Why does NO3^(2-) get reduced instead of H+ in Cu+HNO3

[Vriska]

doesn't make sense, h+ just has a really nice positive charge which is begging electrons but electrons go to the NO3(^2-) instead? why?

 

[Borek]

Beware: there is no such thing as NO3(^2-).

Have you heard about reactivity series? Redox potentials?

 

[Vriska]

Beware: there is no such thing as NO3(^2-).

Have you heard about reactivity series? Redox potentials?

whops HNO3- * then

Beware: there is no such thing as NO3(^2-).

Have you heard about reactivity series? Redox potentials?

whops NO3- *. Yea i have heard of reduction potential and I guess i understand this happens because NO3- is higher in the reduction (I can't find a table that has this though ) potential chart than H+ but why? Is there some structural reason why NO3- tends to form NO2 and oxidize H+ to H2O? or could it be that NO3- is lower than copper?! if that were the case the NO3- could oxidize H+ to form H2O with a NO2+ ion which gets stabilized by electrons donated from the Cu?

 

[Borek]

First of all: you can't oxidize H⁺, so most of what you wrote is off.

Is H⁺ an oxidizer strong enough to oxidize copper?

 

[Vriska]

First of all: you can't oxidize H⁺, so most of what you wrote is off.

Is H⁺ an oxidizer strong enough to oxidize copper?

according to the table, no. okay, so h2 is not formed. It looks like a reaction with NO3- is possible to yield NO + H2O, from here oxidation of NO might give the result but how did this happen ? is there any reaction mechanism or anything?

 

[Borek]

Yes, there is thermodynamic behind, in particular relationship between the reaction ΔG and the potential.

 

[Vriska]

Yes, there is thermodynamic behind, in particular relationship between the reaction ΔG and the potential.

soo no simple mechanism and i have to memorize this?

 

[Borek]

The only thing to remember here is the reactivity series, which will give answer to many other problems as well.

And I believe I told you long ago inorganic chemistry is not based on mechanisms like organic is.

 

[Vriska]

The only thing to remember here is the reactivity series, which will give answer to many other problems as well.

And I believe I told you long ago inorganic chemistry is not based on mechanisms like organic is.

arright, I guess I'll do that then but - is there a reason some structural reason perhaps why NO3- and other compounds keen to accept electrons? I see why the elements are arranged like that - electronegativity but what about the compounds? is it again only based on experimental/gibbs considerations?

yeah, you did, was just hoping there was something.

 

[Borek]

It is no always this way, but you can rationalize things this way: central atom in many inorganic oxoacids has a high oxidation number. That means it will easily accept electrons, but it can't, as it is already screened by electron rich O^2-. In low pH these O^2- can get protonated and removed as water molecules, making the central atom more accessible.

I just made up this explanation, so don't treat it too seriously.

 

[snorkack]

The only thing to remember here is the reactivity series, which will give answer to many other problems as well.

And I believe I told you long ago inorganic chemistry is not based on mechanisms like organic is.

Wrong. Mechanisms are just as essential.
Check reactivity series, and you will find that cold dilute sulphuric acid is easier to reduce than hydrogen. To S₈, not SO₂ - sulphurous acid dismutes. And S₈ itself is a stronger oxidant than hydrogen.
Check reactivity series, and you will find that both NO₂ and NO are unstable to dismutation.
Check reactivity series, and you will find that cold dilute perchloric acid is a strong oxidant, stronger even than nitric acid.

By thermodynamics and reactivity series, the reactions in cold dilute acids ought to be:
4Cu+4H₂SO₄→CuS+3CuSO₄+4H₂O
5Cu+12HNO₃→N₂+5Cu(NO₃)₂+6H₂O
4Cu+8HClO₄→CuCl(ClO₄)+3Cu(ClO₄)₂+4H₂O

None of these reactions take place, for reasons of mechanism.

 

[Borek]

Mechanisms are just as essential.

I think you are missing the context.

Sure, there are cases where details of the mechanism, especially competing kinetics between different reaction paths, matter. But for someone coming from organic chemistry (as OP does), it is better to assume mechanisms are irrelevant, as in 99% of the cases looking for them they will only muddy the water.

 

[Vriska]

Wrong. Mechanisms are just as essential.
Check reactivity series, and you will find that cold dilute sulphuric acid is easier to reduce than hydrogen. To S₈, not SO₂ - sulphurous acid dismutes. And S₈ itself is a stronger oxidant than hydrogen.
Check reactivity series, and you will find that both NO₂ and NO are unstable to dismutation.
Check reactivity series, and you will find that cold dilute perchloric acid is a strong oxidant, stronger even than nitric acid.

By thermodynamics and reactivity series, the reactions in cold dilute acids ought to be:
4Cu+4H₂SO₄→CuS+3CuSO₄+4H₂O
5Cu+12HNO₃→N₂+5Cu(NO₃)₂+6H₂O
4Cu+8HClO₄→CuCl(ClO₄)+3Cu(ClO₄)₂+4H₂O

None of these reactions take place, for reasons of mechanism.

Which table are you using? I'm unable to piece the reaction to what you're getting using this : http://www.webassign.net/zumchemp6/11-table-01.gif

 

[snorkack]

For sulphur, for example this:
http://chemwiki.wikidot.com/standard-electrode-potentials#toc21

SO₄^-2 + 4H⁺ + 2e^- ⇋ H₂SO₃ + H₂O +0.172
H₂SO₃ + 4H⁺ +4e^- ⇋ S + 3H₂O +0.450

A reaction not expressly given there: how would you compute the standard potential for the reaction
SO₄^-2 + 8H⁺ + 6e^- ⇋ S + 4H₂O
You have the necessary data given above.