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Methane
Methane is a hydrocarbon with the formula CH4,
consisting of one carbon and four hydrogen atoms.
It is produced in the laboratory by heating a mixture of
ground mixture of anhydrous sodium ethanoate (sodium acetate) and soda-lime
(this is quicklime soaked with a concentrated solution of sodium hydroxide).
CH3COONa(S)
+ NaOH(S)
®
Na2CO3(S)
+ CH4(g)
from soda-lime
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Note: soda-lime is used because it does not attack
glass readily and is not deliquescent. Soda-lime is therefore a non-deliquescent
form of sodium hydroxide.
Properties of Methane (Physical
and Chemical)
Physical properties
1. Methane is a gas which is colorless and odorless.
2. It is almost insolution in water.
3. It is much less dense than air (vapor density = 8, vapor
density of air = 14.4).
4. It is neutral to litmus.
Chemical Reactions
1. Combustion: On the application of a flame or electric
spark, methane burns or explodes in air with a pale, non-luminous flame. The
products of combustion is steam, carbon(IV) oxide and enormous heat.
CH4(g) + 2O2(g)
® CO2(g) + 2H2O(g)
DH = -890 kJ mol-1
Note: the cause of explosions in coal mines is the
ignition of the mixture of methane (the important constituent of natural gas
produced in coal mines) and oxygen of the air.
2. Reaction with chlorine - methane reacts slowly at ordinary
temperature with chlorine in the presence of light to form a mixture of products
- light acts as catalyst (the reaction is photocatalysis).
Different products are possible:
First products:
CH4(g) + Cl2(g)
® CH3Cl(g)
+ HCl(g)
Chloromethane
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Second products (with excess of chlorine):
CH3Cl(g) +
Cl2(g)
® CH2Cl2(g)
+ HCl(g)
Dichloromethane
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Third products (with excess of chlorine):
CH2Cl2(g)
+ Cl2
®
CHCl3(g) +
HCl(g)
Trichloromethane
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Fourth and final products (with excess of chlorine):
CHCl3(g) + Cl2(g)
® CCl4(g)
+ HCl(g)
Tetrachloromethane
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Note:
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The above reactions are substitution reactions - each hydrogen atom is replaced with an atom of chlorine successively to give the
products. This is so because methane (the alkanes in general) is saturated
(i.e., all the valence electrons of carbon are bonded with hydrogen atoms,
so, there is "no vacancy").
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Successive substitution occurs with increasing difficulty.
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Similar reactions as above
are formed between gaseous bromine and methane - the reactions are however slower.
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Methane and other alkanes are inert, i.e., they are unreactive. The
above two reactions are the only ones shown by alkanes at ordinary temperature
and pressure.
Uses of Methane
1. Used as fuel - for domestic and industrial heating.
Methane is advantageous as fuel because it is nonpoisonous, has no smell and
leaves no residue (i.e. it is environmentally friendly).
2. Used for the production of certain important compounds. Example, carbon
black, hydrogen, alkynes, carbon disulphide, hydrochloric acid, trichloromethane
(also called chloroform) and tetrachloromethane.
Uses of Halogenated Products of Methane
1. Methyl chloride (chloromethane), CH 3Cl,
a gas, used chiefly in the production of silicone resins and rubbers. Most of it
is made from methanol and hydrogen chloride:
CH3OH(l)
+ HCl(aq)
® H2O(l)
+ CH3Cl(g)
2. Methylene chloride (dichloromethane), CH 2Cl2,
is a liquid, used in paint and vanish removers and as a solvent for aerosol
paints.
3. Chloroform (trichloromethane), CHCl 3
- the chief use of chloroform is for the production of chlorodifluoromethane
(Freon-22). Also used in surgical operations as anaesthetic.
4. Tetrachlorometane or carbon tetrachloride, CCl 4.
This is the most important of the chlorinated methanes. The industrial
usefulness of this compound is based largely on its conversion to
dichlorodifluoromethane (Freon-12), trichlorofluoromethane (Freon- 11), and
tetrachloroethylene. Also used as solvent and for removal of grease in the
industry.
Related:
Hydrocarbon and Alkanes
Isomerism
in Alkanes
IUPAC Nomenclature of Alkanes
Petroleum and
Petrol
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