Important Sodium Compounds (Na)

Introduction to Major Compounds of Sodium

Sodium, an alkali metal, forms a variety of industrially and biologically significant compounds. Its high reactivity means it is always found in combined states in nature. Key compounds include its halides, hydroxides, carbonates, and bicarbonates, which are fundamental in various chemical industries and everyday applications.

Sodium Chloride (NaCl)

• Chemical Formula: NaCl
• Common Name: Common Salt, Table Salt, Rock Salt

Occurrence and Preparation

Sodium chloride is one of the most abundant salts found in nature.

• Seawater: Contains approximately 2.7% to 2.9% by mass of NaCl.
• Rock Salt: Large deposits of solid NaCl are found underground, often formed from the evaporation of ancient seas.
• Industrial Preparation: Primarily obtained by the evaporation of seawater or mining of rock salt deposits. Purification of crude salt involves dissolving it in water, filtering to remove insoluble impurities, and then precipitating impurities like magnesium chloride (MgCl₂) and calcium chloride (CaCl₂) using sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃), respectively. Pure NaCl is then obtained by recrystallization.
  • CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq)
  • MgCl₂(aq) + 2NaOH(aq) → Mg(OH)₂(s) + 2NaCl(aq)

Properties

• A white crystalline solid.
• Possesses a high melting point (801 °C) and boiling point (1413 °C).
• Highly soluble in water. Its solubility does not vary significantly with temperature.

Uses

• An essential component of the human diet.
• Used as a food preservative (e.g., in pickling and curing meat).
• A crucial raw material for the industrial manufacture of other important sodium compounds like sodium hydroxide, sodium carbonate, sodium bicarbonate, chlorine, and hydrogen chloride.
• Used in refrigeration mixtures (e.g., with ice).

Sodium Hydroxide (NaOH)

• Chemical Formula: NaOH
• Common Name: Caustic Soda

Industrial Preparation: Castner-Kellner Process

Sodium hydroxide is predominantly manufactured by the electrolysis of an aqueous solution of sodium chloride (brine) using a Castner-Kellner cell.

• Anode (Graphite): Chloride ions are oxidized to chlorine gas. 2Cl⁻(aq) → Cl₂(g) + 2e⁻
• Cathode (Mercury): Sodium ions are reduced to form sodium amalgam with the mercury cathode. Na⁺(aq) + e⁻ → Na(in Hg) (Sodium amalgam)
• Decomposition of Amalgam: The sodium amalgam then reacts with water in a separate chamber to produce sodium hydroxide solution, hydrogen gas, and mercury, which is recycled back to the cathode. 2Na(in Hg) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + 2Hg(l)

Properties

• A white, translucent, deliquescent solid (absorbs moisture and carbon dioxide from the atmosphere).
• A very strong base: Highly soluble in water, forming a strong alkaline solution.
• It is corrosive and can cause severe burns upon contact with skin.

Exam-Relevant Reactions

1. Neutralization with Acids: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
2. Reaction with Acidic Oxides: 2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l) 2NaOH(aq) + SO₂(g) → Na₂SO₃(aq) + H₂O(l)
3. Reaction with Amphoteric Metals (e.g., Aluminium, Zinc): 2Al(s) + 2NaOH(aq) + 6H₂O(l) → 2Na[Al(OH)₄](aq) + 3H₂(g) (Sodium tetrahydroxoaluminate(III)) Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate)
4. Saponification (Hydrolysis of esters/fats): A key reaction in soap manufacturing. RCOOR'(ester) + NaOH → RCOONa(soap) + R'OH(alcohol) (General representation)

Sodium Carbonate (Na₂CO₃)

• Chemical Formula: Na₂CO₃ (anhydrous), Na₂CO₃·10H₂O (decahydrate)
• Common Name: Washing Soda (decahydrate), Soda Ash (anhydrous)

Industrial Preparation: Solvay Process (Ammonia-Soda Process)

The Solvay process is an economical and widely used method for producing sodium carbonate.

