silicon dioxide (sio₂) — jee/neet short notes

1. silica / silicon dioxide

• SiO₂ = silica
• About 95% of earth’s crust is made up of silica and silicates.
• Important crystalline forms:
  • quartz
  • cristobalite
  • tridymite
• These forms are interconvertible at suitable temperatures.

2. structure of sio₂

• SiO₂ is a covalent, 3-dimensional network solid.
• Each Si atom is tetrahedrally bonded to 4 O atoms.
• Each oxygen atom is bonded to 2 silicon atoms.
• Each corner of a SiO₄ tetrahedron is shared with another tetrahedron.
• Forms 8-membered rings containing alternate Si and O atoms.
• Can be considered a giant molecule.

3. chemical properties

• SiO₂ is almost non-reactive because of its very high Si–O bond enthalpy.
• Resists:
  • halogens
  • H₂
  • most acids
  • metals, even at high temperature
• Exceptions: HF and NaOH

With NaOH:$$SiO_2+2NaOH\rightarrow Na_2SiO_3+H_2O$$

With HF:$$SiO_2+4HF\rightarrow SiF_4+2H_2O$$

4. uses of silica

• Quartz: piezoelectric material → accurate clocks, radio/TV broadcasting and mobile communications.
• Silica gel: drying agent; support for chromatographic materials and catalysts.
• Kieselguhr: amorphous silica; used in filtration.

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silicones

• Silicones are organosilicon polymers.
• Repeating unit:

$$\boxed{(R_2SiO)_n}$$

• Starting materials: alkyl/aryl substituted silicon chlorides

$$R_nSiCl_{4-n}$$

• Methyl chloride reacts with silicon in presence of Cu catalyst at 573 K to produce methyl-substituted chlorosilanes, which are used to make silicones.

properties of silicones

• Water-repellent (hydrophobic) due to non-polar alkyl groups.
• High thermal stability.
• High dielectric strength.
• Resistant to oxidation and chemicals.
• Generally chemically stable.

uses

• Sealants
• Greases
• Electrical insulators
• Waterproofing fabrics
• Surgical and cosmetic applications because of biocompatibility

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silicates

• Basic structural unit:

$$\boxed{SiO_4^{4-}}$$

• Si is bonded tetrahedrally to four O atoms.
• SiO₄ units can share 1, 2, 3 or 4 oxygen atoms.
• Depending on sharing, they form:
  • chains
  • rings
  • sheets
  • 3-D networks
• Negative charge is balanced by positively charged metal ions (cations).

examples

• Feldspar
• Zeolites
• Mica
• Asbestos

Important: Glass and cement are important man-made silicates.

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zeolites

• When some Si atoms are replaced by Al atoms in the 3-D SiO₂ network, an aluminosilicate is formed.
• Replacement produces an overall negative charge.
• Charge is balanced by cations such as:
  • Na⁺
  • K⁺
  • Ca²⁺

important uses

1. Catalysts
   • Used in petrochemical industries for:
     • cracking of hydrocarbons
     • isomerisation
   • ZSM-5 converts alcohols directly into gasoline.
2. Ion exchangers
   • Hydrated zeolites are used to soften hard water by ion exchange.