Combustion of saturated hydrocarbons vs combustion of unsaturated hydrocarbons

I read in my science book that saturated hydrocarbons burn with a blue flame (indicates complete combustion) and unsaturated hydrocarbons burn with a yellow flame (indicates incomplete combustion) due to the highier percentage of carbon in unsaturated hydrocarbons as compared to saturated hydrocarbons. So I think the reason why unsaturated hydrocarbons don't burn completely is the presence of double and triple bonds in unsaturated hydrocarbons which are more stronger than single bonds because of which they are difficult to break. There was a question that :-

Why is a mixture of ethyne and oxygen used for welding?

And the answer was(as written in the book) :

So that ethyne burns completely without producing soot but according to me the issue is not about availability of oxygen. As I saw in the reactions of combustion, unsaturated hydrocarbons require less oxygen to burn than saturated ones. For example :- $$\ce{C2H6 + 7/2 O2 = 2CO2 + 3H2O}$$ And $$\ce{C2H2 + 5/2 O2 = 2 CO2 + H2O}$$ Here we can see that unsaturated hydrocarbons require less oxygen as compared to saturated hydrocarbons. So my queation is :-

Is the answer given in the book correct. If yes, why? $$Thank you$$

• very related (possibly a duplicate): chemistry.stackexchange.com/questions/46925/… – matt_black Mar 7 '17 at 16:57
• In fact, according to my answer to the related question, your book is wrong unless it specified specific conditions of burning. – matt_black Mar 7 '17 at 16:58

1 Answer

You are viewing it from the wrong perspective. In the case of normal burning, Generally, the oxygen is the limiting reagent (therefore some of the hydrocarbons left behind ). Let's take 1 mole of Oxygen. So you have to take 0.285 mol of ethane (0.57 mol carbon ) and 0.4 mol of ethyne (0.8 mol of carbon). It's quite clear ethyne has more carbon content for a given amount of oxygen. As if some hydrocarbon get left, Ethyne will give more carbon content.

https://www.youtube.com/watch?v=FJi4ODPQSE0

(Note: If concentration of oxygen is very high and enough temperature is given then both will burn with Blue Flame, as it is done in case of welding https://www.youtube.com/watch?v=hKMDxag8yAo )

Now In the case of welding, welding is done in presence of excess of Oxygen, so there is no question of limiting reagent of oxygen. What matters is Auto Ignition point, Ethyne has much lower auto ignition temperature that is 305 °C as compared to ethane which has 515 °C.

Of course, you pointed out above that for, given 1 mol of ethyne and ethane, ethyne need less oxygen to be burned, so it can be taken in quite concentrated form.

You can take maximum 80% Ethyne (concentration in air), it will burn. Whereas ethane can't be burn as its concentration exceeds 12.5% (in air).

More concentrated heat implies Higher can be temperature achieved.