amalgam.
• The subsequent lack of free sodium allows the continued reduction of sodium ions into the mercury pool.

Mercury electrodes are fundamentally different than those like platinum, which don't interact to form such amalgams, hence leading to different cathode reactions. This aspect highlights an essential chemical behavior that is significant in designing different electrochemical cells, as the electrode materials directly influence the reaction outcomes.

Reduction Reactions

Reduction reactions are a core concept in electrochemistry, involving the gain of electrons by a species. In the context of sodium chloride electrolysis, the type of electrode used can change the reduction reactions occurring at the cathode.

• When a platinum electrode is used at the cathode, hydrogen ions (\(\mathrm{H}^+\)) from water are reduced to form hydrogen gas. This is because platinum provides an ideal surface for hydrogen ion reduction.
• However, when a mercury electrode is employed, sodium ions (\(\mathrm{Na}^+\)) are reduced instead, forming sodium metal that dissolves in mercury to create sodium amalgam.

Understanding reduction reactions is critical for predicting the outcomes of electrolysis processes. The properties of the electrodes determine which ions will be reduced. In the case of mercury, the formation of sodium amalgam is preferred over hydrogen gas production, illustrating how material choice in electrodes can guide and control chemical processes.