In my teaching-lab experiments I've seen that nitric acid solutions are able to facilitate the dissolution of metals such as silver, even though they are more active than hydrogen. Does anyone know why nitric acid is so special? Why is it such a strong oxidizing agent?
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2$\begingroup$ Its not that special- concentrated sulfuric acid and all the oxyacids of chlorine are oxidising particularly perchloric HCL04. $\endgroup$– user2617804May 28, 2014 at 23:14
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2 Answers
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Because, unlike in other metal dissolution reactions, the $\ce{H+}$ of $\ce{HNO3}$ isn't reduced—the $\ce{NO3-}$ is. The following data and balanced reactions are taken from Wikipedia:
\begin{align} \ce{NO3- + 4 H+ + 3 e- &-> NO + 2 H2O} & E^\circ_\mathrm{red} &= \pu{0.96 V} \\ \ce{NO3- + 2 H+ + e- &-> NO2 + H2O} & E^\circ_\mathrm{red} &= \pu{0.79 V} \\ \ce{Ag+(aq) + e- &-> Ag(s)} & E^\circ_\mathrm{red} &= \pu{0.799 V} \end{align}
Since the standard reduction potential (SRP) of the $\ce{NO2}$ reaction looks smaller than that of $\ce{Ag+}$ (I may be wrong, but by significant digits it can't be greater than $0.799$), one can conclude that it's the $\ce{NO}$ reaction that's occurring here. And the $\ce{NO}$ reaction has a large enough SRP to oxidise $\ce{Ag+}$.
Usually nitrogen compounds are pretty versatile when it comes to redox reactions, since nitrogen shows many oxidation states. So the simple reason for why $\ce{HNO3}$ is so strong an oxidising agent (with respect to other acids) is that it has a different, better path available to it to get reduced.
Note that the exact reduction path (i.e. final reduction products/oxidation state) depends upon the concentration of nitric acid—so much that copper can be oxidised in three different ways.
answered Jun 6, 2012 at 1:17
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$\begingroup$ what about platinum? the E reduction of platinum is greater than NO3, yet NO3 still oxidized Pt. also the same case for Ag $\endgroup$– DiniJan 28, 2020 at 13:35
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ManishEarth did a good job explaining already. Perhaps I could point out one more you could think of to understand why nitric acid has such a high reduction potential. The nitrogen centre in nitric acid is in +5 oxidation state, which is also the highest known oxidation state of nitrogen. As a result, the acid exists in high energy state to begin with as it is really unstable.