For example Ethyne in combustion by limited supply of air

$$ \ce{ 2C2H2 + 3O2 ->[\Delta] 2CO2 + 2H2O + 2C } $$ vs.

$$ \ce{ 2C2H2 + 5O2 -> 4CO2 + 2H2O + Energy } $$

  1. What are the limits of air availability to make these combustions?

  2. How to do these experiments in lab?


1 Answer 1


Hmm, I see a couple of problems in here:

Energy in the reaction is misunderstood:

In general, breaking bonds is endothermic, and making them is exothermic. In any combustion reaction we have bonds broken ($\ce{O2}$'s for example) and bonds created (forming $\ce{H2O}$, in this instance). The overall change in the energy defines a reaction to be endo- or exothermic; and in the case of combustion (any type) it's exothermic. If you want to write "heat" and "energy" in both sides of the reactions you face, it's going to look a little awkward. Note that in the biochemical reactions in which you see terms like "energy" and such in only the side of reactants or the products (e.g.: ATP energy releases) it's because they want to put emphasis on that particular intake or release of energy.

The equation you've provided for incomplete combustion is incomplete:

I don't see any balanced equations in there, do you? And, I can't get the reason for why you've written two $\ce{CO2}$s.

However, your second equation is about the complete combustion of ethyne, and is balanced and correct.

As for your answer, take a look at what wikipedia says:

The incomplete (partial) combustion of a hydrocarbon with oxygen produces a gas mixture containing mainly $\ce{CO2, CO, H2O}$, and $\ce{H2}$.

Depending on how much your burning sample will be out of reach of air, the mixture will vary in weight percent of the products of the reaction.

In a lab, you normally will get incomplete combustion reactions, for reasons:

  • Your sample is hardly pure hydrocarbon.
  • The air in the lab isn't in wild currents and overall, is "stale" in simple words.
  • The hydrocarbon doesn't get enough "requirements" for combustion and ignites incompletely.
  • etc.

enter image description here


enter image description here


A good sign (or alarm) of the incomplete combustion is the orange hue of fire, especially if it's accompanied by a visible "smoke". If the experiment requires complete combustion, we'll make use of burners such as Bunsen burner.

So, let's summarize: The incomplete combustion is the normal thing happening in the burning of ethyne. You usually do some modifications if you're to have a complete combustion reaction. When the examiner claims "with contact with adequate supplies of air", s\he only means that a complete combustion takes place, though this is rare in real life. Plus, ethyne is not a very good hydrocarbon to start with, because as CDC mentions, it has a potentially negative effect on the nervous system, causing headaches, asphyxia etc., as well as painful frostbites.

  • 1
    $\begingroup$ Also, avoidance of ethyne is recommended. It is a nasty gas. $\endgroup$
    – permeakra
    Commented Jan 26, 2015 at 14:13
  • $\begingroup$ Great point, @permeakra! $\endgroup$
    – M.A.R.
    Commented Jan 26, 2015 at 14:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.