Some Important Compounds of Carbon and Silicon

Carbon Monoxide, CO

Preparation of Carbon Monoxide:

It is prepared by direct oxidation of C in limited supply of oxygen.

On small scale it is prepared by dehydration of formic acid with Con. H₂SO₄ at 373K.

Properties:

• It is colourless, odourless and almost water insoluble gas.
• It a powerful reducing agent. CO is used in the extraction of many metals from their oxide ores.

Carbon Dioxide CO₂

Preparation of Carbon Dioxide:

It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.

Properties:

• It is a colourless and odourless gas.
• It is slightly soluble in water. When C0₂ dissolves in water only some of the molecules react with water to form carbonic acid.
• It is not poisonous like CO.
• But increase in combustion of fossil fuels and decomposition of limestone for cement manufacture increase of C0₂ in the atomosphere is one of the main reasons of green-house effect.
• Photosynthesis

Silicon Dioxide, SiO₂

Silicon dioxide, commonly known as silica, occurs in various crystallographic forms.
For example, Quartz, Cristobalite and thermite are some of the crystalline forms of silica.

Structure of Silicon Dioxide:

Silicon dioxide is a covalent three-dimensional network solid. Each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom in turn covalently bonded to another silicon atoms as shown below:

Properties of Silicon Dioxide:

• In normal form silica is very less reactive.
• At elevated temperature it does not reacts with halogens, dihydrogen and most of the acids and metals. But it reacts with HF and NaOH.

Uses of Silicon Dioxide:

• Quartz is extensively used as a piezoelectric material.
• Silica gel is used as adsorbent in chromatography.
• An amorphous form of silica, kieselghur is used in filtration plants.

Silicones

Preparation of Silicones:

The linear, cyclic or cross-linked polymeric compounds containing (R₂SIO) as a repeating unit, are known as silicones. They are manufactured from alkyl substituted chlorosilanes.

Properties of Silicones:

• It is chemically inert, water repellent, heat resistant.
• It is good electrical insulators.

Uses of Silicones:

• It is used as lubricants (Vaseline), insulators etc.

Silicates

• A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos. The basic structural unit of silicates is SiO^-4₄.

• Two important man-made silicates are glass and cement.

Zeolites

• Aluminosilicate are formed when few Si atoms are replaced by Al atoms in three-dimensional network of silicon dioxide.
• Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation,
• e.g., ZSM-5 (A type of zeolite) used to convert alcohols directly into gasoline.
• Zeolites are complex aluminium silicates.
• Hydrates zeolites are used as ion exchangers in softening of “hard” water.