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Why is there a reaction when one mixes Copper pennies and Nitric acid together? What is this reaction?
Question Date: 2009-01-03
Answer 1:

Nitric acid is a powerful oxidizer as well as a very strong and somewhat unstable acid. Any acid works by donating Hydrogen to evolve water -- in the case of Nitric acid, the metal causes the nitric acid to break down into Hydrogen and nitrate ions which then combine with the copper to make copper nitrate, Nitrogen dioxide and water. (If there is more water present initially, it is broken up by nitric acid to make Hydronium (H30+) and Hydroxide (OH-) ions coordinated with the nitrate ions. In that case, NO is produced instead of NO2 -- both of these are poisonous gasses, indeed NO2 can react with water to make more NH03 -- a process that can happen in your lungs if you inhale NO2. (NO2 is one of the nastier by-products of automobile operation-- and is closely monitored by modern pollution controls).

As to why this happens, strong acids are very weakly bound to their hydrogen and thus greatly accelerate the corrosion process. (If you place copper in water, it will also be attacked by the naturally occurring H3O in the water -- leading to Copper Hydroxide, but this process is very slow unless there are substantial salts dissolved in the water (they can react with the Copper Hydroxide to make carbonates and other insoluble salts-- like in old water pipes). An alternative way to accelerate corrosion is electrolysis -- i.e. if you put a small amount of salt in water and place two copper wires in the solution in series with a battery, you can generate hydrogen gas while the Chlorine produced will mostly attack the copper to make cupric chloride solution.

Copper is often blue or green in water-coordinated salts-- There is a very nice demo on line to watch what happens at:

The deeper question you ask is "what are the actual mechanisms of the action of the acid?" --this area of study is termed "reaction dynamics or kinetics" and is the subject of a great deal of current research since tools to measure tiny quantities of very short-lived ions have only become available in the last 20 years or so. It is usually easy to determine if a reaction is energetically favorable, but much harder to determine reaction rates, which depend on the detailed process-- a good example is the action of Catalysts or Enzymes which tremendously accelerate reaction rates, without themselves being consumed in the reaction.

Answer 2:

Thank you for submitting your question.When copper is mixed with nitric acid a red/ox reaction occurs. A red/ox reaction is a reaction where something is reduced and something else is oxidized. In the case of copper with nitric acid the copper (oxidation state 0) becomes oxidized to copper nitrate (Cu(NO3)2 where the copper oxidation state now +2). The species reduced is the nitrogen as it forms nitrogen dioxide (NO2), a brown gas (you may have seen this brown color in the smog over LA). So the reaction that occurs here is:Cu + 4HNO3 --> Cu(NO3)2 + 2NO2 + 2H2O

I should also mention that in the case of dilute nitric acid nitric oxide (NO) is formed. The reaction here is: 3Cu + 8HNO3 --> 3Cu(NO3)2 + 2NO + 4H2O.Nitric oxide is also a toxic gas but it is colorless. It reacts quickly with oxygen in the air to form nitrogen dioxide.

Answer 3:

The reaction that occurs is a redox reaction. The Copper metal (Cu 0)on the copper pennies is oxidized in concentrated nitric acid to produce Cu2+ ions. Copper is easily oxidized to Cu2+, which is blue in aqueous solutions (water) so you should see the solution turn blue as the copper penny reacts. Because the nitric acid oxidizes the copper, the Nitric acid itself gets reduced. It is reduced to nitric oxide (NO). Nitric oxide then reacts with oxygen in the air to make nitrogen dioxide (NO2). Nitrogen dioxide is a brown gas, so you should also see this gas forming in the reaction (unless you do the reaction in the absence of air. NO is colorless). This is the same brown gas in the brown haze you see if you've ever flown into Los Angeles, as it is a major component in smog.

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