Copper is a reddish-brown metal, widely used in plumbing and electrical wiring; it is perhaps most familiar to people in the United States in the form of the penny. (Although since 1983, pennies are actually made of zinc surrounded by a paper-thin copper foil to give them the traditional appearance of pennies.) Copper is oxidized by concentrated nitric acid, HNO3, to produce Cu2+ ions; the nitric acid is reduced to nitrogen dioxide, a poisonous brown gas with an irritating odor: Cu(s) + 4HNO3(aq) ——> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
When the copper is first oxidized, the solution is very concentrated, and the Cu2+ product is initially coordinated to nitrate ions from the nitric acid, giving the solution first a green, and then a greenish-brownish color. When the solution is diluted with water, water molecules displace the nitrate ions in the coordinate sites around the copper ions, causing the solution to change to a blue color. In dilute nitric acid, the reaction produces nitric oxide, NO, instead: 3Cu(s) + 8HNO3(aq) ——> 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
In the following demonstration, a balled-up piece of thin copper wire is added to about 100 mL of concentrated nitric acid; once the copper is added the evolution of nitrogen dioxide occurs quickly. Once all of the copper has reacted, the solution is diluted with distilled water, changing the solution from a dark brown to a pale blue color. This demonstration can be done with copper in the form of shot, pellets, thicker wire, or bars, but is a great deal slower than with copper wire. Video Clip: REAL, 7.02 MB
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