Electrolysis Of Brine — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Chlor-Alkali Process Electrolysis of concentrated aqueous sodium chloride (brine) produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in solution. Industrially, the chlor-alkali process is carried out in membrane, diaphragm, or (older) mercury cells to keep Cl2 and OH− separated and to obtain pure NaOH. Greenish-yellow chlorine gas with a characteristic choking smell evolves at the anode; colorless, odourless hydrogen bubbles at the cathode; the catholyte becomes alkaline (formation of NaOH). In membrane cells, products remain separated; if mixed, a bleaching smell (hypochlorite) may appear. The overall process is non-spontaneous (endergonic) with E°cell ≈ −2.19 V; an applied cell voltage typically 2.7–3.2 V (including overpotentials and IR drop) is required. It is driven by electrical energy input; equilibrium is shifted by continuous removal of Cl2, H2, and NaOH. 1. Anodic oxidation (concentrated brine favors chloride discharge due to overpotential): (2 , Cl - (aq) Cl 2 (g) + 2 , e - ); E ( Cl 2/Cl - ) = +1.36 , V . 2. Cathodic reduction of water: (2 , H 2O (l) + 2 , e - H 2 (g) + 2 , OH - (aq) ); E ( H 2O/H 2 (alk.) ) -0.83 , V . 3. Sodium ions migrate through the cation-exchange membrane (in membrane cells) from the anode compartment to the cathode compartment, where they combine with freshly formed OH - to give NaOH(aq): ( Na + (aq) + OH - (aq) NaOH (aq) ). 4. Product separation is maintained: the membrane (or diaphragm) blocks back-diffusion of OH - and Cl - so that ( Cl 2 ) does not react with ( OH - ) to form hypochlorite/chlorate; gases disengage from electrodes and are collected separately. 5. Overall cell reaction (membrane/diaphragm): (2 , NaCl (aq) + 2 , H 2O (l) 2 , NaOH (aq) + Cl 2 (g) + H 2 (g) ); E cell -2.19 , V , so an external potential (> overpotentials) is required. 6. Mercury cell variant (Castner–Kellner): at the cathode, ( Na + ) forms sodium amalgam: ( Na + (aq) + e - + Hg (l) Na(Hg) ), which is then reacted in a separate decomposer with water: (2 , Na(Hg) + 2 , H 2O (l) 2 , NaOH (aq) + H 2 (g) + 2 , Hg (l) ), while chlorine evolves at the anode. Assuming oxygen forms at the anode in concentrated brine—actually Cl2 is produced due to chloride’s lower overpotential. Thinking sodium metal is deposited—water is reduced instead, giving H2 and OH− at the cathode. Forgetting that NaOH is formed and collected in the cathode compartment (not the anode side). Ignoring that mixing Cl2 with hot NaOH yields NaOCl/NaClO3 (bleach)—hence the need for product separation. Confusing cell types: membrane (modern, high-purity NaOH) vs diaphragm (dilute NaOH with brine) vs mercury (amalgam intermediate, environmental hazard). Misplacing gases: Cl2 at anode, H2 at cathode; both are collected separately to avoid explosive mixtures. Overlooking that the process requires external DC; E°cell is negative (non-spontaneous).