Give the mechanism of reaction of conc.HNO3 with Cu.

Asked by Dr Amitava | 9th Sep, 2014, 08:50: PM

Expert Answer:


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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)


Topperlearning Team.

Answered by Prachi Sawant | 23rd Sep, 2014, 12:15: PM