Q.Match the items of Column I and Column II.
Column I:

Concept: Classification of Electrochemical Cells — each cell type has a distinct construction, reaction, and application.

Step 1 – Lechlanche cell (dry cell): The anode reaction is $Zn \rightarrow Zn^{2+} + 2e^-$. This matches (d).

Step 2 – Ni–Cd cell: It is a rechargeable (secondary) cell. This matches (c). …

The Concept: Cell Representation and the Nernst Equation

Before matching, let's understand what each cell type is and why its description fits. The Nernst equation tells us how cell potential depends on concentration, but here we're matching types of cells to their characteristics — so we need to know the chemistry of each.

A Leclanché cell is the common dry cell (zinc-carbon). Its anode reaction is $Zn \to Zn^{2+} + 2e^-$, which is exactly what option (d) says. The cathode uses MnO₂ and carbon, and the electrolyte is a paste of NH₄Cl and ZnCl₂.

A Ni–Cd cell is a rechargeable (secondary) battery. The key word here is "rechargeable" — option (c). It uses nickel hydroxide at the cathode and cadmium at the anode, and you can reverse the reaction by applying an external voltage.

A fuel cell is different: it continuously consumes fuel (like hydrogen) and oxidant (oxygen) from outside. The overall reaction is $2H_2 + O_2 \to 2H_2O$, which matches (a). And because it directly converts the chemical energy of combustion into electrical energy, it also matches (e). So fuel cell has two matches.

A mercury cell uses a zinc anode and HgO cathode, but the electrolyte is a paste of KOH and ZnO — no free-flowing ions in solution. It's small, long-lasting, and used in hearing aids. That's option (b).

Now let's match systematically.

Step-by-Step Matching

1. Leclanché cell (i)

   The defining reaction at the anode is $Zn \to Zn^{2+} + 2e^-$. This is exactly option (d).

   No other cell in Column I has this as its primary description (though mercury cell also uses Zn, its defining feature is different).

   So (i) matches with (d).

2. Ni–Cd cell (ii)

   This is a rechargeable battery. The term "rechargeable" is option (c).

   No other cell here is rechargeable (Leclanché is primary, fuel cell is continuous, mercury is primary).

   So (ii) matches with (c).

3. Fuel cell (iii)

   Two characteristics apply: …

Method: Concept Matching with Electrochemical Cell Classification

This method uses the defining characteristics of each cell type — electrolyte nature, rechargeability, electrode reactions, and application — to match correctly.

Steps:

Step 1: Recall the key features of each cell in Column I

Step 2: Match each cell to its description in Column II

• (i) Lechlanche cell → (d) reaction at anode, Zn → Zn²⁺ + 2e⁻
• (ii) Ni–Cd cell → (c) rechargeable …

Here are the most common mistakes students make when matching these cells with their descriptions, along with the correct reasoning to avoid them.

Mistake 1: Confusing "Fuel Cell" with "Combustion"

• The Error: Students see "converts energy of combustion into electrical energy" (option e) and match it with the Leclanché cell or Ni-Cd cell, thinking any cell that uses a chemical reaction is "combustion."
• Why it’s wrong: Combustion specifically means a reaction with oxygen that produces heat and light (like burning hydrogen or methane). Only a Fuel cell uses a continuous supply of fuel (like $H_2$) and oxidant (like $O_2$) to directly convert the energy of that combustion-like reaction into electricity.
• How to avoid: Remember the keyword: Fuel cell = "converts energy of combustion" (option e) AND its cell reaction is $2H_2 + O_2 \rightarrow 2H_2O$ (option a). These two are always paired.

Mistake 2: Forgetting the "Rechargeable" Feature of Ni-Cd

• The Error: Students match Ni–Cd cell with "does not involve any ion in solution" (option b) because they think "no liquid" means "no ions." Or they match it with "reaction at anode, $Zn \rightarrow Zn^{2+} + 2e^-$" (option d) because they confuse the anode material.
• Why it’s wrong: Ni-Cd cells are rechargeable (option c). They use solid electrodes (Nickel and Cadmium) and a paste electrolyte, but they do involve ions in the paste. The "no ion in solution" description belongs to the Mercury cell (which uses a paste of $KOH$ and $Zn(Hg)$ — but the key is it's used in hearing aids). Also, the anode reaction $Zn \rightarrow Zn^{2+} + 2e^-$ is for the Leclanché cell (dry cell), not Ni-Cd.
• How to avoid: Memorise the key property: Ni-Cd = rechargeable (option c). Don't confuse it with primary (non-rechargeable) cells like Leclanché or Mercury.

Mistake 3: Mixing up the Anode Reaction of Leclanché and Mercury Cells

• The Error: Students match Mercury cell with "reaction at anode, $Zn \rightarrow Zn^{2+} + 2e^-$" (option d) because both use zinc. They forget that the Leclanché cell also uses a zinc anode.
• Why it’s wrong: Both Leclanché and Mercury cells use a zinc anode, but the Leclanché cell is the classic "dry cell" where the anode reaction is exactly $Zn \rightarrow Zn^{2+} + 2e^-$ (option d). The Mercury cell uses a zinc amalgam anode, but its defining feature is that it does not involve any ion in solution (option b) and is used in hearing aids.
• How to avoid: Link the anode reaction (option d) specifically to the Leclanché cell (the most common primary cell). Link the "no ion in solution / hearing aid" (option b) specifically to the Mercury cell.

Mistake 4: Forgetting the Fuel Cell Reaction …