Nitric Acid is a Powerful Oxidizer
Nitric acid (HNO₃) is a strong oxidizing agent. This means it readily accepts electrons from other substances, causing them to become oxidized. When metals react with nitric acid, the nitric acid acts as an oxidizer, not an acid.
The Reaction Pathway
1. Nitric acid oxidizes the metal: Instead of releasing hydrogen ions (H⁺) like other acids, nitric acid uses its own nitrogen atoms to accept electrons from the metal. This results in the metal losing electrons and becoming oxidized.
2. Nitrogen reduction: The nitrogen in nitric acid gets reduced (gains electrons) and forms various nitrogen oxides like nitrogen dioxide (NO₂) or nitrogen monoxide (NO).
3. Metal nitrate formation: The oxidized metal ions combine with nitrate ions (NO₃⁻) to form a metal nitrate salt.
Example: Reaction of Copper with Nitric Acid
Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
* Copper (Cu) is oxidized to copper(II) ions (Cu²⁺).
* Nitrogen in nitric acid is reduced to nitrogen dioxide (NO₂).
* Copper(II) nitrate (Cu(NO₃)₂) is formed.
Exceptions:
* Very dilute nitric acid: In very dilute nitric acid, a small amount of hydrogen gas might be produced along with other nitrogen oxides. This is because the concentration of nitric acid is too low to effectively act as an oxidizer.
* Certain metals: Some metals, like gold and platinum, are not readily oxidized by nitric acid. These metals require a mixture of nitric acid and hydrochloric acid (aqua regia) to dissolve them.
In Summary:
The oxidizing nature of nitric acid prevents the typical acid-metal reaction of hydrogen gas production. Instead, the metal is oxidized and nitrogen in nitric acid is reduced, leading to the formation of nitrogen oxides and metal nitrates.
