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Question
Illustrate how copper metal can give different products on reaction with HNO3.
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Solution 1
Concentrated nitric acid is a strong oxidising agent. It is used for oxidising most metals. The products of oxidation depend on the concentration of the acid, temperature, and also on the material undergoing oxidation.
\[\ce{3Cu + 8HNO3(dilute) -> 3Cu(NO3)2 + 2NO + 4H2O}\]
\[\ce{Cu + 4HNO3(conc) -> Cu(NO3)2 + 2NO2 + 2H2O}\]
Solution 2
Copper reacts with cold and dilute nitric acid; copper nitrate and nitric oxide are obtained.
\[\ce{2HNO3 -> 2NO + H2O + 3O}\]
\[\ce{[Cu + O -> CuO] \times 3}\]
\[\ce{[CuO + 2HNO3 -> Cu(NO3)2 + H2O] \times}\]
\[\ce{3Cu + 8HNO3 -> \underset{(cold and dilute)}{3Cu(NO3)2} + 2NO + 4H2O}\]
Copper reacts with hot and concentrated nitric acid, nitrogen dioxide is obtained.
\[\ce{2HNO3 -> 2NO2 + H2O + O}\]
\[\ce{Cu + O -> CuO}\]
\[\ce{CuO + 2HNO3 -> Cu(NO3)2 + H2O}\]
\[\ce{Cu + \underset{(hot and conc)}{4HNO3} -> Cu(NO3)2 + 2NO2 + 2H2O}\]
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