Identifying Elements Undergoing Oxidation/Reduction

[Janet Nguy 2C]

How do you identify elements that undergo oxidation/reduction in a reaction?
For example, in 6K1 part a, it asks:

Identify elements undergoing oxidation/reduction and indicate their initial and final oxidation numbers for the equation

H+ (aq) + Cr2O7 [2-] (aq) + C2H5OH (aq) -> Cr [3+] (aq) + C2H4O (aq) + H2O (l)

[Jessica Castellanos]

A good way to remember how to identify elements undergoing oxidation/reduction is
"LEO the lion goes GER"
LEO: Losing Electrons=Oxidation
GER: Gaining Electrons=Reduction

[MinuChoi]

You can also look at the element's oxidation number before and after the equation. Elements whose oxidation number increases are oxidized, and vice versa for reduced.
For 6K1:
Given H is typically +1 oxidation and oxygen is typically -2 oxidation number, we can tell that our two elements undergoing change are Cr and C. in Cr₂O₂^2-, Cr has oxidation number +6 since O2 is -2. In the products, the Cr number is +3. Cr is reduced.
In the reactant C2H5OH, C has oxidation number -2 (H is +1 and O is -2). In the products, C's oxidation number increases to -1. C is oxidized.

[Janet Nguy 2C]

You can also look at the element's oxidation number before and after the equation. Elements whose oxidation number increases are oxidized, and vice versa for reduced.
For 6K1:
Given H is typically +1 oxidation and oxygen is typically -2 oxidation number, we can tell that our two elements undergoing change are Cr and C. in Cr₂O₂^2-, Cr has oxidation number +6 since O2 is -2. In the products, the Cr number is +3. Cr is reduced.
In the reactant C2H5OH, C has oxidation number -2 (H is +1 and O is -2). In the products, C's oxidation number increases to -1. C is oxidized.

Are you ignoring the H+ and H2O in the reactant and product? Do they not matter for this problem?

[Jessica Castellanos]

I forgot to mention the example in my last post. So for the example in your post, to identify the elements, calculate the oxidation state for each of the elements in the reactants and and for each of the elements in the products and if you use the rules from the previous post you can figure out which elements are going under oxidation and which are going under reduction.

[MinuChoi]

Are you ignoring the H+ and H2O in the reactant and product? Do they not matter for this problem?

They don't really matter for part a. To find the oxidation number of a single element in a compound, you don't really need to look at the other reactants. But you will definitely need to consider them for balancing the reaction, which is the rest of the problem. Keep in mind the actual compounds don't change when you balance the reaction; you're just altering the amounts.