Is there a mistake in my homework question?

[Femme_physics]

Hey guys. I know that oxidation is a atom/molecule/ion losing electrons, therefor whenever that happens the minus decreases until it may become a plus.

Reduction is just the opposite.

But in the following question there are 3 options for oxidation reaction and 1 for reduction! And I can only pick 1 answer. I'm confused, and not sure what to do. Any help?

The question:

Which of the following is an oxidation reaction?

MnO₂ >>>>> Mn²⁺
MnO₄²- >>>>> MnO₂
MnO₂ >>>>> MnO₄^-
MnO₄^- >>>>> MnO₂

 

[Rajini]

Hi,
I guess you need to consider balancing of reactions. Maybe you have to check which equation is balanced and choose the oxidation reaction from them.
I maybe wrong but i don't have any clue!

 

[Borek]

Hey guys. I know that oxidation is a atom/molecule/ion losing electrons, therefor whenever that happens the minus decreases until it may become a plus.

That would work when comparing oxidation state of two otherwise identical ions (that is, differing only by charge). When composition changes, it is no longer correct.

Do you know what oxidation numbers are? Try to assign them to Mn in all compounds. Where does the ON go up?

 

[Femme_physics]

Sorry, I'm having tough time. I know what oxidation numbers are... this question is just befuddling me. I still see 1 reduction and 3 oxidations... sigh

 

[Borek]

List all oxidation numbers, for all substances.

 

[Femme_physics]

(Sorry it takes me a while to reply...am a busy woman)

Oxidation Numbers:

Mn+4 O-4 >>> Mn+2
Mn+6 O-8 >>> Mn+4 O-4
Mn+4 O-4 >>> Mn+3 O-4
Mn+3 O4 >>> Mn+4 O-4

I'm not sure what are they supposed to tell me with respect to the answer. Looking at the reactions as is and determining whether they're oxidation or reduction based on what we we know (OIL RIG - Oxidation Is Loss of e-, Reduction is Gain of e-) makes more sense to me.

 

[Borek]

You are wrong when it comes to oxidation number of oxygen - it is per atom, not per all atoms, so in each case ON for oxygen is -2. But you have correctly calculated sums, so it shouldn't matter much. However, there is a problem with ONs for Mn in different compounds:

Mn+4 O-4 >>> Mn+2

OK

Mn+6 O-8 >>> Mn+4 O-4

OK

Mn+4 O-4 >>> Mn+3 O-4

OK for MnO₂, but wrong for MnO₄^- - 4 oxygens at -2 mean -8, not -4.

Mn+3 O4 >>> Mn+4 O-4

Same problem.

I'm not sure what are they supposed to tell me with respect to the answer. Looking at the reactions as is and determining whether they're oxidation or reduction based on what we we know (OIL RIG - Oxidation Is Loss of e-, Reduction is Gain of e-) makes more sense to me.

Same approach will help. Even if you don't see all oxygens in the not fully balanced reaction as those presented, you can safely assume ON for oxygen doesn't change - it is always -2. That means oxygen neither gains nor losses electrons, so any change that happens must be either electron gain or loss for Mn. Knowing oxidation numbers for Mn before and after reaction you can tell whether it gained or lost electrons, then OIL RIG will give you answer you are looking for.

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