Reactions of Nitric Acid ($$\text{HNO}_3$$) with Metals

Chemical Redox Equations:

$$\begin{aligned} \text{Dilute } \text{HNO}_3 \text{ with Cu:} &\quad 3\text{Cu} + 8\text{HNO}_3 \longrightarrow 3\text{Cu(NO}_3)_2 + 2\text{NO} + 4\text{H}_2\text{O} \\ \text{Conc. } \text{HNO}_3 \text{ with Cu:} &\quad \text{Cu} + 4\text{HNO}_3 \longrightarrow \text{Cu(NO}_3)_2 + 2\text{NO}_2 + 2\text{H}_2\text{O} \\ \text{Dilute } \text{HNO}_3 \text{ with Zn:} &\quad 4\text{Zn} + 10\text{HNO}_3 \longrightarrow 4\text{Zn(NO}_3)_2 + \text{N}_2\text{O} + 5\text{H}_2\text{O} \\ \text{Conc. } \text{HNO}_3 \text{ with Zn:} &\quad \text{Zn} + 4\text{HNO}_3 \longrightarrow \text{Zn(NO}_3)_2 + 2\text{NO}_2 + 2\text{H}_2\text{O} \end{aligned}$$

Concept: The identity of the gaseous reduction product depends heavily on both the concentration of the acid and the electropositive nature of the metal substrate.

Notation:

• $$\text{NO}$$ = Colorless nitric oxide gas;
• $$\text{NO}_2$$ = Pungent brown gas;
• $$\text{N}_2\text{O}$$ = Colorless laughing gas;
• $$\text{Cu}$$ = Less electropositive metal;
• $$\text{Zn}$$ = More electropositive metal.