What happens when Cu reacts with cold dilute HN{O_3}?

When copper (Cu) reacts with cold and dilute nitric acid (HNO3), several chemical reactions occur due to the oxidation properties of nitric acid. The primary products of this reaction are copper nitrate (Cu(NO3)2), nitrogen dioxide (NO2), and water (H2O). The reaction can be summarized by the following equation:

3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO2 + 4H2O

In this reaction, copper is oxidized from its elemental state (Cu) to copper(II) ions (Cu^2+), while nitric acid is reduced to nitrogen dioxide gas (NO2). The presence of water helps facilitate the reaction and also forms as a product.

The brownish-red gas NO2 is responsible for the characteristic reddish-brown fumes often observed during the reaction. These fumes are a mixture of NO2 and nitrogen tetroxide (N2O4), which can further react with atmospheric moisture to form nitric acid (HNO3), contributing to the acidic properties of the fumes.

It's important to note that the reaction of copper with concentrated nitric acid can lead to a different set of reactions, where the nitric acid acts as a strong oxidizing agent and can potentially lead to the formation of copper(II) nitrate, nitrogen oxides, and even elemental nitrogen. The reaction products can vary depending on factors such as the concentration of nitric acid, temperature, and reaction conditions.