Industrial Electrolysis

Electrolysis of aqueous NaCl (chlor-alkali process): $H_{2}$ at cathode (2$H_{2}$O + 2$e^{-}$ → $H_{2}$ + 2$OH^{-}$), $Cl_{2}$ at anode (2$Cl^{-}$ → $Cl_{2}$ + 2$e^{-}$), NaOH in solution. Products $H_{2}$, $Cl_{2}$, and NaOH are all industrially valuable. Electrolysis of molten NaCl (Downs process): produces Na metal at cathode and $Cl_{2}$ at anode. Electrolytic refining of copper: impure Cu acts as anode (dissolves), pure Cu deposits at cathode; impurities (Ag, Au) collect as "anode mud."

Electroplating

The object to be plated is the cathode; the plating metal is the anode; electrolyte contains the metal ion. To gold-plate a piece of jewelry: jewelry = cathode, gold = anode, gold salt solution = electrolyte. Thickness of plating = mass deposited controlled by Faraday's law (w = MIt/nF). Used for: decorative finishes, corrosion resistance (chromium plating), electrical conductivity (silver contacts).

Battery Technology and Energy Storage

Dry cell (Leclanché): Zn container/anode, $MnO_{2}$ + carbon cathode, $NH_{4}Cl$/Zn$Cl_{2}$ electrolyte (paste), 1.5 V. Used in remote controls, flashlights. Non-rechargeable because cell reaction products cannot be easily reversed.

Lead storage battery: 6 series cells × 2 V = 12 V. Pb anode, $PbO_{2}$ cathode, 38% H_{2}$$SO_{4} electrolyte. During discharge, both electrodes become Pb$SO_{4}$ and H_{2}$$SO_{4} is consumed (electrolyte dilutes). State of charge monitored by measuring H_{2}$$SO_{4} density (hydrometer). During charging: external electrical power reverses the reactions, regenerating Pb, $PbO_{2}$, and H_{2}$$SO_{4}.

Nickel-cadmium (NiCd): compact, high drain, rechargeable; used in power tools and cordless devices. Cadmium is toxic — disposal is an environmental concern.

Lithium-ion: (beyond NEET syllabus but noteworthy) highest energy density among common rechargeable batteries; used in smartphones and EVs.

Hydrogen-oxygen fuel cell: not a battery (does not store energy) but converts continuously supplied $H_{2}$ and $O_{2}$ to electricity. Only by-product is water. ~70% efficient vs. ~40% for combustion engines. Used in space shuttles (provided both electricity and drinking water from water by-product), forklifts, buses, and experimental vehicles.

Corrosion in Engineering

Corrosion costs global economies billions annually. Critical prevention methods:

• Galvanization: hot-dip zinc coating on iron. Zn (E° = −0.76 V) acts as sacrificial anode even when coating is scratched.
• Sacrificial anode / cathodic protection: Mg blocks buried adjacent to iron pipelines — Mg (E° = −2.37 V) oxidizes preferentially, protecting Fe.
• Stainless steel (Fe + 10-18% Cr): Cr forms passive $Cr_{2}O_{3}$ layer.
• Painting and polymer coatings: physical barrier preventing moisture and $O_{2}$ access.
• Alloying with noble metals (Pt, Pd): impractical for bulk iron but used in specialized applications.