# Finding the oxidized and reduced element

1. Sep 24, 2013

### 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

2. Sep 24, 2013

### Staff: Mentor

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

3. Sep 24, 2013

### 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

4. Sep 24, 2013

### Staff: Mentor

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

5. Sep 24, 2013

### 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.

6. Sep 24, 2013

### Staff: Mentor

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.

7. Sep 24, 2013

### physics=world

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

8. Sep 24, 2013

### Staff: Mentor

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.

9. Sep 24, 2013

### physics=world

yes I noticed that as well

10. Sep 24, 2013

### Staff: Mentor

So, what is being oxidized?

11. Sep 24, 2013

### physics=world

carbon

12. Sep 24, 2013

### Staff: Mentor

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).

13. Sep 24, 2013

### physics=world

that is surprising. I did not know about that.

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

14. Sep 25, 2013

### Staff: Mentor

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

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