AAMC 10 - oxidizing agent question?

[AOT11]

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I got tihs question right originally, but noticed something while reviewing my exam. In H2O2, O is -1, while in H2O, O is -2. So isn't it possible that H2O2 could be the oxidizing agent. I know O2 has a 0 oxidation state, but why do we assume it's going from -1 to 0 (in O2) and not -1 to -2 (in H2O)?

 

[AOT11]

Anyone? 🙁

 

[MedPR]

If you attach it to SDN I might be able to help. I can't see imgur while at work.

 

[AOT11]

bump 🙁

 

[pfaction]

HCL + H2O2 + MnO4 -> MnCL2 + O2 + 2H2O
+1/-1---+1-1-----+8-8------>+2/-2---------0------+1/-2

 

[AOT11]

HCL + H2O2 + MnO4 -> MnCL2 + O2 + 2H2O
+1/-1---+1-1-----+8-8------>+2/-2---------0------+1/-2

Correct. So in this case can't you say the O is going from -1 to -2 (-1 in H2O2 to -2 in H2O)??? That would mean O got reduced and is therefore the oxidizing agent. Do you see what I'm saying?

 

[pfaction]

Well technically H2O2 is overall +2/-2.
If we look at O2, it's pure 0. So it's the reducing agent.

 

[fas376]

Well technically H2O2 is overall +2/-2.
If we look at O2, it's pure 0. So it's the reducing agent.

Pfac - You don't look at "overall" for redox chemistry, OP is right.

OP - I found the Mn first and didn't even bother looking at H2O2. I see what you're saying though. Hm...I really don't know :/

 

[piojita63]

I got tihs question right originally, but noticed something while reviewing my exam. In H2O2, O is -1, while in H2O, O is -2. So isn't it possible that H2O2 could be the oxidizing agent. I know O2 has a 0 oxidation state, but why do we assume it's going from -1 to 0 (in O2) and not -1 to -2 (in H2O)?

Bumppp!!! All of this was going through my mind when I took AAMC 10 and I decided to guess between the 2 possible ones. I guess wrong 🙁
I would like a decent explanation to this too!
Bump!!!

 

[movax]

Bumppp!!! All of this was going through my mind when I took AAMC 10 and I decided to guess between the 2 possible ones. I guess wrong 🙁
I would like a decent explanation to this too!
Bump!!!

I think this one is pretty easy, if you don't over think it. Even if you forget that in a peroxide, oxygen has a -1 charge, not a -2, it should be readily apparent that Mn is changing states.

That leaves you C & D. If you recall the peroxide rule for O2, you pick D right away. If you don't, I imagine you could pick D with some fudge-logic; when's the last time you saw HCl as a oxidizing/reducing agent?

 

[fas376]

I think this one is pretty easy, if you don't over think it. Even if you forget that in a peroxide, oxygen has a -1 charge, not a -2, it should be readily apparent that Mn is changing states.

That leaves you C & D. If you recall the peroxide rule for O2, you pick D right away. If you don't, I imagine you could pick D with some fudge-logic; when's the last time you saw HCl as a oxidizing/reducing agent?

Yeah I get that, but now I'm curious as to why we don't look at H2O2 --> H2O because that completely changes the answer. What's the actual reason?

 

[bdc142]

Unabashed bump.

 

[Night Hawk]

I got tihs question right originally, but noticed something while reviewing my exam. In H2O2, O is -1, while in H2O, O is -2. So isn't it possible that H2O2 could be the oxidizing agent. I know O2 has a 0 oxidation state, but why do we assume it's going from -1 to 0 (in O2) and not -1 to -2 (in H2O)?

You could assume that but it doesn't fit with the information given. Metals are the easiest to start with, in this case Mn(4+)---->Mn(2+) Gains Electrons Reduced = Oxidant. HCl is easy to eliminate since Cl is minus one on both sides and H is (1+) on both sides, POE gives you D.

To answer your question, H2O, OH-, & H+ are (95% of the time) used for balancing the half reactions. In this case:

Recduction Half Reaction: MnO2 + 2e- + 2HCl ---> MnCl2 + 2OH-
Oxidation Half Reaction: ..H2O2 + 2OH- ---> O2 + 2e- + 2H2O

Which Gives: HCL + H2O2 + MnO2 -> MnCL2 + O2 + 2-H2O (the OH-'s cancel and the e-'s cancel when you add the two half reactions)

You'll have to look in your Gen Chem text since, from what i've seen, EK and TPR don't address balancing Redox RXN's. Hope that helps.

 

[Night Hawk]

HCL + H2O2 + MnO4 -> MnCL2 + O2 + 2H2O
+1/-1---+1-1-----+8-8------>+2/-2---------0------+1/-2

The charges you put aren't correct and it's MnO2

HCL + H2O2 + MnO2 -> MnCL2 + O2 + 2-H2O
+1/-1... +1/-1... +4/-4---->+2/-2........ 0..... +1/-2

 

[osprey099]

You could assume that but it doesn't fit with the information given. Metals are the easiest to start with, in this case Mn(4+)---->Mn(2+) Gains Electrons Reduced = Oxidant. HCl is easy to eliminate since Cl is minus one on both sides and H is (1+) on both sides, POE gives you D.

To answer your question, H2O, OH-, & H+ are (95% of the time) used for balancing the half reactions. In this case:

Recduction Half Reaction: MnO2 + 2e- + 2HCl ---> MnCl2 + 2OH-
Oxidation Half Reaction: ..H2O2 + 2OH- ---> O2 + 2e- + 2H2O

Which Gives: HCL + H2O2 + MnO2 -> MnCL2 + O2 + 2-H2O (the OH-'s cancel and the e-'s cancel when you add the two half reactions)

You'll have to look in your Gen Chem text since, from what i've seen, EK and TPR don't address balancing Redox RXN's. Hope that helps.

This. H2O, OH-, and H+ should not be considered when trying to determine oxidizing/reducing agents. They are only used at the end of a reaction to make both sides balanced (make the reaction look prettier).