Understanding Mercury Cell Properties

This section details the properties of a Mercury cell, analyzing each statement provided in the question to identify the accurate characteristics of this type of battery.

Statement (A) Analysis: Energy Conversion

Statement (A) claims the Mercury cell converts the energy of combustion into electrical energy. This description accurately fits a fuel cell, which continuously consumes fuel and an oxidant. However, a Mercury cell is a type of galvanic cell (or voltaic cell) that stores chemical energy within its reactants and converts it into electrical energy, but it does not involve combustion in the way a fuel cell does. Therefore, statement (A) is incorrect for a Mercury cell.

Statement (B) Analysis: Rechargeability

Statement (B) suggests that the Mercury cell is rechargeable. Mercury cells belong to the category of primary batteries. Primary batteries are designed for single use; once their chemical reactants are depleted, they cannot be efficiently or safely recharged. In contrast, secondary batteries, like lithium-ion or lead-acid batteries, are rechargeable. Thus, statement (B) is incorrect.

Statement (C) Analysis: Cell Reaction

Statement (C) provides the cell reaction: $Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$. This is the characteristic overall electrochemical reaction for a standard Mercury cell. The anode reaction involves the oxidation of zinc amalgam ($Zn(Hg)$), and the cathode reaction involves the reduction of mercury(II) oxide ($HgO$). The products are zinc oxide ($ZnO$) and liquid mercury ($Hg$). This reaction accurately represents the chemistry occurring within a Mercury cell. Therefore, statement (C) is correct.

Statement (D) Analysis: Device Application

Statement (D) states that the Mercury cell is a low current device used in hearing aids. Historically, Mercury cells were widely used in small electronic devices such as hearing aids, calculators, and watches. They provided a stable voltage output and had a long shelf life, making them suitable for low-drain applications. While they offered advantages, they also posed environmental concerns due to mercury content. This statement accurately reflects a common application and characteristic of Mercury cells. Therefore, statement (D) is correct.

Conclusion on Mercury Cell Characteristics

Based on the analysis of each statement:

• Statement (A) is incorrect as it describes a fuel cell, not a Mercury cell.
• Statement (B) is incorrect because Mercury cells are primary (non-rechargeable) batteries.
• Statement (C) is correct, representing the actual cell reaction.
• Statement (D) is correct, highlighting a common application and characteristic.

Therefore, the properties correctly describing a Mercury cell are (C) and (D).