## Knowing which atom was oxidized/ reduced

melissa_dis4K
Posts: 106
Joined: Fri Sep 28, 2018 12:28 am

### Knowing which atom was oxidized/ reduced

I notice that to determine which element was oxidized or reduced you need to find the charge of the atom. For example, Cr(subscript2)O(subscript 7)^2- being turned into Cr^3+. I found that it was being reduced from 6+ to 3+ but my question is: If we have the final redox reaction instead of finding the charges again like I did for the part mentioned before, can I just look at the final redox reaction and see that Cr(subscript2)O(subscript 7)^2- was turned into 2Cr^3+ and I see the oxidation number decreases can I automatically say it was reduced?

melissa_dis4K
Posts: 106
Joined: Fri Sep 28, 2018 12:28 am

### Re: Knowing which atom was oxidized/ reduced

But then for 3C(subscript 2)H(subscript 5)OH -----> 3C(subscript 2)H(subscript 4)O how would I know that C was oxidized since they have zero charge. So should I always have to calculate the charges like mentioned above or can I assume that because in the redox reaction the Cr was reduced then the C had to be oxidized? Thanks!

megan blatt 2B
Posts: 61
Joined: Fri Sep 28, 2018 12:28 am

### Re: Knowing which atom was oxidized/ reduced

The charges that you are referring to are actually the oxidation numbers of the elements so even if it is not explicitly written, all elements in the reaction have an oxidation number. You can find the oxidation numbers by calculating the numbers of the elements you know then by adding and subtracting, you can find the oxidation number of the element that you don't know. For the example you gave of 3C2H5OH, we know that H always has an oxidation number of +1. O, unless it is in a dioxide molecule, has an oxidation number of -2. The molecule overall has a charge of 0 because it is neutral, which means all of the elements in the molecule oxidation numbers should add up to 0. If we add the ones we know, we get a leftover charge of +4 for 2 C atoms. Therefore, you can determine that the oxidation number of a C atom in that molecule is +2. Also, like you said, you can assume since it is a redox reaction and the Cr is being reduced that C is being oxidized.

Aurbal Popal
Posts: 63
Joined: Fri Sep 28, 2018 12:27 am

### Re: Knowing which atom was oxidized/ reduced

When I do half equations, I usually do not look for the oxidation numbers. I just note where the electrons are placed. If it is a product, that means that the substance lost electrons, which means that it was oxidized. If it is a reactant, then the substance gained electrons, which means reduction took place. Otherwise, if a question does not require half reactions, I would look at the oxidation numbers.

Hope this helps!