Match List I with List II:

Choose the correct answer from the options given below:

(A)-(III), (B)-(IV), (C)-(II) D-(I)

The correct answer is option 3. (A)-(III), (B)-(IV), (C)-(II) D-(I).

Let us go through each option:

(A) Fuel cell: (III) Cell reaction, \(2H_2 + O_2 \longrightarrow 2H_2O\)

The reaction is the overall reaction that occurs in a hydrogen fuel cell. This type of fuel cell, known as a proton exchange membrane (PEM) fuel cell, operates through the electrochemical reaction of hydrogen and oxygen to produce water and electrical energy. Here's the balanced equation for the cell reaction:

\[ 2H_2 + O_2 \longrightarrow 2H_2O \]

In a PEM fuel cell, hydrogen gas (H₂) is fed to the anode, where it dissociates into protons (H⁺) and electrons (e⁻) through a process called oxidation:

\[ H_2 \longrightarrow 2H^+ + 2e^- \]

The protons then migrate through the proton exchange membrane to the cathode, while the electrons flow through an external circuit, generating electrical power. At the cathode, oxygen gas (O₂) and protons from the anode combine with electrons from the external circuit to form water:

\[ O_2 + 4H^+ + 4e^- \longrightarrow 2H_2O \]

Overall, the combination of these reactions results in the net reaction shown above:

\[ 2H_2 + O_2 \longrightarrow 2H_2O \]

This process is highly efficient and produces electricity without the combustion of hydrogen, making fuel cells a clean and sustainable energy source.

(B) Mercury cell : (IV) Gives steady potential

Mercury cells offer a steady potential due to the nature of their chemical reactions. In a mercury cell, the anode consists of zinc (Zn), while the cathode contains a paste made of mercury oxide (HgO) mixed with graphite or other conductive materials.

The overall reaction in a mercury cell can be summarized as:

\[ \text{Anode (Oxidation)}: \quad Zn \longrightarrow Zn^{2+} + 2e^- \]

\[ \text{Cathode (Reduction)}: \quad 2HgO + 2e^- \longrightarrow 2Hg + O_2 \]

Combining these two half-reactions, we get the overall cell reaction:

\[ Zn + 2HgO \longrightarrow Zn^{2+} + 2Hg + O_2 \]

This reaction produces a relatively steady potential difference between the electrodes, providing a consistent voltage output over time. The steady potential of the mercury cell is one of its key characteristics, making it suitable for applications where a stable voltage source is required, such as in certain medical devices, cameras, and some types of hearing aids. However, due to environmental concerns associated with mercury, mercury cells have become less common and are being replaced by alternative technologies.

(C) Lechlanche cell : Reaction at anode, \(Zn \longrightarrow Zn^{2+} + 2e^-\)

The Leclanché cell actually operates via a different reaction at the anode. The Leclanché cell is a type of primary battery, often used in devices like flashlights. The reaction at the anode typically involves the oxidation of zinc:

\[ Zn \longrightarrow Zn^{2+} + 2e^- \]

This reaction releases electrons, which flow through the external circuit to the cathode, producing electrical energy. Meanwhile, the zinc ions produced in the anode reaction migrate to the cathode through the electrolyte. At the cathode, usually made of manganese dioxide, reduction occurs:

\[ 2NH_4^+ + 2MnO_2 + 2e^- \longrightarrow Mn_2O_3 + 2NH_3 + H_2O \]

This reaction consumes the electrons and completes the electrical circuit. The overall reaction in a Leclanché cell can be summarized as:

\[ Zn + 2NH_4Cl + 2MnO_2 \longrightarrow ZnCl_2 + 2NH_3 + Mn_2O_3 \]

This reaction liberates electrical energy, producing zinc chloride, ammonia, and manganese(III) oxide.

(D) Ni-Cd cell : (I) Rechargeable 

Nickel-Cadmium (Ni-Cd) cells are rechargeable. They are a type of rechargeable battery commonly used in various electronic devices such as cordless phones, power tools, and portable electronic devices.

The chemical reactions that occur in a Ni-Cd cell during discharge and recharge are as follows:

Discharge Reaction:

At the negative electrode (anode), typically made of cadmium (Cd):

\[ \text{Anode (Discharge)}: \quad \text{Cd} + 2OH^- \longrightarrow \text{Cd(OH)}_2 + 2e^- \]

At the positive electrode (cathode), typically made of nickel oxyhydroxide (NiOOH):

\[ \text{Cathode (Discharge)}: \quad \text{NiOOH} + H_2O + e^- \longrightarrow \text{Ni(OH)}_2 + OH^- \]

The overall discharge reaction in a Ni-Cd cell:

\[ \text{Cd} + \text{NiOOH} + H_2O \longrightarrow \text{Cd(OH)}_2 + \text{Ni(OH)}_2 \]

Recharge Reaction:

During the recharge process, the reactions are reversed. The cadmium hydroxide (\( \text{Cd(OH)}_2 \)) and nickel oxyhydroxide (\( \text{Ni(OH)}_2 \)) are converted back to cadmium and nickel oxyhydroxide, respectively, by applying an external electrical current.

Therefore, the correct match is: 3. (A)-(III), (B)-(IV), (C)-(II), (D)-(I).