Copper Metal and Nitric Acid

Many of the less metals (copper is one example) that do not react with hydrochloric acid will react with concentrated nitric acid. They do not produce hydrogen gas (as is the case when reactive metals such as zinc or aluminum react with hydrochloric acid) but react to produce brown nitrogen dioxide gas. The nitrogen dioxide results from reduction of the nitrate ion. This is considered to be a reduction because in the nitrate ion nitrogen has an oxidation state of +5 but in nitrogen dioxide it has an oxidation state of +4; the oxidation state of the nitrogen is therefore decreased or "reduced." We can represent this process with the half-reaction shown below.

2H⁺ + NO₃^- = NO₂ + H₂O

In turn, the solid copper is oxidized to copper(II), which dissolves in the solution. We can represent this process with the hald-reaaction showm below.

Cu(s) = Cu²⁺(aq) + 2e^-

Most students of chemsitry are familiar with the blue color of the copper(II) ion. However, copper is only blue when water moelecules are coordinated to it. In concentrated nitric acid, the nitrate ions are coordinated to the copper(II) ion, imparting a green color to the solution as the reaction proceeds. Finally, in the last portion of the video, the solution turns blue as water is added and the nitric acid is diluted.

Copper metal is added to concentrated nitric acid	Evolution of brown nitrogen dioxide gas begins
Evolution of gas continues	Water is added; acid is diluted and reaction is quenched

Hazards: concentrated nitric acid is extremely corrosive and is a strong oxidizer; it should be handed with care. This nitrogen dioxide gas generated by this reaction is hazardous. This reaction should be performed only in a fully functional fume hood.

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(Windows AVI formation, Microsoft Video9 Compression)

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