why alkene will produce more soot than alkane?

Question

nechirwan · Answer

The reason is different fractions of carbon in the molecules. 
e.g. 
Ethane 
2 C2H6 + 7 O2 --> 4 CO2 + 6 H2O 
Ethene 
2 C2H4 + 6 O2 --> 4 CO2 + 4H2O 

In the oxidation reaction of ethane you need 57% of the oxygen for the carbon. In the oxidation reaction of ethene you need 67% of the oxygen for the carbon. 
So if a partial combustion takes part (and only then soot is formed) this has more inluence on the carbon fraction in alkenes than in alkanes and therefor more soot is formed. 
To distinguish between alkanes and alkenes you can use several methods of analysis. e.g. addition of Br2 (rapid decolourization with alkenes) or Baeyer reaction (addition KMnO4/Na2CO3 yields in formation of MnO2 in presence of alkenes/alkines).