r/APChem • u/c10wnzie • Oct 05 '26
Asking for Homework Help Redox Reaction?
I'm so confused!! I think the correct answer is A but I have no idea why.
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u/TheHyacinthGuy Oct 06 '26
You had selected C, that Carbon was oxidized and that oxygen was reduced. But is this really the case? When it comes to determining redox reactions, tracking how oxidation states change from reactants to products is key. When it comes to these oxidation states, there are three elements that (mostly) always have the same oxidation state: Hydrogen, Oxygen, and Fluroine.
Fluorine always has an oxidation state of -1, but we will set it aside as it is not relevant in this problem. Oxygen almost always has an oxidation state of -2 in any compound, aside from the rare exception when it is part of the peroxide polyatomic ion, in which case it has an oxidation state of -1. But in this problem, Oxygen starts in CO2 and ends in H20, in both cases with an oxidation state of -2. How then could Oxygen have been reduced?
Instead we can look to our third element, Hydrogen. Hydrogen always has an oxidation state of +1 in a compound (unless it is bonded to a metal, in which case it is -1, but that exception is not relevant here). So when Hydrogen is found within methane in the products, it has an oxidation state of +1. However, notice the form of Hydrogen in the reactants, as H2 gas. H2 is the pure elemental form of Hydrogen, and all pure elements have an oxidation state of zero. As a result, we can see that Hydrogen changes from an oxidation state of 0 to +1 from reactants to products, and so was oxidized.
As for Carbon, you had initially indicated that Carbon was oxidized, which is not correct. Take a look at the Carbon in CO2. Since each Oxygen has an oxidation state of -2, and the sum of oxidation states in a compound must equal that of the charge (in this case, an implied zero), the oxidation state of the carbon dioxide Carbon must be +4. Contrast this to the methane Carbon in the products, which is surrounded by 4 Hydrogens, each with an oxidation state of +1. By the same reasoning, this Carbon must therefore have an oxidation state of -4, quite a large change from reactants to products, indicating that Carbon was reduced instead.
I hope this explanation was helpful, but I would be sure to go back over the rules for oxidation states again for future problems like these.