1. Reduction of Nitric Acid:
Nitric acid (HNO3) undergoes reduction during the reaction with a metal. In this process, the nitrogen atom in nitric acid is reduced from a +5 oxidation state to a lower oxidation state, usually +2 or +1. This reduction of nitric acid consumes electrons and prevents the reduction of hydrogen ions (H+) to hydrogen gas.
2. Formation of Water:
During the reaction between a metal and nitric acid, water (H2O) is formed as a byproduct. This is because nitric acid contains both hydrogen and oxygen atoms, and when the hydrogen ions (H+) are not reduced to hydrogen gas, they combine with oxygen from the nitrate ion (NO3-) to form water molecules.
3. Oxidizing Properties of Nitrogen Oxides:
Nitric acid also releases nitrogen oxides, such as nitrogen dioxide (NO2) and nitric oxide (NO), during the reaction. These nitrogen oxides are strong oxidizing agents that can further oxidize the metal ions and prevent the evolution of hydrogen gas.
4. Passivation of Metals:
Some metals, such as iron, aluminum, and chromium, form a protective oxide layer on their surfaces when they react with nitric acid. This oxide layer acts as a barrier, preventing further reaction of the metal with the acid and inhibiting the evolution of hydrogen gas.
In summary, the strong oxidizing nature of nitric acid, the formation of water and nitrogen oxides, and the passivation of metals all contribute to the absence of hydrogen gas evolution when a metal reacts with nitric acid. Instead, different products, such as metal nitrates, water, and nitrogen oxides, are formed.
