Copper reacts with nitric acid according to this equation:

$$\ce{Cu + 4 HNO3 -> Cu(NO3)2 + 2 NO2 + 2 H2O}$$

This is a redox reaction with copper getting oxidized, and nitrogen getting reduced. If we think of the $\ce{H+}$ as a spectator ion (which I know is not exactly right, since it forms covalent bonds in water in the products), then you would have the following net ionic reaction:

$$\ce{Cu + 2 NO3- -> Cu^2+ + 2 NO2 + 2 O^2-}$$

I'm wondering why this reaction doesn't happen with solutions of nitrate salts (like $\ce{KNO3}$, for example). If nitrate is such a strong oxidizing agent, shouldn't the dissolved nitrate ions still be able to oxidize the copper, even without the $\ce{H+}$ around? It wouldn't be able to form water as a product, but should be able to form hydroxide ions ... something like this:

$$\ce{Cu + 2 KNO3 + 2 H2O -> 2 KOH + Cu(OH)2 + 2 NO2}$$

Is it just that forming water is necessary to make the reaction favorable, or is there something else that I'm missing about the process? (Maybe something in the mechanism where the H+ speeds it up?)

  • 5
    $\begingroup$ Without H+ it isn't strong anymore. $\endgroup$
    – Mithoron
    May 19, 2023 at 21:38
  • $\begingroup$ Chem+Math Expression formatting reference: MathJax Basics / Chem+Math expressions/formulas/equations / Upright vs italic / Math SE Mathjax tutorial // MathJax is preferred not to be used in CH SE Q titles. $\endgroup$
    – Poutnik
    May 20, 2023 at 5:24
  • $\begingroup$ O^2- is a very strong base, reacting with water as acid, forming OH-. NO2 reacts with OH-, forming NO3- and NO2-. $\endgroup$
    – Poutnik
    May 20, 2023 at 5:31
  • $\begingroup$ The simultaneous formation of $\ce{NO2}$ and hydroxides is impossible in chemistry. It will rather produce nitrites and nitrates. $\endgroup$
    – Maurice
    May 20, 2023 at 8:54
  • $\begingroup$ Your $\ce{HNO3}$ acid acts as an acid but in your last reaction there is no acid did you try adding some $\ce{H2SO4 }$ acid to dissolve that precipitate $\ce{Cu(OH)2}$. Which blocks the reaction. Not even pure $\ce{HNO3}$ acid will react continuously with $\ce{Cu}$ because $\ce{Cu(NO3)2}$ is a precipitate. (I was refereeing to fuming nitric acid with no water). Normal $\ce{HNO3}$ acid has water and it dose react. Main cause is you are lacking an acidic medium. $\endgroup$
    – Avon97
    May 20, 2023 at 13:34


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