- 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.
- 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.
H2C=CH2 + H2 H3C-CH3
(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.
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
CH4 + C12 CH3C1 + HC1
CH3C1 + C12 CH2C12 + HC1
CH2C12 + C12 CHCl3 + HCl
CHC13 + C12 CCl4 + HC1
(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
OH OH -2H2O ║ ║
OH OH Glyoxal