\(Zn(s) \longrightarrow Zn^{2+} + 2e^-\)

Above reaction at anode is found in :

Leclanche cell

The correct answer is option 1. Leclanche cell.

Let us dive into the details of each cell type and how they relate to the reaction:

\(\text{Zn(s)} \rightarrow \text{Zn}^{2+} + 2e^-\)

This reaction describes the oxidation of zinc metal to zinc ions, releasing two electrons. This is an important aspect of electrochemical cells where zinc serves as the anode.

1. Leclanché Cell:

The Leclanché cell consists of a zinc anode and a carbon rod (often with manganese dioxide) as the cathode. The electrolyte is typically ammonium chloride (NH₄Cl) or zinc chloride (ZnCl₂).

Anode Reaction: The oxidation reaction at the anode in a Leclanché cell is:

\(\text{Zn(s)} \rightarrow \text{Zn}^{2+} + 2e^-\)

Here, zinc metal oxidizes to zinc ions, and the electrons released flow through the external circuit.

Cathode Reaction: At the cathode, the reduction reaction involves manganese dioxide and ammonium chloride:

\(\text{2MnO}_2 + \text{2NH}_4\text{Cl} + 2e^- \rightarrow \text{Mn}_2\text{O}_3 + \text{2H}_2\text{O} + 2\text{Cl}^-\)

Electrolyte: The electrolyte provides ionic conductivity between the anode and cathode, allowing the flow of electrons and completion of the circuit.

Usage: The Leclanché cell is a type of dry cell used in various applications, including portable batteries.

2.Mercury Cell:

Structure: The mercury cell, also known as the zinc-mercury cell, uses zinc as the anode and mercury oxide (HgO) as the cathode. The electrolyte is usually a strong alkali such as potassium hydroxide (KOH).

Anode Reaction: The oxidation reaction at the anode is:

\(\text{Zn(s)} \rightarrow \text{Zn}^{2+} + 2e^-\)

Zinc metal oxidizes to zinc ions, releasing electrons.

Cathode Reaction: At the cathode, the reduction reaction involves mercury oxide:

\(\text{HgO} + \text{H}_2\text{O} + 2e^- \rightarrow \text{Hg} + 2\text{OH}^-\)

Electrolyte: Potassium hydroxide (KOH) provides the ionic medium necessary for the reaction to occur.

Usage: Mercury cells are used in devices that require a stable voltage and long shelf life, such as hearing aids and small electronic devices.

3. Ni-Cd Cell:

The nickel-cadmium (Ni-Cd) cell uses cadmium as the anode and nickel oxide hydroxide (NiOOH) as the cathode. The electrolyte is usually potassium hydroxide (KOH).

Anode Reaction: The oxidation reaction at the anode is:

\(\text{Cd(s)} \rightarrow \text{Cd}^{2+} + 2e^-\)

This is different from the zinc oxidation reaction.

Cathode Reaction: At the cathode, the reduction reaction is:

\(\text{NiOOH} + \text{H}_2\text{O} + e^- \rightarrow \text{Ni(OH)}_2 + \text{OH}^-\)

Electrolyte: Potassium hydroxide (KOH) facilitates ion transport between the electrodes.

Usage: Ni-Cd cells are commonly used in rechargeable batteries for various electronic devices, such as cordless phones and power tools.

4. Fuel Cell:

Fuel cells convert chemical energy directly into electrical energy through the reactions of hydrogen and oxygen.

Anode Reaction: In a typical hydrogen fuel cell, the anode reaction is:

\(\text{2H}_2 + 4\text{OH}^- \rightarrow 4\text{H}_2\text{O} + 4e^-\)

This is not related to zinc oxidation.

Cathode Reaction: At the cathode, oxygen reacts with water to form hydroxide ions.

Electrolyte: The electrolyte is typically a proton exchange membrane (PEM) or an alkaline solution.

Usage: Fuel cells are used in various applications, including power generation and transportation, due to their high efficiency and low environmental impact.

Conclusion:

The given reaction:

\(\text{Zn(s)} \rightarrow \text{Zn}^{2+} + 2e^-\)

is characteristic of the Leclanché cell and the Mercury cell, where zinc serves as the anode and undergoes oxidation. This reaction is not typical for the Ni-Cd cell or the Fuel cell.

Thus, the correct answer is 1. Leclanché cell.