Question

draw the electron dot structure for ethyne .A mixture of ethyne and oxygen is burnt for welding > IN your opinion why cannot we use a mixture of ethyne and air for this purpose ??

Answer 1

1) The Ethyne (C2 H2) is made up of 2 carbon atoms and 2 hydrogen atoms.
The electron dot structure of Ethyne:
          
 H :  C ::: C : H        
 
2)Ethyne is an unsaturated hydrocarbon. When ethyne(acteylene) is burned in air, it burns with a very sooty flame due to incomplete combustion. The temperature of flame produced is also not high.
If a mixture of ethyne and pure oxygen is burned, then ethyne burn with completely producing blue flame.
The oxy acetylene flame is extremely hot and produces a very high temperature which is used for welding metals.

Answer 2

Electron dot structure of ethyne(C2H2)

H : C ::: C : H

Ethane is burnt with oxygen for getting high temperature because if we use air it will produce low heat, also it will give smoky flame due to incomplete combustion.

2HC:::CH+5O2 = 4CO2+ 2H2O + HEAT