Commercial cells

1. Primary cell: - Those in which the reaction taking place cannot be reversed. So cell can’t be recharged. e.g. - (1) Dry cell

Oxidation at Anode → Zn → Zn²⁺ + 2e^-

Reduction at cathode → 2MnO₂ + 2NH₄⁺ + 2e^- → Mn₂O₃ + 2NH₃ +H₂O

 Overall reaction → Zn + 2MnO₂ + 2NH₄⁺ → Mn₂O₃ + 2NH₃ + H₂O + Zn²⁺

The Ammonia (NH₃) evolved combine immediately with ZnCl₂ to form a complex-

ZnCl₂ + 2NH₃ → [Zn (NH₃)₂ Cl]

Hence cell will not burst, due to NH₃

As the overall reaction involves ions so the cell can’t provide a constant EMF. Because with time Zn⁺² and NH₄⁺ decreases The EMF ranges from 1.25 to 1.50 V

The ammonium chloride (NH₄Cl) used is corrosive because it produces HCl which corrodes the Zn metal and there is change of leakage of cell.

NH₄Cl + H₂O → NH₄OH + HCl

Paste include H₂O, due to which HCl produced will make the container to leak

(2) Mercury cell (Ruben – Mallory cell) or Button cell: - used in devices which require low current.

At Anode

Zn → Zn²⁺ + 2e^-

2 Zn + 4 OH^- → 2 ZnO + 2 H₂O + 4e^-

At cathode

2 HgO + 4 e^- + 2 H₂O → 2 Hg + 4 OH^-

2 Zn + 2 HgO → 2 ZnO + 2 Hg

Or

Zn + HgO → Hg + ZnO

As the overall reaction is not involving any kind of ions so it can provide a constant EMF of 1.35 volt.