Q.Write anode and cathode reactions that occur in dry cell. How does a dry cell differ from a mercury cell ?

Concept: Electrochemical cells — understanding the redox reactions in primary batteries and their design differences.

Anode (oxidation) in a dry cell:

The zinc container acts as the anode:

$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$

Cathode (reduction) in a dry cell:

A graphite rod surrounded by $\text{MnO}_2$ and carbon black serves as the cathode. The reduction involves ammonium ions:

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

Key difference from a mercury cell: …

The Concept: How a Voltaic Cell Works

Both dry cells and mercury cells are primary cells (non-rechargeable) that convert chemical energy into electrical energy through spontaneous redox reactions. The anode is where oxidation occurs (electrons are lost), and the cathode is where reduction occurs (electrons are gained). The electrolyte provides ions to complete the circuit internally.

The dry cell is the common "Leclanché cell" — cheap but with a falling voltage. The mercury cell is a more advanced design, giving a remarkably stable voltage until it is nearly exhausted.

1. Dry Cell Reactions

The dry cell has a zinc can as the anode and a carbon (graphite) rod as the cathode, surrounded by a paste of MnO₂, NH₄Cl, and ZnCl₂.

• Anode (oxidation): Zinc metal loses electrons to form Zn²⁺ ions.

• Cathode (reduction): Ammonium ions (NH₄⁺) from the paste are reduced at the carbon rod. The MnO₂ acts as a depolariser, preventing hydrogen gas buildup.

Overall cell reaction:

The dry cell gives about 1.5 V initially, but the voltage drops gradually as the cell is used because the internal resistance increases and the electrolyte becomes less effective.

2. Mercury Cell Reactions

The mercury cell uses a zinc amalgam (Zn/Hg) as the anode and a paste of HgO (mercury(II) oxide) mixed with carbon as the cathode. The electrolyte is a KOH (potassium hydroxide) paste — a basic medium.

• Anode (oxidation): Zinc from the amalgam is oxidised to zincate ion in the basic medium.

(Often written as: Zn + 2OH⁻ → ZnO + H₂O + 2e⁻)

• Cathode (reduction): Mercury(II) oxide is reduced to metallic mercury.

Overall cell reaction:

The mercury cell produces a steady 1.35 V throughout its life because the internal resistance remains nearly constant and the reactions do not produce gases or change the electrolyte composition.

3. How They Differ: Dry Cell vs. Mercury Cell