Finding the oxidized and reduced element

[physics=world]

1. I need to find the element being oxidized and reduced for:

CO2 + C = 2CO

I don't know how to do it for this one.





3. I know how to do it for ones like this:

2Al + 3Cl2 = 2Al(3+) 6Cl(-)

it would be

zero for elemental Al and Cl

and the oxidation number for the product would be its charge.

but how do I do it for ones like this.

CO2 + C = 2CO

 

[Borek]

Have you tried to calculate oxidation numbers for all atoms present in the equation?

 

[physics=world]

Would it be

for the left side:

C=+4, O =+2 for CO2

and elemental C = 0

and for the right side:

C=+2, O =+2 for 2CO

 

[Borek]

So far, so good. Which atoms oxidation numbers changed?

 

[physics=world]

If I add the C atom and subtract from the other side would this be correct:

(4+0) - 2 = 2

So, C would be reduced.

 

[Borek]

I am not sure what you are doing, but your conclusion is partially right. Carbon is being reduced, but you can't have reduction without oxidation.

What is being oxidized?

Hint: I was referring to atoms (not elements) for a purpose.

 

[physics=world]

Oxygen is being oxidized, but how much is the oxidation number increased by?

 

[Borek]

No, oxygen oxidation number doesn't change - it was -2 before and it is -2 after.

Sigh, I just realized you calculated oxidation numbers for oxygen wrong - you wrote them as +2. No, it was -2 in all compounds.

 

[physics=world]

yes I noticed that as well

 

[Borek]

So, what is being oxidized?

 

[physics=world]

carbon

 

[Borek]

Yep. You have two different carbon atoms on the left - one is getting oxidized, the other is getting reduced.

Sometimes it can get even more surprising, when you have two identical atoms on the left and they get reduced and oxidized at the same time - it is called disproportionation. The simplest example is probably

Cl₂ + 2OH^- → Cl^- + OCl^- + H₂O

(try to assign oxidation numbers and see what is happening here).

 

[physics=world]

that is surprising. I did not know about that.

So, is the other C going from 0 to 2?

 

[Borek]

Yes, one C is going from +4 to +2, the other one from 0 to +2. First is reduced, the other is oxidized.