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I have been recently studying electrochemistry and I came across the electrochemical series. A question was asked saying what will happen if a copper spoon is used to stir an aluminium nitrate solution,the answer was no reaction will take place as Aluminium is more stable in its ionic form and copper cannot reduce aluminium ion, but can't nitrate ion which is above copper in the spectrochemical series oxidize copper and be transformed into NO which is more stable than Cu. Why doesn't it happen?

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Minor correction: It is not the spectrochemical series but rather the electrode potential tables. Those tabulated values are given under standard conditions

You are indeed correct in thinking that nitrate ion should oxidize copper. It is a game of pH. If you have very low pH and nitrate ions, the following reaction takes place (in the absence of other ions)...

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

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  • $\begingroup$ I used Mathpix Snip to convert an equation image into LaTeX. It is very useful tool for converting scanned equations/ mathematical papers into editable format. See mathpix.com if interested. $\endgroup$
    – M. Farooq
    Dec 7 '20 at 20:00
  • $\begingroup$ Well, Mathpix Snip does well for plain math, but fails horribly at chemical expressions. Please check the output for sanity before copy-pasting. $\endgroup$
    – andselisk
    Dec 8 '20 at 6:24
  • $\begingroup$ Mathpix Snip is an excellent tool for scanned math equations. I do not know of any better freeware. It is not meant for chemical structures. Anyway the sky did not fall apart if the state symbol for liquid appeared as 1 by my oversight. $\endgroup$
    – M. Farooq
    Dec 8 '20 at 6:48
  • $\begingroup$ I just would like the post by a 20k user to look professionally. Don't blindly accept what some black box tools spoon-feed you with. As for Mathpix, it's not the first and not the greatest OCR tool for LaTeX documents. Just as many others, it fails at recognizing anything complex and compiled with extra packages. What's even worse, it's not a tool, but a service which doesn't let you do anything before you sign up for it, which makes it useless for me. $\endgroup$
    – andselisk
    Dec 8 '20 at 9:14
  • $\begingroup$ There was an important question in medieval times "How many angels can stand on the point of a pin?" en.wikipedia.org/wiki/… Mathpix Snip is a tool or service, after 100 years, if SE survives, people would view this $excessive$ obsession with LaTeX, fonts sizes of symbols or trivial terminologies exactly like the above question. We should focus on the concept. $\endgroup$
    – M. Farooq
    Dec 8 '20 at 14:48
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First, an observation that Aluminum nitrate is the product of a weak base and a strong acid. As such, expect the aqueous $\ce{Al(NO3)3}$ salt solution to be but mildly acidic.

Now, if the copper metal is fully immersed in a mildly acidic salt solution with no other factors (like dissolved oxygen exposure), I suspect this pH level alone is not likely sufficient to result in any noticeable chemical reaction including gaseous $\ce{NO}$ generation.

However, exposure to an oxygen source introducing some dissolve ${O_2}$, could in time result in an electrochemical based reaction resulting in the observed formation of an insoluble basic salt (see relatedly, for example, Equation (7) in this Wikipedia discussion on the industrial production of basic copper chloride).

So possible corrosion of the copper metal, especially if aged with a protective cuprous oxide coating. This is based on the electrochemical half-cell reactions:

$\ce{ Cu -> Cu+ + e- }$

$\ce{ Cu+ -> Cu++ + e- }$

$\ce{4 H+ + O2 + 4 e- <-> 2 H2O }$ (Source, see for example, p. 21-2 here)

resulting, in the current case, the following net reaction, for example:

$\ce{4 Cu+ + O2 + 2 H+ -> 4 Cu++ + 2 OH-}$

with potentially some insoluble basic nitrate formation observed.

As such, an aerated version of this acidic ionic mix may still not be a completely inert (non-corrosive) situation for the copper metal with time. As added support, I reference an educational source on the history of The Statue of Liberty, to quote:

The Statue of Liberty is a landmark every American recognizes. The Statue of Liberty is easily identified by its height, stance, and unique blue-green color (Figure 1). When this statue was first delivered from France, its appearance was not green. It was brown, the color of its copper “skin.” So how did the Statue of Liberty change colors? The change in appearance was a direct result of corrosion. The copper that is the primary component of the statue slowly underwent oxidation from the air. The oxidation-reduction reactions of copper metal in the environment occur in several steps...

A more technical reference Atmospheric corrosion prediction: a review makes several important observations, to quote in part:

For another example, when the nitric acid content in the atmosphere is high, the corrosion rate of copper is also independent on temperature (Samie et al. 2007)... When the temperature increases, the solubility of oxygen decreases and the corrosion product is more compact, the corrosion reaction slowed down as a consequence. However, temperature increase also leads to the increase of diffusion rate of oxygen and chloride ions, which accelerates the corrosion reaction.

Also:

Acidified aerosol is another important pollutant which has the potential to make a significant contribution to the atmospheric corrosion process (Cole et al. 2009)...While gas absorption may occur relatively close to the source, acidified aerosols can be transported over some distance from the original gaseous source. The aerosols are characterized by low pH, fine size (typically 1–100 mm) and high dissolved ionic salt content...Take zinc as an example, the presence of acidified aerosols may lead to enhanced corrosion by disrupting any protective oxide films and subsequently establishing electrochemical cells, relatively uniform oxide layers are formed, which may be dissolved...

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  • $\begingroup$ The OP is asking " but can't nitrate ion which is above copper in the spectrochemical series oxidize copper" not whether copper will corrode or not. Copper even starts to dissolve slowly in vinegar in the presence of oxygen. $\endgroup$
    – M. Farooq
    Dec 8 '20 at 16:52
  • $\begingroup$ @Farooq: Yes, and answered, to quote: "if the copper metal is fully immersed in a mildly acidic salt solution with no other factors (like dissolved oxygen exposure), I suspect this pH level alone is not likely sufficient to result in any noticeable chemical reaction including gaseous NO generation." However, is it clear that the copper is completely insulated from air? And if not, the claim of no reaction is, in time, not likely accurate, as claimed per my sources and presented chemistry. $\endgroup$
    – AJKOER
    Dec 8 '20 at 17:59
  • $\begingroup$ I should also add per the question itself, to quote: "when a copper spoon is used to stir the solution", which implies a partial exposure of the copper spoon to both air and the acidic ionic salt, which would likely display corrosive effects at the interface of the copper metal spoon and the solution with time, or upon removal of the spoon and left to dry (especially a wet-dry cycle). $\endgroup$
    – AJKOER
    Dec 8 '20 at 18:17

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