Understanding the Mercury Cell Components

The question asks about the electrolyte paste used in a mercury cell. A mercury cell is a type of primary cell, meaning it is not rechargeable. It is known for providing a very stable voltage during its discharge life, which makes it useful in devices like watches, calculators, and hearing aids.

A typical mercury cell consists of the following main components:

• Anode: This is the negative electrode where oxidation occurs. In a mercury cell, the anode is usually made of zinc-mercury amalgam.
• Cathode: This is the positive electrode where reduction occurs. The cathode is typically made of mercuric oxide (HgO) mixed with graphite (carbon). The graphite helps in improving conductivity.
• Electrolyte: This is the medium that conducts ions between the electrodes. In a mercury cell, the electrolyte is an alkaline paste, usually consisting of potassium hydroxide (KOH) and zinc oxide (ZnO). A paste containing mercuric oxide (HgO) is associated with the cathode side, facilitating the cathode reaction.

Electrochemical Reactions in a Mercury Cell

Let's look at the chemical reactions that happen at the electrodes:

• Anode Reaction (Oxidation of Zinc):
  Zn(Hg) + 2OH^- → ZnO(s) + H₂O(l) + 2e^-
  Here, zinc from the amalgam is oxidized to zinc oxide, releasing electrons.
• Cathode Reaction (Reduction of Mercuric Oxide):
  HgO(s) + H₂O(l) + 2e^- → Hg(l) + 2OH^-
  Here, mercuric oxide is reduced to liquid mercury, consuming electrons.
• Overall Cell Reaction: By adding the anode and cathode reactions, we get the overall reaction:
  Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)
  Notice that the concentration of ions in the electrolyte (like OH^-) does not change during the overall reaction, which helps maintain a stable cell voltage.

Identifying the Electrolyte Paste Component

The question states that a paste of ________ is used as an electrolyte. While the electrolyte itself is primarily KOH and ZnO, a paste containing mercuric oxide (HgO) is crucial and is in contact with the cathode and the electrolyte. Given the options, HgO is the component that is typically present as a paste alongside the electrolyte in the cathode compartment, participating in the reduction reaction.

Let's examine the given options:

• HgO: Mercuric oxide is the active material in the cathode paste that gets reduced during discharge. While not the electrolyte itself, it is present as a paste and reacts. This aligns best with the provided correct answer.
• LiFe: This is not a standard component of a mercury cell. Lithium iron compounds are often associated with lithium-ion batteries.
• H₂O: Water is present as part of the electrolyte paste, but it's not the primary component described as "a paste of".
• H₂SO₄: Sulfuric acid is an electrolyte used in lead-acid batteries, not alkaline mercury cells.

Considering the options and the typical composition of a mercury cell, the paste referred to, particularly in the context of the cathode and its reaction, contains HgO.

Conclusion

In a mercury cell with a zinc-mercury amalgam anode and a carbon cathode, the electrolyte is a paste of KOH and ZnO. A paste containing mercuric oxide (HgO) is used as the cathode material which reacts with the electrolyte components.

Therefore, among the given options, HgO is the correct answer representing a key component of the paste used in conjunction with the electrolyte and cathode in a mercury cell.

Revision Table: Mercury Cell Components

Additional Information about Mercury Cells and Primary Batteries

Mercury cells were widely used but are now less common due to environmental concerns regarding mercury disposal. They belong to the class of primary batteries, which means the electrochemical reaction is irreversible, and the battery cannot be recharged. Their main advantage is the stable voltage output throughout most of their lifespan.

Other examples of primary batteries include:

• Dry Cells (Leclanché cell): Uses a zinc anode and a carbon cathode in an ammonium chloride and zinc chloride electrolyte paste.
• Alkaline Batteries: Uses a zinc anode and a manganese dioxide cathode in an alkaline electrolyte (KOH or NaOH). They generally have a longer shelf life and higher energy density than dry cells.
• Lithium Batteries: Use lithium as the anode. Different cathode materials and electrolytes are used, providing high energy density and voltage.

Understanding the components and reactions of different battery types like the mercury cell is important for comprehending electrochemistry and energy storage principles.