SS2 Chemistry Lesson Note on Methane

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METHANE

Occurrence: Methane is the major constituent of natural gas, petroleum gas and, is usually known as Marsh gas. It is also found in poorly ventilated coal mines as fire dump.

LABORATORY PREPARATION

Procedure: A finely powdered mixture of equal amount of anhydrous sodium ethanoate and soda lime is heated in a hard glass test tube. The methane evolved is collected over water.

Equation of reaction is shown below:

CH3COONa_(s)   +   NaOH_(s)    →   Na₂CO_3(s) +   CH_{4 (g)}

To prepared other members, the same method is applied. For example, the preparation of ethane required heating sodium propanoate according to the equation below:

CH₃CH₂COONa  →   Na₂CO₃  +  C₂H₆                                          

PROPERTIES OF ALKANES

PHYSICAL PROPERTIES

1. The first members (i.e. C₁ –C₄) are colourless gases. From (C₅ –C₁₇) are colourless liquids. Higher members greater than C₁₇ are colourless, wax like solids.
2. They are insoluble in water
3. Branched alkanes have lower boiling and melting points than their corresponding straight chain alkanes. Branching makes the molecules spherical and therefore reduces the intermolecular forces between the molecules. For example, the boiling point of 2-methylpropane is lower than that of the straight chain isomer butane.

 CHEMICAL PROPERTIES

Alkanes are generally inert but they can undergo two types of reactions: combustion and substitution reactions

1. Combustion reaction. Alkanes burn in air (or oxygen) to form CO₂ and H₂O (steam). The reaction is exothermic e.g

Combustion of Methane, CH₄

CH_4(g) + 2O_2(g)  →  2H₂O_(g) +CO_2(g)

2. Substitution reaction.: Alkanes undergo substitution reaction with halogens. The reaction of methane with chlorine takes place in the presence of ultra-violet light which serves as catalyst. The continues until all hydrogen atoms in methane are replaced with chlorine

CH_4(g) + Cl_2(g)  →  CH₃Cl_(g) + HCl_(g)

CH₃Cl + Cl₂    CH₂Cl₂ + HCl

CH₂Cl₂  + Cl₂  → CHCl₃  + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

Uses of methane

1. It is mainly used fuel
2. It is used to make carbon black, carbon (iv) sulphide, trichloromethane or tetrachloromethane.

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