Carbon Compounds - Revision Guide

Introduction to Major Compounds of Carbon

Carbon, a non-metal, forms a vast array of compounds due to its unique ability to catenate and form strong covalent bonds with other elements. In high school chemistry, the most frequently studied inorganic compounds of carbon include its oxides (carbon dioxide, carbon monoxide) and carbonates. These compounds are vital to numerous industrial processes, biological cycles, and everyday life.

1. Carbon Dioxide (CO₂)

Chemical Formula and Common Name

• Chemical Formula: CO₂
• Common Name: Carbon dioxide; Dry ice (solid form)

Laboratory Preparation

Carbon dioxide is typically prepared in the laboratory by the action of dilute hydrochloric acid on calcium carbonate (e.g., marble chips).

• Reaction:

  CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

• Principle: The acid reacts with the carbonate to produce a salt, water, and carbon dioxide gas. The gas is usually collected by downward displacement of water or upward displacement of air due to its higher density than air.

Properties

• Physical: Colourless, odourless gas; denser than air; soluble in water; solid CO₂ sublimes directly into gas at room temperature and pressure.
• Chemical:
  • Acidic Oxide: Dissolves in water to form carbonic acid (H₂CO₃), a weak acid.

    CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

  • Non-combustible: Does not burn and extinguishes fires, hence used in fire extinguishers.
  • Reaction with bases: Reacts with bases to form carbonates or bicarbonates.
    • With limewater (calcium hydroxide solution):

      Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

      (White precipitate of calcium carbonate, used as a test for CO₂)
      • If excess CO₂ is passed:

      CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

      (Soluble calcium bicarbonate, causing the white precipitate to disappear)

2. Carbon Monoxide (CO)

Chemical Formula and Common Name

• Chemical Formula: CO
• Common Name: Carbon monoxide

Laboratory Preparation

Carbon monoxide can be prepared in the laboratory by the dehydration of formic acid (methanoic acid) using concentrated sulfuric acid as a dehydrating agent.

• Reaction:

  HCOOH(conc.) --(conc. H₂SO₄, 373K)--> CO(g) + H₂O(l)

• Principle: Concentrated sulfuric acid removes water from formic acid, leaving carbon monoxide.

Properties and Exam-Relevant Reactions

• Physical: Colourless, odourless, tasteless gas; slightly less dense than air; sparingly soluble in water.
• Chemical:
  • Neutral Oxide: Does not react with acids or bases to form salts.
  • Highly Toxic: Binds irreversibly with haemoglobin in blood to form carboxyhaemoglobin, preventing oxygen transport.
  • Powerful Reducing Agent: Especially at high temperatures.
    • Reduction of metal oxides (e.g., in blast furnace for iron extraction):

      Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)

      ZnO(s) + CO(g) → Zn(s) + CO₂(g)

  • Combustible: Burns with a blue flame to form carbon dioxide.

    2CO(g) + O₂(g) → 2CO₂(g)

  • Reaction with Chlorine: Forms phosgene (carbonyl chloride), a highly poisonous gas.

    CO(g) + Cl₂(g) → COCl₂(g)

3. Important Carbonates

A. Calcium Carbonate (CaCO₃)

• Chemical Formula: CaCO₃
• Common Names: Limestone, Marble, Chalk

Preparation

Occurs naturally in abundance. Can be prepared in the lab by passing carbon dioxide through limewater.

• Reaction:

  Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Properties and Exam-Relevant Reactions

• Physical: White solid, practically insoluble in water.
• Chemical:
  • Reaction with Acids: Decomposes with dilute acids to produce carbon dioxide gas.

    CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

    CaCO₃(s) + H₂SO₄(dilute) → CaSO₄(s) + H₂O(l) + CO₂(g)

  • Thermal Decomposition: Decomposes on strong heating (calcination) to produce calcium oxide (quicklime) and carbon dioxide.

    CaCO₃(s) --(Δ)--> CaO(s) + CO₂(g)

B. Sodium Carbonate (Na₂CO₃)

• Chemical Formula: Na₂CO₃
• Common Names: Washing soda (Na₂CO₃·10H₂O), Soda ash (anhydrous Na₂CO₃)

Preparation (Solvay Process – Key step)

The Solvay process is an industrial method for producing sodium carbonate. A key step involves the thermal decomposition of sodium bicarbonate.

• Reaction:

  2NaHCO₃(s) --(Δ)--> Na₂CO₃(s) + H₂O(l) + CO₂(g)

Properties and Exam-Relevant Reactions

• Physical: White crystalline solid, highly soluble in water, forms alkaline solutions.
  • Efflorescence: Washing soda (decahydrate) loses water of crystallisation on exposure to air.

    Na₂CO₃·10H₂O(s) → Na₂CO₃·H₂O(s) + 9H₂O(g)

• Chemical:
  • Hydrolysis: Dissolves in water to produce an alkaline solution due to hydrolysis of the carbonate ion.

    Na₂CO₃(aq) + 2H₂O(l) ⇌ 2NaOH(aq) + H₂CO₃(aq)

    Or more precisely, for the carbonate ion:

    CO₃²⁻(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + OH⁻(aq)

  • Reaction with Acids: Reacts with acids to liberate carbon dioxide.

    Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

Comparative Properties of Carbon Oxides

Property	Carbon Dioxide (CO₂)	Carbon Monoxide (CO)
Nature of Oxide	Acidic oxide	Neutral oxide
Solubility in Water	Soluble, forms carbonic acid	Sparingly soluble
Density (vs. Air)	Denser than air	Slightly less dense than air
Combustibility	Non-combustible, extinguishes fires	Combustible, burns with a blue flame
Reducing Property	Very weak/none	Strong reducing agent
Physiological Effect	Suffocating at high concentrations	Highly toxic (forms carboxyhaemoglobin)
Reaction with NaOH	Reacts to form sodium carbonate/bicarbonate	Does not react
Common Uses	Fire extinguishers, carbonated drinks, photosynthesis	Fuel gas, reducing agent in metallurgy, synthesis gas