CBSE · Class 12 · Chemistry · ElectrochemistryWhat are primary and secondary batteries? Explain the structure, operation, and chemical reactions of Leclanché cell (dry cell) and lead-storage battery with suitable equations.

Introduction to Batteries

Batteries are essentially galvanic cells where chemical energy of redox reactions is converted into electrical energy. They are mainly classified into two categories:

• Primary Batteries: In these batteries, the reaction occurs only once, and after some time of use, the battery dies, and it cannot be recharged or reused. Example: dry cell, mercury cell.
• Secondary Batteries: These batteries can be recharged by passing current in the opposite direction so that they can be used repeatedly. Example: lead-storage battery, nickel-cadmium cell.

1. Leclanché Cell (Dry Cell)

• Structure: A standard dry cell consists of a zinc cylinder acting as the anode. The cylinder is filled with a moist paste of ammonium chloride ($NH_4Cl$) and zinc chloride ($ZnCl_2$). Inside the paste, a carbon (graphite) rod is placed, acting as the cathode. Around the carbon rod, a mixture of manganese dioxide ($MnO_2$) and powdered carbon is present.
• Operation and Reactions:
  • Anode (Oxidation): Zinc is oxidized to form zinc ions. $$Zn(s) ightarrow Zn^{2+} + 2e^{-}$$
  • Cathode (Reduction): Manganese dioxide undergoes reduction, where the oxidation state of $Mn$ changes from +4 to +3, and $NH_4^+$ ions are consumed. $$MnO_2 + NH_4^+ + e^{-} ightarrow MnO(OH) + NH_3$$
  • Complex Formation: The ammonia ($NH_3$) formed in the reaction combines with $Zn^{2+}$ ions to form $[Zn(NH_3)_4]^{2+}$ complex ions, maintaining the cell potential.
• Potential: The potential of this cell is approximately $1.5 V$.

2. Lead-Storage Battery

• Structure: This is the most important secondary cell widely used in automobiles and inverters. It consists of a lead anode and a grid cathode made of lead dioxide ($PbO_2$). Sulfuric acid ($H_2SO_4$) at 38% concentration is used as the electrolyte.
• Discharge (Discharging) Operation and Reactions:
  • Anode Reaction: Lead sulfate ions react with lead to form lead sulfate. $$Pb(s) + SO_4^{2-}(aq) ightarrow PbSO_4(s) + 2e^{-}$$
  • Cathode Reaction: Lead dioxide undergoes reduction in the presence of sulfuric acid and protons to form lead sulfate. $$PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^{-} ightarrow PbSO_4(s) + 2H_2O(l)$$
  • Overall Cell Reaction: $$Pb(s) + PbO_2(s) + 2H_2SO_4(aq) ightarrow 2PbSO_4(s) + 2H_2O(l)$$
• Charging: When the battery is charged, the cell reactions occur in the opposite direction, and lead sulfate is converted back to lead and lead dioxide.