Reaction of Copper With Nitric Acid: A Step-by-Step Guide

When you drop a piece of copper into nitric acid, the metal does not simply dissolve. It reacts vigorously, producing a blue-green solution of copper nitrate and releasing toxic nitrogen dioxide gas, which appears as a dense, reddish-brown fume. This is a classic example of an oxidation-reduction reaction, and it is one of the few common acid-metal reactions that does not produce hydrogen gas.

What Happens During the Reaction

The reaction of copper with nitric acid is not straightforward because nitric acid is a strong oxidizing agent. Unlike hydrochloric or dilute sulfuric acid, which attack metals by displacing hydrogen, nitric acid oxidizes the copper metal directly.

The overall chemical equation depends on the concentration of the acid. For concentrated nitric acid (roughly 16 M or above), the reaction is:

Cu + 4 HNO₃ → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O

For dilute nitric acid (around 6 M), the reaction produces nitric oxide instead of nitrogen dioxide:

3 Cu + 8 HNO₃ → 3 Cu(NO₃)₂ + 2 NO + 4 H₂O

In both cases, the copper atoms lose electrons (they are oxidized) to form Cu²⁺ ions, which give the solution its characteristic blue color. The nitrate ion from the acid is reduced, meaning it gains electrons, to form nitrogen oxides. The concentrated acid reaction is faster and more dramatic because the nitrogen dioxide gas is produced immediately.

The Role of Nitric Acid as an Oxidizer

Nitric acid is unusual among common acids because the nitrate ion is a powerful oxidizing agent. In the reaction of copper with nitric acid, the nitrate ion is more easily reduced than the hydrogen ion. That is why you see brown fumes of NO₂ instead of bubbles of hydrogen gas. This property makes nitric acid useful for dissolving metals that are resistant to other acids, such as copper and silver. The Royal Society of Chemistry notes that nitric acid can oxidize most metals except gold and platinum Royal Society of Chemistry.

Observing the Reaction in the Lab

If you perform this reaction in a laboratory setting, you will notice several distinct changes. The copper metal, which is a shiny reddish-brown solid, begins to darken and then break apart as it reacts. The solution around the copper turns a pale blue and then deepens to a rich turquoise or blue-green as copper(II) nitrate forms.

The most visible sign is the gas. With concentrated acid, reddish-brown fumes of nitrogen dioxide rise immediately from the surface of the metal. These fumes are toxic and corrosive, so the reaction must be carried out in a fume hood or a well-ventilated area. With dilute acid, the gas is colorless nitric oxide, but it quickly reacts with oxygen in the air to form nitrogen dioxide, so a brown color still appears above the solution after a short delay.

The reaction also generates heat. The process is exothermic, and the container will feel warm to the touch. In some cases, if the acid is concentrated enough and the copper is finely divided, the heat can be enough to boil the acid locally, accelerating the reaction further.

Safety Considerations

Nitric acid is a strong corrosive and a strong oxidizer. It can cause severe burns on contact with skin or eyes. The gases produced, especially nitrogen dioxide, are highly toxic and can cause serious lung damage if inhaled. Always handle nitric acid in a fume hood with appropriate personal protective equipment, including chemical-resistant gloves, a lab coat, and safety goggles. According to PubChem, nitrogen dioxide is a pulmonary irritant and can be fatal at high concentrations PubChem. Do not attempt this reaction outside of a proper laboratory setup.

Practical Applications and Implications

The reaction of copper with nitric acid is not just a demonstration for chemistry students. It has real-world uses in metal etching and refining. Because nitric acid dissolves copper but does not attack gold or platinum, it is used in the recovery of precious metals from electronic scrap. The copper is removed as copper nitrate, leaving the noble metals behind.

The reaction is also relevant in environmental chemistry. Nitrogen dioxide produced from industrial processes, including metal treatment with nitric acid, contributes to acid rain and smog. Understanding the reaction helps chemists design better scrubbers and control systems to capture these gases before they are released into the atmosphere.

A Simple Worked Example

Suppose you want to dissolve 6.35 grams of copper in concentrated nitric acid. Using the balanced equation for concentrated acid (1 mole Cu to 4 moles HNO₃), you can estimate the acid needed. The atomic mass of copper is about 63.5 g/mol, so 6.35 g is 0.1 moles of copper. You would need 0.4 moles of nitric acid. Concentrated nitric acid is about 16 M, so the volume required is 0.4 moles divided by 16 mol/L, which is 0.025 L, or 25 mL. In practice, you would use a slight excess of acid to ensure the reaction goes to completion.

Key Takeaways

• The reaction of copper with nitric acid is an oxidation-reduction process that produces copper(II) nitrate and nitrogen oxides, not hydrogen gas.
• Concentrated nitric acid yields brown nitrogen dioxide gas, while dilute nitric acid yields colorless nitric oxide that turns brown in air.
• The reaction is exothermic and must be performed in a fume hood with proper safety gear due to toxic gases and corrosive acid.
• This reaction has practical uses in metal etching and precious metal recovery because nitric acid dissolves copper but not noble metals like gold or platinum.
• The blue-green color of the solution comes from hydrated Cu²⁺ ions, a clear visual indicator that the reaction has occurred.