Sodium Hydroxide ($NaOH$)
The Castner-Kellner Cell, Halogen reactions, and Amphoterism.
Sodium Hydroxide ($NaOH$), commonly known as Caustic Soda, is one of the strongest and most widely used industrial bases. Its preparation involves highly specific electrochemical principles, and its chemical reactions serve as the foundation for testing advanced inorganic concepts in JEE and NEET.
1. Preparation: The Castner-Kellner Cell
Sodium hydroxide is commercially manufactured by the electrolysis of aqueous sodium chloride (brine) in a Castner-Kellner cell.
Cell Setup:
- Cathode: A flowing stream of Mercury ($Hg$).
- Anode: Carbon (or Titanium) rods.
- Electrolyte: Concentrated aqueous $NaCl$ (Brine).
The Reaction Mechanism (The JEE Trap):
In aqueous $NaCl$, both $Na^+$ and $H^+$ ions are present. Standard electrochemistry suggests $H^+$ should discharge at the cathode first (since $H^+$ is easier to reduce than $Na^+$). However, because a mercury cathode is used, the overvoltage of hydrogen on mercury is extremely high. This forces the $Na^+$ ions to discharge instead.
$Na^+ + e^- \xrightarrow{Hg} Na-\text{amalgam}$
$Cl^- \rightarrow \frac{1}{2}Cl_2 \uparrow + e^-$
Formation of $NaOH$:
The sodium amalgam ($Na-Hg$) is removed from the cell and treated with water to yield Sodium Hydroxide, Hydrogen gas, and regenerating the Mercury.
2. Physical Properties & Deliquescence
- Sodium hydroxide is a white, translucent solid. It melts at $591\text{ K}$.
- Highly Deliquescent: It readily absorbs moisture from the atmosphere to form a concentrated aqueous puddle.
- Reaction with Atmospheric $CO_2$: If left exposed to air, the surface of solid $NaOH$ absorbs carbon dioxide to form a crust of Sodium Carbonate ($Na_2CO_3$).
$2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O$
- Its dissolution in water is highly exothermic.
3. Reaction with Halogens (Disproportionation)
The reaction of $NaOH$ with halogens (like $Cl_2, Br_2, I_2$) is a classic example of a disproportionation reaction (where the halogen is simultaneously oxidized and reduced). The products depend strictly on the temperature and concentration of the $NaOH$.
A. With Cold & Dilute $NaOH$
Produces Sodium Halide (oxidation state $-1$) and Sodium Hypohalite (oxidation state $+1$).
$NaOCl$ is Sodium Hypochlorite (the active ingredient in bleach).
B. With Hot & Concentrated $NaOH$
Produces Sodium Halide (oxidation state $-1$) and Sodium Halate (oxidation state $+5$).
$NaClO_3$ is Sodium Chlorate.
4. Reactions with Amphoteric Metals & Non-Metals
Reaction with Amphoteric Metals
While most metals only react with acids, amphoteric metals ($Zn, Al, Sn, Pb, Be$) react with strong bases like aqueous $NaOH$ to form complex soluble salts and liberate Hydrogen gas ($H_2$).
$Zn + 2NaOH + 2H_2O \rightarrow Na_2[Zn(OH)_4] + H_2 \uparrow$
(Sodium zincate, often written as $Na_2ZnO_2$)
$2Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2 \uparrow$
(Sodium aluminate, often written as $NaAlO_2$)
Reaction with Non-Metals (White Phosphorus Trap)
When White Phosphorus ($P_4$) is boiled with an aqueous solution of $NaOH$ in an inert atmosphere (like $CO_2$), it undergoes disproportionation to form highly toxic Phosphine gas ($PH_3$).
(Sodium hypophosphite)
5. Industrial Uses
- Soap & Detergents: Used in the saponification of fats and oils to manufacture soap.
- Bauxite Purification: Crucial in the Baeyer's process for purifying bauxite ore to extract Aluminum.
- Textile Industry: Used for mercerizing cotton (improving its strength and luster).
- Petroleum Refining: Used to neutralize acid impurities.
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