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Why do saturated hydrocarbons burn with a clean flame while unsaturated hydrocarbons burn with a sooty flame when burnt in air?

My chemistry book says that it happens due to the amount of carbon being more than the amount of hydrogen in unsaturated hydrocarbons. But isn't that also the case with saturated hydrocarbons?

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  • $\begingroup$ Also, am i wrong in thinking that it happens due to the bonds of unsaturated compounds being stronger than the single bonds present in saturated hydrocarbons due to which incomplete combustion of unsaturated hydrocarbons takes place? $\endgroup$ – Aniansh Feb 25 '16 at 13:43
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    $\begingroup$ Your observation is not true in general. The flame character depends a lot of how the hydrocarbon is burned. If the amount of air or oxygen is right, all hydrocarbons will burn cleanly. Eg Acetylene (or ethyne or C2H2) will burn cleanly with one of the hottest flames known if the gas/oxygen mixture is well controlled as it is in oxygen-acetylene welding torches. $\endgroup$ – matt_black Dec 18 '16 at 11:00
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Your observation that unsaturated hydrocarbons burn with a sooty flame is not always true and depends on the circumstances. There are more factors at play than the nature of the hydrocarbon that is burning.

The most important reason why flames are sometimes smoky is that the combustion is incomplete. This means that not enough oxygen was present to fully oxidise all the carbon to carbon dioxide or carbon monoxide gases (and the amount of CO will vary with the ratio as well as, given enough oxygen, it should also be completely combusted to carbon dioxide). If there is not enough oxygen in the flame there are many competing side reactions that generate sooty resides (the reactions are pretty complex). Some pyrolysis reactions of hydrocarbons may occur before combustion itself (if the mixing in the flame is less than perfect) and these may contribute to the generation of sooty products which may be harder to burn than the original hydrocarbon.

Incomplete combustion is as much a function of the environment of burning as it is the thing that burns. Some environments don't enable enough oxygen to get into the gas mixture to drive complete combustion and the reactions can easily leave a lot of sooty emissions. A candle, for example, burns with a smoky flame despite being made from mostly saturated long chain hydrocarbons. It does so because a candle wick doesn't do a good job of mixing air with the vapours of the burning hydrocarbon (this is deliberate as the prime role of candles is to provide light and this comes from incandescent particles of soot in the flame: a pure flame burning efficiently would provide little light by itself).

Even well controlled environment such as engines can't completely avoid side reactions. But Diesel engines (which use mostly saturated hydrocarbons) produce more sooty particles than petrol (gasoline) engines though petrol engines have far more unsaturated fuels. The exact reasons are complex and depend on the precise environment inside the engine.

In well controlled environment where the oxygen-fuel ration is set carefully most hydrocarbons burn cleanly with little soot. Gas-based cookers (using mostly saturated methane, ethane, propane or butane) are very clean because they control the gas mix very precisely (use the wrong burner and they may well be sooty!). Welding and cutting torches can happily use the highly unsaturated acetylene with a clean, non-sooty, flame (the oxygen-acetylene flame is one of the hottest achievable) because they control the gas mix very carefully leaving little room for side reactions.

Uncontrolled flames might well show your general observation that unsaturated fuels burn more sootily, but this is as much a product of the environment as it is the product of the hydrocarbon. It is hard, without a carefully controlled experiment, to tell whether the general observation is true at all.

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  • $\begingroup$ Thanks for the answer, the comparison between burning acetylene in welding and the candle example clearly shows that the circumstances of combustion play a more significant role in deciding the soot produced as compared to the type of hydrocarbon. $\endgroup$ – Aniansh Mar 1 '17 at 11:37
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I think that your book is actually saying the percentage of carbon in unsaturated hydrocarbon is more than saturated hydrocarbon. Let's take the hydrocarbon with two carbon, which are saturated hydrocarbon(ethane), unsaturated hydrocarbons(ethene and ethyne).

The percentage of carbon in ethane($\ce{C_2H_6}$) =$\ce{[2(12)/{2(12) + 6(1)}]*100}$ = 80 %

The percentage of carbon in ethene($\ce{C_2H_4}$) =$\ce{[2(12)/{2(12) + 4(1)]*100}}$ = 86 %

The percentage of carbon in ethyne($\ce{C_2H_2}$) =$\ce{[2(12)/{2(12) + 2(1)}]*100}$ = 92 %

According from above, we can see unsaturated hydrocarbon have a higher percentage of carbon than saturated hydrocarbon. So, there is a sooty flame when burning unsaturated hydrocarbons and a clean flame when burning saturated hydrocarbon

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  • $\begingroup$ How exactly does the increase in carbon content translate to incomplete combustion? $\endgroup$ – Aniansh Feb 25 '16 at 13:56
  • $\begingroup$ @Aniansh Is this about complete or incomplete combustion? $\endgroup$ – Simon-Nail-It Feb 25 '16 at 13:58
  • $\begingroup$ you just mentioned in your answer that the increased carbon content of unsaturated hydrocarbons leads to them burning with a sooty flame and that only happens when incomplete combustion takes place. I am asking you how is increase in carbon content related to sooty flame? By this regard, ethane should burn less efficiently as compared to methane. Also, does the carbon to hydrogen ratio in a hydrocarbons affect its combustion properties? $\endgroup$ – Aniansh Feb 25 '16 at 14:02
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    $\begingroup$ What I can say is the soot is carbon, if a higher content of carbon will produce a higher chance of soot. Second, the carbon to hydrogen ratio in a hydrocarbons affect the efficiency of combustion. Regarding to the question of methane vs ethane, I still haven't have much research on it but I will try find the answer $\endgroup$ – Simon-Nail-It Feb 25 '16 at 14:14
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    $\begingroup$ Thanks for the answer ! It was helpful! I am sure someone else will post a complete answer in the future. $\endgroup$ – Aniansh Feb 25 '16 at 14:17

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