Aamc 5 # 24

[orthoboy]

---

Members do not see this ad.

.

 

[liveoak]

Painful question that I still haven't figured out a good answer for.

Apparently NO3 decomposes into O2 and NO because it has a higher affinity for electrons compared to H+

Thus, NO3 (where N has an oxidation of 5) has a higher affinity for those electrons than H+, and thus, it gets reduced to NO before H gets reduced to H2.

But I probably have no clue what i'm talking about.

 

[kehlsh]

N gets put in, N gets out

 

[tendiw]

Its a question about experiments really. When the Cu metal is reacted with HNO3, a redox reaction occurs. Remember H is always +1 except in metal halides where it is -1.
Now copper is oxidized to Cu 2+ which mean something must be reduced.

The are some elements that dont change states often like oxygen (except in peroxides where it is -1). The only element that can be reduced to complete the redox reaction is nitrogen. In NO2, nitrogen's oxidation is +3 as opposed to NO3 where its oxidation state is +5. Nitrogen must be reduced for copper to be oxidized.

The NO2 gas evolved is brown in color and the copper solution is blue.

THIS IS A REDOX QUESTION

 

[tendiw]

N gets put in, N gets out

That would also follow "H gets put in H gets out", creating a dilemma

 

[liveoak]

Its a question about experiments really. When the Cu metal is reacted with HNO3, a redox reaction occurs. Remember H is always +1 except in metal halides where it is -1.
Now copper is oxidized to Cu 2+ which mean something must be reduced.

The are some elements that dont change states often like oxygen (except in peroxides where it is -1). The only element that can be reduced to complete the redox reaction is nitrogen. In NO2, nitrogen's oxidation is +3 as opposed to NO3 where its oxidation state is +5. Nitrogen must be reduced for copper to be oxidized.

The NO2 gas evolved is brown in color and the copper solution is blue.

THIS IS A REDOX QUESTION

why couldn't hydrogen be changed to complete the reduction? From h+ -> H2

thanks.

 

[tendiw]

why couldn't hydrogen be changed to complete the reduction? From h+ -> H2

thanks.

The question tells you when Cu is place in H20, no reaction takes place. This is a clue to inform you that the redox reaction fails with hydrogen alone as it wont be reduced.

HNO3(aq) also contains H20 as it is aqueous, but its the HNO3 part that lets the reaction take place.

If hydrogen could be reduced in the reaction, the Cu would have reacted with H20 in the first part of the question. That clue they give you eliminates hydrogen.

To further elaborate. The standard potential for Cu2+ to Cu(s) is positive, form H+ to H2(g) is actually zero. For a redox reaction to be spontaneous, E>0. Converting Cu(s) to Cu2+ is gives a negative E so the reduction reaction that must occur with the oxidation of copper must be positive an MUCH larger than zero.

It should be known for the MCAT, that Cu2+ to Cu(s) is positive and Zn2+ to Zn(s) as well as Pb2+ to Pb(s) are negative

 

[liveoak]

The question tells you when Cu is place in H20, no reaction takes place. This is a clue to inform you that the redox reaction fails with hydrogen alone as it wont be reduced.

HNO3(aq) also contains H20 as it is aqueous, but its the HNO3 part that lets the reaction take place.

If hydrogen could be reduced in the reaction, the Cu would have reacted with H20 in the first part of the question. That clue they give you eliminates hydrogen.

To further elaborate. The standard potential for Cu2+ to Cu(s) is positive, form H+ to H2(g) is actually zero. For a redox reaction to be spontaneous, E>0. Converting Cu(s) to Cu2+ is gives a negative E so the reduction reaction that must occur with the oxidation of copper must be positive an MUCH larger than zero.

It should be known for the MCAT, that Cu2+ to Cu(s) is positive and Zn2+ to Zn(s) as well as Pb2+ to Pb(s) are negative

best explanation yet. thx. so,