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?


marked as duplicate by Mithoron, Tyberius, Todd Minehardt, andselisk, Buck Thorn May 14 at 14:57

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