What does it mean "to lose electrons" in a combustion (redox) reaction?

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

saoj

Full Member
10+ Year Member
Joined
Jan 23, 2012
Messages
129
Reaction score
0
The redox reaction below: (combustion)

C3H8 + 5 O2 ---> 3 CO2 + 4 H2O

Oxygen is the oxidizing agent, in other words, it is oxidizing C3H8, in other words, it is making C3H8 lose electrons.

I would like to understand what do they mean by "losing electrons". It makes sense when the products are formed through ionic bonds, but I don't understand what is the rationale when the products are formed through covalent bonds like in the combustion above.

Members don't see this ad.
 
just remember 'oil rig' and you'll be set. Oxidation Is Losing (electrons), and Reduction Is Gaining (electrons). past that, what else could you need? cihv: covalent, ionic, hydrogen bonds, van der waals interactions.. the 'Co' in covalent always sounds liek sharing to me so it's easy to remember whats what (and that ionic is donating).. probably i didnt answer your question at all but hey, +1 post for me.🙄



btw: by 'losing electrons' it's losing hydrogens usually. at least in my understanding of it.
 
You can look at the polarity of the bonds between carbon and oxygen and carbon and hydrogen. C-O bonds are more polar than C-H bonds (obviously) and because the oxygen is more electronegative, it pulls electron density away from the carbon...which can be seen as "losing electrons."

In general, for redox reactions involving organic molecules (like ethane), the addition of oxygen directly to a carbon is an oxidation while addition of hydrogens is a reduction.
 
The redox reaction below: (combustion)

C3H8 + 5 O2 ---> 3 CO2 + 4 H2O

Oxygen is the oxidizing agent, in other words, it is oxidizing C3H8, in other words, it is making C3H8 lose electrons.

I would like to understand what do they mean by "losing electrons". It makes sense when the products are formed through ionic bonds, but I don't understand what is the rationale when the products are formed through covalent bonds like in the combustion above.

Look at what C is bonded to on the left, then look at what C is bonded to on the right. A different way to look at redox is gain/loss of oxygen and hydrogen. Gain oxygens = oxidized, gain hydrogens = reduced. O2 becomes H2O = gains hydrogens = reduced. Propane=CO2 = oxidized.
 
Members don't see this ad :)
You can look at the polarity of the bonds between carbon and oxygen and carbon and hydrogen. C-O bonds are more polar than C-H bonds (obviously) and because the oxygen is more electronegative, it pulls electron density away from the carbon...which can be seen as "losing electrons."

In general, for redox reactions involving organic molecules (like ethane), the addition of oxygen directly to a carbon is an oxidation while addition of hydrogens is a reduction.

O2 became H2O and guess what, H2O is more polar than O2. 🙁

So the propane is being oxidized because it is gaining oxygens.

So the O2 is being reduced because it is gaining hydrogens.

The fact that the O2 also gained a Carbon to become CO2 is what? How do you go by telling if it was the C that gained O2 or if it was the O2 that gained a C?
 
The fact that the O2 also gained a Carbon to become CO2 is what? How do you go by telling if it was the C that gained O2 or if it was the O2 that gained a C?

In this case it doesn't matter. You have 5 O2s on the reactant side. Those O2s are completely oxidized. The only thing that can happen to them is reduction.
 
In this case it doesn't matter. You have 5 O2s on the reactant side. Those O2s are completely oxidized. The only thing that can happen to them is reduction.

Your last sentence can lead to confusion.

The O2s are being reduced and not oxidized. You mean they were oxidized before the reaction? Can you elaborate?
 
In this case it doesn't matter. You have 5 O2s on the reactant side. Those O2s are completely oxidized. The only thing that can happen to them is reduction.

Your last sentence can lead to confusion.

The O2s are being reduced and not oxidized. You mean they were oxidized before the reaction? Can you elaborate?


I didn't say they were being oxidized. "The only thing that can happen to them is reduction." Reduction = being reduced = gaining electrons.

In O2, how many oxygens is each oxygen bound to? One.

In CO2, how many oxygens is each oxygen bound to? Zero.

In H2O, how many oxygens is each oxygen bound to? Zero.

In C3H8, how many oxygens is C bound to? Zero.

In CO2, how many oxygens is C bound to? Two.

In O2, how many non-oxygens is each oxygen bound to? Zero.

In H2O, how many non-oxygens is each oxygen bound to? Two.

In CO2, how many non-oxygens is each oxygen bound to? One.

Therefore, propane is oxidized, and O2 is reduced.

Edit: I see where you might get confused now. What I was saying earlier was that, prior to the reaction, O2 was completely oxidized relative to everything else in the reaction. You must oxidize something (say, H2O) to make an oxidized substance like O2. If that's confusing, just ignore it. It's not that important.

When I'm doing redox problems, if I see oxygens and hydrogens moving around, I use the rule that more oxygens on the right than on the left means that it got oxidized. In addition, if something gains hydrogens, it was reduced. Obviously these rules don't work in reactions that don't have oxygen and hydrogen in them, but it is a useful shortcut (for me, at least) when there is oxygen and hydrogen. Propane became CO2, therefore propane was oxidized. O2 became H2O, therefore it was reduced.

You asked how we know if O2 gained a carbon or if propane gained an O2. It doesn't matter which way you think about it. O2 didn't gain an oxygen, so it was not oxidized. Propane did not gain any hydrogens or lose any oxygens, so it couldn't have been reduced.
 
Last edited:
I think it would be worthwhile to go back to your organic text/notes to review how you calculate formal charge. For reactions like this you're typically not concerned with the oxidation state of anything besides carbon.
 
Top