 |
| Alkanes - Methane, A Typical Example |
|
|
| |
| Molecular formula : CH4 |
| |
| Electronic formula : |
| |
 |
| |
| Structural formula: |
| |
 |
| |
| Shape of the methane molecule: Tetrahedral |
| |
| Bond angle: 109.5o |
| |
 |
| |
| Molecular mass: 16 amu. |
| |
| Common name: Marsh gas. |
| |
|
| |
| 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. |
| |
|
| 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. |
| |
|
|
| |
 Colourless, odourless, tasteless |
| |
 Lighter than air and water |
| |
 Insoluble in water |
| |
 Green-house gas - traps infrared radiations |
| |
|
| |
 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 |
| |
|
 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. |
| |
| |
| |
