1. Batteries in Everyday Devices

Dry Cell (Leclanché):

• Anode: Zn (oxidizes); Cathode: $MnO_{2}$ + C graphite rod
• Electrolyte: $NH_{4}Cl$ + $ZnCl_{2}$ paste (moist, not liquid)
• EMF: 1.5 V; Non-rechargeable; Used in torches, remotes

Lead Storage Battery:

• Anode: Spongy Pb; Cathode: $PbO_{2}$; Electrolyte: $H_{2}SO_{4}$ (38%)
• EMF: 2 V/cell; 6 cells × 2 V = 12 V (car battery)
• Rechargeable; Discharge converts both electrodes to $PbSO_{4}$ ($H_{2}SO_{4}$ becomes dilute)
• Charging reverses: $PbSO_{4}$ → Pb (cathode) and $PbSO_{4}$ → $PbO_{2}$ (anode)
• Density of $H_{2}SO_{4}$ indicates state of charge (denser = more charged)

Mercury Cell:

• Anode: Zn-Hg amalgam; Cathode: HgO; Electrolyte: KOH/ZnO paste
• EMF: 1.35 V; Constant voltage (ideal for hearing aids, pacemakers, watches)
• Non-rechargeable; Environmentally hazardous (mercury disposal issues)

Nickel-Cadmium (NiCd):

• Anode: Cd; Cathode: NiO(OH); EMF: 1.2 V; Rechargeable
• Used in power tools; Memory effect issue; Cd is toxic

2. Hydrogen-Oxygen Fuel Cell

• Anode ($H_{2}$ side): $H_{2}$ + 2$OH^{-}$ → 2$H_{2}$O + 2$e^{-}$ (alkaline type)
• Cathode ($O_{2}$ side): $O_{2}$ + 2$H_{2}$O + 4$e^{-}$ → 4$OH^{-}$
• Overall: 2$H_{2}$ + $O_{2}$ → 2$H_{2}$O (only by-product is water)
• Efficiency: ~70% (far exceeds Carnot-limited heat engines ~40%)
• Application: Space shuttles, zero-emission vehicles, stationary power

3. Electroplating

• Electrolytic process: Object to be plated = cathode; Plating metal = anode
• Example: Nickel plating on iron — cathode: iron object; anode: Ni; electrolyte: $NiSO_{4}$ solution
• Applications: Decorative finishes, corrosion protection, aerospace parts

4. Corrosion Prevention in Real Life

• Galvanization (Zn coating): Even if scratched, Zn (E° = −0.76 V) corrodes sacrificially before Fe (E° = −0.44 V). Used for iron pipes, roofing sheets.
• Cathodic protection: Mg blocks buried near iron pipelines. Mg (E° = −2.37 V) oxidizes as anode; pipe is cathode (protected).
• Painting/coating: Barrier method — prevents moisture and $O_{2}$ contact with iron.
• Alloying: Stainless steel (Fe + Cr) — Cr forms a protective passive oxide layer.