Match the entries of column I with appropriate entries of column II and choose the correct option out of the four options given.

(i)-(d), (ii)-(c), (iii)-(a), (iv)-(b)

The correct answer is option 2. (i)-(d), (ii)-(c), (iii)-(a), (iv)-(b). 

(i) Leclanche cell(d) reaction at anode, \(Zn \longrightarrow Zn^{2+} + 2e^-\):

Components of the Cell:

The anode is typically made of zinc (Zn). Cathode, usually a carbon rod surrounded by manganese dioxide \((MnO_2)\). The electrolyte is an aqueous solution of ammonium chloride \((NH_4Cl)\).

At the anode (Zinc side): \(Zn \longrightarrow Zn^{2+} + 2e^-\)

Zinc metal (Zn) is the material used for the anode. During the operation of the cell (discharge process), zinc undergoes oxidation. The zinc atoms on the anode lose electrons \((e^-)\). This loss of electrons leads to the formation of zinc ions \((Zn^{2+})\). The electrons released during this process flow through the external circuit, providing the electric current used by the cell.

Overall Cell Operation:

In a Leclanche cell, the zinc anode undergoes oxidation as described above. The released electrons travel through an external circuit, powering devices connected to the cell. Meanwhile, inside the cell, manganese dioxide \((MnO_2)\) at the cathode reduces, capturing the electrons and protons to form manganese oxide and water. The ammonium chloride \((NH_4Cl)\) electrolyte facilitates ion movement and completes the electrochemical circuit by allowing ions to migrate between the anode and cathode, maintaining charge balance. In summary, the reaction at the anode

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

represents the oxidation half-reaction where zinc metal loses electrons, producing zinc ions and releasing energy used to power external devices connected to the cell.

(ii) Ni-Cd cell(c) rechargeable:

The \(Ni-Cd\) (Nickel-Cadmium) cell is a type of rechargeable battery commonly used in various applications due to its reliability and relatively low cost. Here’s how it works and some key features:

Discharge Reaction(when the battery is used):

Anode (Negative electrode): Cadmium (Cd) undergoes oxidation:

\(Cd \longrightarrow Cd^{2+} + 2e^- \)

Cathode (Positive electrode): Nickel oxide-hydroxide (\(NiOOH)\) undergoes reduction:

\(NiOOH + H_2O + e^- \longrightarrow Ni(OH)_2 + OH^-\)

Overall Discharge Reaction:

\(Cd + NiOOH + H_2O \longrightarrow Cd(OH)_2 + Ni(OH)_2\)

Recharge Process:

During recharge, an external electrical current is applied to reverse the chemical reactions. The nickel hydroxide \((Ni(OH)_2)\) at the cathode is oxidized back to nickel oxide-hydroxide \((NiOOH)\):

\(Ni(OH)_2 + OH^- \longrightarrow NiOOH + H_2O + e^- \)

The cadmium hydroxide \((Cd(OH)_2)\) at the anode is reduced back to cadmium metal (Cd):

\(Cd^{2+} + 2e^- \longrightarrow Cd\)

Overall Recharge Reaction:

\(Cd(OH)_2 + NiOOH \longrightarrow Cd + Ni(OH)_2 \)

Advantages of Ni-Cd Cells:

Rechargeability: Can be recharged many times without significant loss of capacity.

Reliability: Good performance over a wide range of temperatures and under varying loads.

Low Maintenance: Requires minimal maintenance compared to other battery types.

High Current Output: Capable of delivering high currents, making them suitable for applications that require bursts of power.

Overall, Ni-Cd cells are valued for their robustness and reliability, making them suitable for applications such as portable electronics, power tools, and emergency backup systems. However, advancements in technology have led to the adoption of other rechargeable battery types like NiMH (Nickel-Metal Hydride) and Li-ion (Lithium-ion) batteries in many modern devices due to their higher energy density and lower environmental impact.

(iii) Fuel cell(a) cell reaction 2H₂ + O₂ → 2H₂O:

The reaction you've mentioned is the overall reaction for a hydrogen fuel cell, specifically a proton exchange membrane fuel cell (PEMFC) or alkaline fuel cell (AFC). Let's delve into it:

Overall Reaction:

\(2H_2 + O_2 \longrightarrow 2H_2O\)

Hydrogen \((H_2)\) is supplied to the anode (negative electrode) of the fuel cell. At the anode, hydrogen molecules dissociate into protons \((H^+)\) and electrons \((e^-)\):

\(H_2 \longrightarrow 2H^+ + 2e^- \)

The protons \((H^+)\) migrate through the electrolyte (proton exchange membrane in PEMFC or potassium hydroxide in AFC) to the cathode. Meanwhile, the electrons \((e^-)\) travel through an external circuit, generating an electric current that can be used to power devices. At the cathode (positive electrode), oxygen (usually from air) combines with the protons \((H^+)\) and electrons \((e^-)\) to form water:

\(O_2 + 4H^+ + 4e^- \longrightarrow 2H_2O\)

Overall Cell Reaction:

\(2H_2 + O_2 \longrightarrow 2H_2O\)

The only byproduct of this reaction is water \((H_2O)\), making fuel cells environmentally friendly. Fuel cells can achieve higher efficiencies than traditional combustion engines because they directly convert chemical energy into electrical energy.

Used in various applications ranging from transportation (like hydrogen-powered vehicles) to stationary power generation (backup power for buildings).

Fuel cells offer a promising alternative to conventional energy sources due to their efficiency, low emissions, and versatility across various applications.

(iv) Mercury cell(b) does not involve any ion in solution and is used in hearing aids:

Mercury cells, also known as mercury oxide batteries, operate without involving ions in solution.

Chemical Composition:

Anode Reaction:

\(Zn + HgO \longrightarrow ZnO + Hg \)

Zinc (Zn) reacts with mercury oxide \((HgO)\) at the anode.This reaction produces zinc oxide \((ZnO)\) and elemental mercury \((Hg)\).

Cathode Reaction: The cathode typically involves reducing agents present within the electrolyte, which can include potassium hydroxide \((KOH)\) or sodium hydroxide \((NaOH)\).

Characteristics:

No Ion Exchange: Unlike typical batteries where ions move through an electrolyte, mercury cells rely solely on chemical reactions at the electrodes.

Steady Voltage: They provide a steady voltage output throughout their lifespan, making them suitable for devices like hearing aids that require consistent power.

Compact Size: Due to their compact size and reliable performance, they have been traditionally used in hearing aids.

In summary, mercury cells do not rely on ions in solution for their operation, making them unique compared to other types of batteries. Their use in hearing aids highlights their reliability and stability but also underscores the importance of transitioning to more sustainable battery technologies.