(Original post by jadelamlam) Why alkenes produce a sootier flame than alkanes? But alkenes require less O2 to burn!
ethane:2C2H6(g) + 7O2(g) --> 4CO2(g) + 6H2O(l)
ethene:C2H4(g) + 3O2(g) --> 4CO2(g) + 2H2O(l)
The C=C bond is harder to break (higher bond energy) than a C-C bond and so there is more likelihood of elemental carbon being produced as soot by alkenes. The hydrogen from both compounds always is burned.