When copper reacts with cold, dilute nitric acid, it produces copper nitrate, water, and nitric oxide gas. This nitric oxide gas then further reacts with oxygen in the air to form nitrogen dioxide, a distinct brown gas.
Reaction with Cold, Dilute Nitric Acid
Copper (Cu) is a metal that reacts with nitric acid (HNO₃). The outcome of this reaction depends on the concentration and temperature of the acid. Based on the provided information, focusing specifically on the reaction with cold, dilute nitric acid:
The reaction is a redox process where copper is oxidized and nitric acid is reduced. The products formed are:
- Copper(II) Nitrate (Cu(NO₃)₂): A soluble salt, typically producing a blue solution.
- Water (H₂O): A common product in acid-base and redox reactions involving oxygen-containing compounds.
- Nitric Oxide (NO): A colorless gas.
Here is a summary of the reaction based on the reference:
| Reactants | Conditions | Products |
|---|---|---|
| Copper (Cu) | Cold, Dilute | Copper Nitrate (Cu(NO₃)₂) |
| Nitric Acid (HNO₃) | Nitric Acid | Water (H₂O) |
| Nitric Oxide (NO) |
The chemical equation for this specific reaction is often represented as:
3Cu(s) + 8HNO₃(dilute) → 3Cu(NO₃)₂(aq) + 4H₂O(l) + 2NO(g)
Subsequent Reaction of Nitric Oxide with Air
The nitric oxide (NO) gas produced in the primary reaction is unstable in the presence of air (which contains oxygen, O₂). According to the reference, nitric oxide undergoes a further reaction:
Nitric oxide reacts with oxygen from the air.
2NO(g) + O₂(g) → 2NO₂(g)
Properties of Nitrogen Dioxide
The product of the reaction between nitric oxide and air is nitrogen dioxide (NO₂).
- Color: Nitrogen dioxide is a poisonous brown gas.
- Odor: It has an irritating odor.
Therefore, while the initial reaction with cold, dilute nitric acid produces colorless nitric oxide, the moment this gas comes into contact with the air above the solution, it quickly turns brown due to the formation of nitrogen dioxide. This visible change is a key indicator that the reaction has occurred.
