Alkanes are a source of heat. Explain it.

  1. No.2. Alkanes are a source of heat. Explain it.

Answer. The main constituents of fuels (coal, petroleum and natural gas) are hydrocarbons. When hydrocarbons-are burnt in air, CO2 and H2O along with heat are produced. This reaction is called combustion. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. It is highly exothermic reaction, i.e. it produces a lot of heat. The basic combustion reaction is

CH4 + 2O2 ———> CO2 + 2H20 + heat

The heat energy produced is used for various processes, in homes, transportations, as well as in industries.

  1. No.3. Prepare the following as directed.

(a) ethane from ethene

When ethene reacts with hydrogen in the presence of Ni at 250 to 300 °C ethane is formed.

Ni

H2C=CH2   +   H2             H3C-CH3

250-300°C

(b) Acetylene from tetra-halide

When an alkyl tetra-halide is heated with zinc dust, the elimination of halogen atoms takes place and acetylene is formed.

Cl  Cl

H-C―C-H        +2Zn(dust)     heat           HC≡CH   +  2ZnCI2 Cl

Cl   Cl                                    (Ethyne)

 

(c) Carbon tetra-chloride from methane

Alkanes react fairly with halogens in diffused sunlight only. CH4 reacts with Cl2 in diffused sun light to form chloromethane, dichloromethane tri chloromethane and finally Tetrachloromethane

diffused

CH4      +     C12                     CH3C1   +   HC1

Sunlight                Chloromethane

 

CH3C1 +    C12                        CH2C12 +    HC1

Dichloromethane

 

CH2C12 +   C12                        CHCl3   +    HCl

Trichloromethane

(Chloroform)

 

CHC13 +   C12                         CCl4    +    HC1

Tetrachloromethane

(Carbon tetrachloride)

 

 

 (d) Ethylene glycol from ethene

When ethene is treated with aq. alkaline KMn04 solution, ethylene glycol is formed in cold state.

3CH2 = CH2   + 2KMnO4 +   4H2O         3CH2 ― CH2   +   2KMnO4   + 2KOH

(e)  1,2-dibromoethane from ethene

When ethene is treated with bromine 1, 2-dibromoethane is formed.

H2C=CH2 + Br2 + Br-CH2-CH2-Br

(f) glyoxal from acetylene

When acetylene is oxidized by alkaline KMn04 hydroxyl groups add to triple bond. This intermediate product eliminates water molecules to form glyoxal.

HC≡CH   +   2KMnO   +   2H20   OH OH   +   2MnO2   +   2KOH

H-C—C-H

OH OH

 

O O

OH OH                            -2H2O         ║ ║

H-C—C-H                             H-C-C-H

OH OH                                  Glyoxal

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