Chem Q pack #55-oxidizing and reducing agents

[pch]

What are the oxidizing and reducing agents, respectively, for this reaction?

2HCl + H2O2 + MnO2--> O2 + MnCl2 + 2H2O

a) H2O2 ; HCl
b) H2O2 ; MnO2
c) MnO2; HCl
d) MnO2; H2O2

The answer is D. After reading the explanation I understand that Mn's oxidation number went from +4 to +2 therefore became reduced and therefore makes MnO2 an oxidizing agent. O2 in H2O2 went from -1 to 0 therefore is a reducing agent.

My question though, is how do I know which of those compounds I need to find oxidation numbers for in order to find the oxidizing and reducing agents? For example, why wouldn't we look for oxidation states of H or Cl in HCl?

Also, when finding oxidation of O in H2O2, I was able to calculate oxidation state as -1 on the reactant side but I was stuck when I tried to find O oxidation state in the product side because there is O2 and 2H2O both in the products and I wasn't sure if I was supposed to calculate for O2 or H2O

Thank you for your help!

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[cocomos]

Hello, I believe I can answer your questions.

For your first question regarding which compounds should you choose, when doing redox problems that involve metals, oxidation/reduction will pretty much always happen with the metals so draw your attention to them. You did well at finding the oxidation states for Mn. Otherwise, it is best to assign oxidation states to all atoms for all molecules to get an idea of what is going on.

For assigning oxidations states for H and Cl, they will almost always be +1 and -1 respectively (emphasis on almost). There is a general oxidation state ordering list that you can find in your general chemistry textbook that will tell you which atoms you should assign oxidation states to first. Hydrogen and halogens are at the top. In the case of this problem, H and Cl do not change oxidation state.

Regarding your last question: yes, it can get tricky as to which oxygen goes to which molecule - in fact, why can't it be a mix of both? The real answer is you need to know the complete mechanism for the reaction. This particular reaction is not elementary, that is, it actually happens in several steps. So you cannot know which oxygen in the reactants goes to which oxygen in the products. But luckily for this question, we do not need this knowledge. You can deduce from the problem that H and Cl do not change oxidation state, so HCl is not an option, eliminate choices A and B. You determined Mn goes from +4 to +2, thus MnO2 must be oxidizing agent. Therefore D must be the answer.

I teach my students test taking strategies like this all the time because logical deduction can be as useful as mastery of the subject when it comes to testing environments.

Hope that helped!

 

[aldol16]

One of the ways would just be to write down the oxidation states of everything in the reaction. If you're good at it, it won't take long at all. But there are a few shortcuts. As the above poster mentioned, metals love changing oxidation state because it's easy for them to do so (d-orbital electrons aren't held as tightly). Further, if you ever see a compound of a pure element (e.g. O2, H2, F2, N2, etc.) on one side that's not on the other side, the chances are that it has changed oxidation state.

You should also be able to recognize common oxidizing/reducing agents. Molecular oxygen is a common redox agent and so are most metal species.