1. Formation of Ammonium Bicarbonate: Brine (sodium chloride solution) is saturated first with ammonia (NH₃) and then with carbon dioxide (CO₂). NH₃(g) + H₂O(l) + CO₂(g) → NH₄HCO₃(aq)
2. Precipitation of Sodium Bicarbonate: The ammonium bicarbonate solution reacts with sodium chloride to form sodium bicarbonate, which is sparingly soluble and precipitates out. NH₄HCO₃(aq) + NaCl(aq) → NaHCO₃(s) + NH₄Cl(aq)
3. Conversion to Sodium Carbonate: The precipitated sodium bicarbonate is filtered, washed, and then heated to produce anhydrous sodium carbonate, along with water and carbon dioxide. The CO₂ is recycled. 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)
4. Ammonia Recovery: Ammonium chloride (NH₄Cl), a byproduct, is reacted with calcium hydroxide (Ca(OH)₂) to regenerate ammonia (NH₃), which is recycled back into the process, making it highly efficient. 2NH₄Cl(aq) + Ca(OH)₂(aq) → 2NH₃(g) + CaCl₂(aq) + 2H₂O(l)

Properties

• A white crystalline solid.
• Decahydrate (Washing Soda): Na₂CO₃·10H₂O is efflorescent, meaning it loses water of crystallization when exposed to air, forming the monohydrate.
• Its aqueous solution is alkaline due to the hydrolysis of the carbonate ion: CO₃²⁻(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + OH⁻(aq)

Exam-Relevant Reactions

1. Reaction with Acids: It reacts with acids to produce salt, water, and carbon dioxide gas. Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) Na₂CO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)
2. Water Softening: Used to precipitate calcium and magnesium salts (which cause water hardness), thereby softening water. Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na⁺(aq) Mg²⁺(aq) + Na₂CO₃(aq) → MgCO₃(s) + 2Na⁺(aq)

Sodium Bicarbonate (NaHCO₃)

• Chemical Formula: NaHCO₃
• Common Name: Baking Soda

Preparation

• It is obtained as an intermediate product in the Solvay process (Step 2).
• It can also be prepared by passing carbon dioxide gas through a concentrated solution of sodium carbonate: Na₂CO₃(aq) + H₂O(l) + CO₂(g) → 2NaHCO₃(aq)

Properties

• A white crystalline solid that is sparingly soluble in water.
• Its aqueous solution is mildly alkaline.
• Decomposes on heating above 100°C, releasing carbon dioxide gas, which is a key property for its uses. 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)

Uses

• Baking: The carbon dioxide gas released upon heating makes cakes and bread rise, giving them a porous texture.
• Antacid: As a mild base, it is used to neutralize excess stomach acid, providing relief from indigestion. NaHCO₃(aq) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
• Fire Extinguishers: It is used in soda-acid fire extinguishers, where it reacts with an acid to produce CO₂ which smothers fires.

Comparative Properties of Important Sodium Compounds

Property	Sodium Chloride (NaCl)	Sodium Hydroxide (NaOH)	Sodium Carbonate (Na₂CO₃)	Sodium Bicarbonate (NaHCO₃)
Common Name	Common Salt	Caustic Soda	Washing Soda / Soda Ash	Baking Soda
Nature	Neutral salt	Strong base	Weak base (aqueous solution)	Mild base
Solubility in Water	Highly soluble	Highly soluble	Soluble	Sparingly soluble
Deliquescent/Efflorescent	Neither	Deliquescent	Efflorescent (decahydrate)	Neither
Effect of Heat	Stable (melts at high temp)	Stable (melts at 318 °C)	Stable (anhydrous), decahydrate loses water, then stable	Decomposes to Na₂CO₃, H₂O, CO₂ above 100°C
pH of Aq. Sol.	~7	>13	~11	~8.5
Key Uses	Food, raw material for other Na cpds	Soap, paper, dyes, petroleum refining	Glass, soap, detergents, water softening	Baking, antacid, fire extinguishers