Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Oct 20, 2017 #1
    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?
     
  2. jcsd
  3. Oct 20, 2017 #2

    Borek

    User Avatar

    Staff: Mentor

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

    Have you heard about reactivity series? Redox potentials?
     
  4. Oct 20, 2017 #3
    whops HNO3- * then
    whops NO3- *. Yea i have heard of reduction potential and I guess i understand this happens because NO3- is higher in the reduction (I cant 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?
     
  5. Oct 20, 2017 #4

    Borek

    User Avatar

    Staff: Mentor

    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?
     
  6. Oct 20, 2017 #5
    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?
     
  7. Oct 20, 2017 #6

    Borek

    User Avatar

    Staff: Mentor

    Yes, there is thermodynamic behind, in particular relationship between the reaction ΔG and the potential.
     
  8. Oct 20, 2017 #7
    soo no simple mechanism and i have to memorize this?
     
  9. Oct 20, 2017 #8

    Borek

    User Avatar

    Staff: Mentor

    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.
     
  10. Oct 20, 2017 #9
    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.
     
  11. Oct 20, 2017 #10

    Borek

    User Avatar

    Staff: Mentor

    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 O2-. In low pH these O2- 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.
     
  12. Nov 4, 2017 #11
    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.
     
  13. Nov 4, 2017 #12

    Borek

    User Avatar

    Staff: Mentor

    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.
     
  14. Nov 5, 2017 #13
    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
     
  15. Nov 5, 2017 #14
    For sulphur, for example this:
    http://chemwiki.wikidot.com/standard-electrode-potentials#toc21
    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why does NO3^(2-) get reduced instead of H+ in Cu+HNO3
  1. Zn(NO3)2 problem (Replies: 2)

  2. Cu(en)2 ? (Replies: 7)

Loading...