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I am confused on the products of reaction between copper nitrate and ascorbic acid. When I searched this online, one of the reactions I got was: $$\ce{2Cu(NO3)2 + C6H8O6 <=> 2CuO + 4H2O + CO2}$$

I am really confused as to how they determined that to be the product. My thinking was that the ascorbic acid breaks into $$\ce{HC6H7O6 <=> H+ + C6H6O6-}$$ and so the hydrogen ion, whose activity is higher than copper, replaces the copper to form nitric acid. I was thinking that this could be a double displacement reaction and so the copper ascorbate would become a precipitate. What am I thinking wrong here?

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    $\begingroup$ You have somehow managed nitrogen to annihilate with carbon and oxygen. You should really take your time for enumeration of chemical equations. BTW, such reaction would need high temperature and nitrate would decompose before it would be effective to fully oxidize organic matter. Definitely not happening in solution. $\endgroup$ – Poutnik May 4 at 3:40
  • $\begingroup$ @Poutnik copper ascorbate? $\endgroup$ – Nilay Ghosh May 4 at 13:32
  • $\begingroup$ Yes, would copper ascorbate + nitric acid product be possible? $\endgroup$ – user510 May 4 at 14:57
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    $\begingroup$ Where is the nitrogen on the R.H.S.? $\endgroup$ – Andrew Morton May 4 at 19:10
  • $\begingroup$ Nitrogen from the $\ce{NO3-}$ combining with the hydrogen ion of ascorbic acid to form nitric acid? $\endgroup$ – user510 May 5 at 14:37
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It is always good to cite the link when you mention that you saw a redox reaction online. It is wrong anyway.

Aqueous Cu(II) is not an strong enough oxidizing agent that it will convert organic matter into carbon dioxide. In other words, it is a weak oxidizing agent. Ascorbic acid is oxidized to dehydroascorbic acid and Cu(II) should be reduced to Cu(I). If you heat the solution strongly, Cu(I) may convert into Cu$_2$O.

Ascorbic acid is indeed an acid, but there is nothing which prevents its conjugate base to behave like a reducing agent. Many organic acids can behave like that.

I am really confused as to how they determined that to be the product.

By conducting the experiment. There is no other simple way to predict what will happen in a complex redox reaction.

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  • $\begingroup$ Would the way I am thinking about it make sense i.e. you get $\ce{Cu(NO3)2 + C6H8O6 <=> HNO3 + CuC6H7O6}$ or would the copper precipitate out? Is there a way to theoretically predict the product of this reaction with some confidence without an experiment? $\endgroup$ – user510 May 4 at 4:45
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    $\begingroup$ "Is there a way to theoretically predict the product of this reaction with some confidence without an experiment? –" I am afraid not. Chemistry has not progressed enough to predict the products theoretically from scratch. May in another 50 or 100 years this will be possible for complex reactions as yours. Today, you need to do an experiment to find out what the products are. $\endgroup$ – M. Farooq May 4 at 5:00
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    $\begingroup$ @M.Farooq Come now... There are many methods within thermodynamics, reaction kinetics and quantum chemistry which not only tell us which products occur with which probability but also enables us to model the properties of these products. We have been able to predict fairly well the outcomes of reactions for 300 years now - So while we do not have a common method, or methods for all kinds of reactions, I find your statement unnecessarily defeatist... Suffice to say it is a complicated problem, this predicting business, and there is no single perfect solution found yet. $\endgroup$ – Stian Yttervik May 4 at 13:22
  • $\begingroup$ @StianYttervik, Of course the human mind has been predicting the outcomes of new reaction by interpolation and extrapolation of known chemical reactions, my point was can an ab initio calculation predict the outcome of this reaction in the OP yet. I don't think so. $\endgroup$ – M. Farooq May 4 at 14:52
  • $\begingroup$ would a reaction I proposed be a possibility? $\endgroup$ – user510 May 4 at 14:57
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In my assessment, an advanced reaction involving a cyclic REDOX reaction occurring with Copper ions, which I have previously discussed in this work 'Generation of Hydroxyl Radicals from Dissolved Transition Metals in Surrogate Lung Fluid Solutions' by Edgar Vidrio relating to disease (cancer) likely induced from progressive damage to DNA (in the new equation this is now an attack on the nitrate ion,...) arising from the transition metal copper interacting with oxygen (in the new reaction this is replaced by HNO3,...) leading to the powerful hydroxyl radical.

That reaction becomes cyclic (at least, to a limited degree depending on the respective concentration of Ascorbic acid) per Vidrio's to quote:

“Similar reactions can occur with Cu, Cr and Ni. Furthermore, biological chelators and reductants can greatly enhance the production of ROS (Burkitt et al., 1991; Engelmann et al., 2003; Wenk et al., 2001). For example, in the presence of ascorbate (Asc), a biological reductant, the oxidized form of the transition metal produced by the Fenton reaction can be reactivated (R2 and R3), thus allowing additional ROS to be produced.

$\ce{Fe(III) + Asc (n) → Fe(II) + Asc (n') (R2)}$

$\ce{Cu(II) + Asc (n) → Cu(I) + Asc (n') (R3)}$

which cycles the cupric back to cuprous to feed a REDOX, which, for our new reaction involves aqueous cupric nitrate and Ascorbic acid, that is detailed in this reference, "Fenton chemistry in biology and medicine" by Josef Prousek, to quote reaction (15) on page 2330, a general depiction of Fenton-type reactions, to quote:

"For Fe(II) and Cu(I), this situation can be generally depicted as follows [20,39],

$\ce{ Fe(II)/Cu(I) + HOX → Fe(III)/Cu(II) + ·OH + X- }$ (15)

where X = Cl, ONO, and SCN. "

In the current context, the hydrolysis of cupric nitrate introduces HNO3, which interacts with cuprous (created from the Ascorbic acid presence) reforming cupric and, importantly, always hydroxyl radicals. The latter radical can attack nitrogen oxides compounds which can further progressively be broken down. Some expected reactions:

$\ce{ .OH + NO3- -> OH- + ·NO2 }$

$\ce{ .NO2 + .NO2 -> N2O4 }$

$\ce{ N2O4 + 2 H2O <=> HNO2 + HNO3}$

$\ce{ 2 Cu(I) + HONO -> 2 Cu(II) + ·OH + N2O2^{2-} }$

etc., so the nitrate, nitrite,..., could be progressively decomposed (even to NH3), which may account for its notable absence in the cited text.

Lastly, the REDOX consumes H+ and select radical reactions could add OH-, all of which could eventually lead to either a basic copper salt, or Cu(OH)2. The latter on warming, forms CuO, as was claimed.

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