Alkanes - Methane, A Typical Example

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General features

Molecular formula : CH₄

Electronic formula :

Structural formula:

Shape of the methane molecule: Tetrahedral

Bond angle: 109.5^o

Molecular mass: 16 amu.

Common name: Marsh gas.

Occurrence

Methane occurs in:

• Natural gas

• Marshy areas where plant matter rots and decomposes. (paddy fields, stagnant ponds)

• Coal mines

• Bodies of termites (cellulose digesting insect)

• Intestines of ruminating animals (animals that chew the cud e.g., cows)

• Constituent of biogas.

Preparation of methane in the laboratory

Methane is prepared in the laboratory by the action of sodium acetate on soda lime. (Soda lime = NaOH + CaO)

Procedure

• A mixture of sodium acetate and soda lime is taken in a hard glass test tube fitted with a rubber stopper carrying a delivery tube.

• The test tube is clamped to a clamp stand.

• The apparatus is arranged as shown in the figure.

• The test tube is heated strongly.

• Sodium acetate reacts with sodium hydroxide to form methane gas. The chemical reaction taking place is given by the following reaction:

This reaction is called decarboxylation.

Mode of collection

Methane is collected by the downward displacement of water.

Note:

Methane forms an explosive mixture with air and is therefore dangerous. Methane being insoluble in water, is covenient and safer to collect by the downward displacement of water.

Properties of Methane

Physical properties

• Colourless, odourless, tasteless

• Lighter than air and water

• Insoluble in water

• Green-house gas - traps infrared radiations

Chemical Properties

Combustion

Methane forms an explosive mixture with air. It burns with a violent explosion forming carbon dioxide and water vapour.

Pyrolysis (Pyro - heat ; lysis - break down)

Thermal decomposition of methane in the absence of air forming its constituent elements, carbon and hydrogen is called pyrolysis.

In a limited supply of air carbon monoxide is formed.

Substitution with Chlorine (Chlorination)

The hydrogen atoms of methane get substituted one after the other by chlorine atoms to form four substitution products as follows:

i.

Methyl chloride

IUPAC name - monochloro methane

ii.

Methylene dichloride

IUPAC name - Dichloro methane

iii.

Chloroform

IUPAC name - Trichloro methane

iv

Carbon tetra chloride

IUPAC name - Tetra chloro methane

Uses of methane

• Domestic fuel (It is a component of natural gas, biogas, coal gas, etc.)

• Carbon black produced by pyrolysis of methane is used as a filler for natural rubber.It is also used in paints and printer's ink.

• Hydrogen produced by pyrolysis of methane is used in the manufacture of ammonia, a raw material for nitrogenous fertilizers.

• Chloroform, a substitution product of methane has anaesthetic properties.

• Carbon tetrachloride, a substitution product of methane is used as a solvent.