Question

29. Copper does not dissolve in HCl(aq) but dissolves
HNO, (aq) producing Cu" ions. Explain the differen
in behaviour.
[Given
E = 0.34V:E = 1.36 V an
Cl/2CI
NO, +4H* +3E - NO(g) + 2H,0
E
No;''No = 0.97V)​

Answer 1

Answer:

Dissolution of a metal in HCl actually means that the metal has displaced H + ions and form its own salt. However, copper is unable to displace H sine copper lies below H in electropositive character series. HNO ^3 is a strong oxidising agent also. Hence it rather oxidises copper to Cu ^2+ ions and hence copper dissolves in HNO 3

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Answer 2

Answer:

Copper does not dissolve in a typical strong acid because the overall cell potential for the oxidation of copper metal to Cu2+ ions coupled with the reduction of H+ ions to H2 is negative.

Cu ---> Cu2+ + 2 e- Eoox = -(0.34 V)

+ 2 H+ + 2 e- ---> H2 Eored = 0.000... V

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Cu + 2 H+ --->Cu2+ + H2 Eo = Eoox + Eored = -0.34 V

Copper dissolves in nitric acid because the reaction at the cathode now involves the reduction of nitric acid to NO gas, and the potential for this half-reaction is strong enough to overcome the half-cell potential for oxidation of copper metal to Cu2+ ions.

3 (Cu ---> Cu2+ + 2 e-) Eoox = -(0.34 V)

+ 2(HNO3 + 3 H+ + 3 e- ---> NO + 2 H2O) Eored = 0.96 V

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3 Cu + 2 HNO3 + 6 H+ ---> 3 Cu2+ + 2 NO + 4 H2O Eo = Eoox + Eored = 0.62 V