Combustion reaction question

[a weaponized ikea chair]

Please look at image attachment.

Equation a) is complete combustion, whereas equation b) is incomplete combustion. How come, even though there is more oxygen in equation b) it is incomplete? Isn't incomplete combustion where there is not enough oxygen?

[fun_jirachi]

Hey there!

It is true that incomplete combustion occurs when there is insufficient oxygen. The result of this is that carbon monoxide is produced instead of carbon dioxide, in addition to water. If you take a look at the amount of oxygen per mole of methane burned, there isn't technically more oxygen, rather, there is less. It's important to note that the amount of methane is also a determining factor in whether there is excess oxygen or not - I could potentially write 100 moles of methane + 3 moles of oxygen => carbon + water + carbon monoxide - and under the explanation you provide there would still be more oxygen. The takeaway here is that it's the mole ratio that matters, rather than the literal amount you see in the question

Hope this helps

[a weaponized ikea chair]

Hmm, so how could you infer that from just looking at the equations?

Thanks

[whys]

Hmm, so how could you infer that from just looking at the equations?

Basically just what fun_jirachi said.

In equation a, for every mol of methane, there is 2 mol of oxygen. This means a ratio of at least 1:2 (methane : oxygen) is required for complete combustion. In equation b, for every 2 mol of methane, there is 3 mol of oxygen. For there to be enough oxygen for complete combustion, you would need 4 mols of oxygen, but we are short of 1 mol. Therefore, there is not enough oxygen for complete combustion in reaction b. For this, you need to determine stoichiometric mol rations.

This mole ratio is not the first thing you should look at to determine if it's complete or incomplete combustion though. A rule that applies to all combustion reactions is that complete combustion always results in CO2 and incomplete combustion always results in CO. I suggest you take this into consideration first, because if you see CO as a product, then you know for sure it is incomplete combustion and can rule out complete combustion entirely.

Hope this helped!