The Reaction of Copper with Nitric Acid: Concentrated vs Dilute

Copper reacts with nitric acid because nitrate can act as an oxidizing agent. Copper metal is oxidized to copper(II) ions, which form copper(II) nitrate in solution. The nitrogen-containing product depends mainly on the acid conditions: concentrated nitric acid typically produces brown nitrogen dioxide (NO₂), while dilute nitric acid typically produces colorless nitric oxide (NO). Nitric oxide turns brown when it reacts with oxygen in the air.

These are the commonly used overall equations:

Concentrated nitric acid

Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

Dilute nitric acid

3Cu(s) + 8HNO₃(aq) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)

In both reactions, copper changes from oxidation state 0 to +2. Nitrogen in nitrate has oxidation state +5. It is reduced to +4 in NO₂ or +2 in NO. The equations show the different electron balances and the amounts of gas produced for each amount of copper.

The distinction between concentrated and dilute acid is a useful guide, not a guarantee that every reaction produces only one nitrogen oxide. Product formation can depend on the reaction conditions, and very dilute acid may give other nitrogen-containing products. A mechanistic study examines steps involved in copper dissolution and the formation of NO₂, nitrous acid, and NO: Mechanistic Study of the Production of NOx Gases from the Reaction of Copper with Nitric Acid.

What you observe

With concentrated nitric acid, copper dissolves and brown fumes of NO₂ form above the reaction mixture. The solution contains Cu²⁺ ions and is usually blue or blue-green. The reaction can release heat, so the mixture may become hot.

With dilute nitric acid, copper also dissolves and the solution becomes blue or blue-green. The gas leaving the reaction is usually colorless NO. When it mixes with oxygen in the air, it forms brown NO₂:

2NO(g) + O₂(g) → 2NO₂(g)

This change explains why brown fumes may appear above an open vessel even when the gas produced at the copper surface was initially colorless. NO is difficult to detect by sight before it reacts with oxygen.

Nitrogen dioxide is toxic and can injure the lungs. Do not use smell or a direct visual test to identify an unknown gas. Any demonstration that produces nitrogen oxides requires suitable controls and ventilation, such as a functioning fume hood, along with appropriate eye and skin protection. Nitric acid is corrosive and a strong oxidizer; consult its safety information before handling it. PubChem: Nitric Acid

Copper does not normally release hydrogen when placed in dilute, non-oxidizing acids such as hydrochloric acid. Nitric acid can dissolve it because nitrate acts as an oxidizing agent. Copper’s position below hydrogen in the activity series helps explain why acidity alone is not enough to produce the same reaction. Royal Society of Chemistry Periodic Table: Copper

Worked example: estimating the gas produced

Consider 1.0 g of copper reacting completely with excess concentrated nitric acid. Using a molar mass of about 63.55 g/mol, this amount is approximately 0.0157 mol of copper. The concentrated-acid equation produces 2 mol of NO₂ for every 1 mol of copper:

0.0157 mol Cu × 2 = 0.0314 mol NO₂

At room temperature, an ideal-gas estimate using a molar volume of about 24.5 L/mol gives:

0.0314 mol × 24.5 L/mol ≈ 0.77 L NO₂

This is an estimate, not a prediction of the exact volume from a particular experiment. Actual gas volume depends on temperature, pressure, and how the gas is collected. The calculation also makes clear why even a small mass of copper can produce a hazardous amount of nitrogen dioxide. This reaction should not be carried out in an open room.

For comparison, the dilute-acid equation produces 2 mol of NO for every 3 mol of copper. The same 1.0 g of copper would therefore produce about 0.0105 mol of NO, or roughly 0.26 L at the same assumed conditions. This comparison follows from the balanced equations. It does not make the dilute-acid reaction safe: NO can turn into toxic NO₂ when exposed to air.

The acid concentration affects the nitrogen oxide product, while other conditions can affect how quickly the reaction proceeds. For example, copper with more exposed surface area can react faster than a thick piece of wire. The balanced equation describes the overall stoichiometry, not the reaction rate.

Key Takeaways

• Copper is oxidized to Cu²⁺, which forms copper(II) nitrate in solution.
• Concentrated nitric acid typically produces brown NO₂; dilute nitric acid typically produces colorless NO, which turns brown in air.
• The overall equations are:
  • Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O for concentrated acid.
  • 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O for dilute acid.
• Nitrate is the oxidizing agent. The product can vary with the reaction conditions, especially in very dilute solutions.
• Nitrogen oxides are hazardous. Do not perform these reactions without appropriate ventilation and chemical safety controls.