In the reaction between copper (Cu) and nitric acid (HNO3), the concentration of HNO3 can affect the oxidation state of the nitrogen oxide products. The reaction is commonly represented as follows:
Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
When concentrated nitric acid is used, the reaction produces nitrogen dioxide (NO2) instead of nitric oxide (NO). This is due to the oxidizing properties of concentrated nitric acid.
Concentrated nitric acid is a strong oxidizing agent. It contains a high concentration of nitrate ions (NO3-) and protons (H+). When copper reacts with nitric acid, copper undergoes oxidation, losing electrons to form Cu2+ ions. The nitrate ions (NO3-) are reduced, accepting the electrons released by copper and being converted to nitrogen oxide species.
In the presence of concentrated nitric acid, the high concentration of nitrate ions and protons facilitates the further oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The reaction can be represented as follows:
2 NO + O2 → 2 NO2
The oxidizing properties of concentrated nitric acid favor this oxidation reaction, leading to the formation of nitrogen dioxide. Therefore, when concentrated nitric acid is used, the resulting product is NO2 rather than NO.
In contrast, with dilute nitric acid or in the absence of excess oxidizing agents, the reaction may produce nitric oxide (NO) instead of nitrogen dioxide (NO2). The concentration and oxidizing power of the nitric acid can significantly influence the products formed in the reaction between copper and nitric acid.