How would a large alkane burn compared to a small alkane?

Answer 1

large or long chain alkanes will burn with a sooty flame with more soot being produced the longer the chain. they also burn with a darker flame that is more orange than bright yellow (pentane) or blue (methane)

this is because with a long chain alkane (large alkane) their is more carbon and hydrogen present per molecule and so more oxygen is needed.

for example in complete combustion of methane compared with nonane, nonane requires 7 times more oxygen.

CH4 + 2O2 -> CO2 + 2H2O C9H20 + 14O2 -> 9CO2 + 10H2O

in reality nonane burns incompletely (unless an oxidiser is used which will provide more oxygen eg. KNO3 or NaClO3) and the oxygen required is not their so carbon and carbon monoxide is produced.

C9H20 + 14O2 -> H2O + CO2 + CO + C

it is impossible to tell how many mols of each product is created.

as less of the molecule has been burned less energy is released so more carbon is being produced and so is also a colder flame.

As it is colder their is less heat to excite the electrons in the carbon molecules so they in turn release less light energy resulting in the darkening of the flame and reddening of the colour.

this is also why gas burners must have adequate ventilation other wise their is not enough oxygen (as it has been burned) and so CO is produced which is poisonous.

take for example a candle and a lighter the lighter employs methane/propane and the candle uses a much longer chain hydrocarbon.

the candle burns with a yellow to orange flame.

the lighter burns with a blue flame at the bottom and a yellow flame at the top, the yellow flame is due to the oxygen running out at that point and so carbon is produced. (unless it is a turbo-lighter which sucks up extra air via a carburetter to create a perfect fuel and oxygen mix).

I do hope this has helped and not just confused you