Question

Both aqueous HCl and HNO3 are strong acids (equal acidity in water); yet CuO will dissolve more easily in 1 M HCl than in 1 M HNO3. What accounts for this difference?

Answer 1

Copper is below H on the activity series. Which implies that copper will not reduce H⁺, nor will H⁺ oxidize copper metal. In other hand there are some side reactions that can occur.

For instance, a copper penny is usually covered with some copper (II) oxide. Copper (II) oxide is soluble in HCl and makes Cu2+ and OH-. Cu metal and Cu2+ will react to make Cu+. Cu+ will promptly react with water and O2 dissolved in the solution to make Cu2+ and OH-. The OH- reacts with H+ to make water. So there are lots of things going on.

But HNO3, which we can say that copper really reacts with is nitric acid, and that is because of the nitrate ion.

3 Cu(s) + 2 NO₃^- + 8 H⁺ 3 Cu²⁺ + 2 NO(g) + 4 H₂O

CuO + H⁺ Cu²⁺ + OH^- Copper (II) oxide dissolves in HCl.
OH^- + H⁺ H₂O ..... OH^- Immediately reacts with H+ to make water.
Cu²⁺ + 4Cl^- CuCl₄^2- In the presence of chloride ion, water molecules will be replaced.
Cu(s) + CuCl₄^2- CuCl(s) + 3Cl^- A comproportionation reaction occurs. CuCl is insoluble in water
4CuCl(s) + 2H⁺ + O₂ 4Cu²⁺ + 2OH^- Cu(I) is oxidized.
OH^- + H⁺ H₂O Immediate reaction between OH^- and H⁺
Cu²⁺ + 4Cl^- CuCl₄^2- Since Cl^- is present, copper exists as the tetrachlorocopper(II) ion.

The bottom line is that copper metal appears to dissolve in HCl in the presence of oxygen.

Cu(s) + O₂(g) + 2HCl(aq) --> CuCl₂(aq) + 2H₂O(l)

The reaction is slow, and requires a fairly hefty excess of HCl.


As for the reaction between copper and nitric acid. With absolutely no oxygen available, it will form NO which is colorless. With O₂ present dissolved in the solution and in the air, the NO will oxidize to the familiar reddish-brown NO₂ that we see coming off a reaction of copper and nitric acid.

So I should have put:
3Cu(s) + 2NO₃^- + 8H⁺ 3Cu²⁺ + 2NO(g) + 4H₂O
2NO(g) + O₂(g) 2NO₂(g)