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Depending on the concentration of nitric acid, copper undergoes two distinct reactions. For dilute concentrations of nitric acid, the reaction is

$$3\text{Cu}+8\text{HNO}_3\rightarrow 3\text{Cu(NO}_3\text{)}_2+2\text{NO}+4\text{H}_2\text{O}$$

For concentrated solutions of nitric acid, the reaction is

$$\text{Cu}+4\text{HNO}_3\rightarrow \text{Cu(NO}_3\text{)}_2+2\text{NO}_2+2\text{H}_2\text{O}$$

I'm curious what causes the reaction to change? Is it that the solution becomes more oxidizing at higher concentrations? Is it that the concentration of nitric acid makes a particular mechanism more probable. Additionally, when using these reactions for modeling, is really only one or the other that is occurring, or is the stoichiometry a mix in reality? If it really is binary, that is, only one of the two reactions is occurring, what concentration does this change occur?

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    $\begingroup$ The stoichiometry is indeed a wild mix, and not just of these two. $\endgroup$ – Ivan Neretin May 13 '19 at 22:44