Answer to Question #154029 in Organic Chemistry for lee

Question

Describe the reactions of separate portions of aqueous copper(II) ions with aqueous sodium hydroxide solution, with excess aqueous ammonia solution and with concentrated hydrochloric acid. In your answer you should link observations with equations which include the formulae of any copper-containing complex ions. Include state symbols.

Answer 1

Reaction of Copper(II) with aqueous NaOH

Copper(II) ion reacts with stoichiometric quantities of aqueous ammonia to precipitate light blue Cu(OH)₂. Some basic salts may also form.

The chemical reaction is;

$Cu^+_{2(aq)}+2NH_{3(aq)}+3H_2O_{(l)} \to Cu(OH)_{2(s)} +2NH^+_{4(aq)}$

Reaction of Copper(II) with excess aqueous NaOH

When an excess of ammonia is added to the Copper(II) ions, you get a dark-blue color. This happens when ammonia becomes a ligand and surrounds the copper ion. The equation then becomes;

$Cu(OH)_{2(aq)} +4NH_{3(aq)} + 5H_2O_{(l)} \to [Cu(NH_3)_4(H_2O)_2]^{2+}_{(aq)} +2H_2O_{(l)} + 2OH^−_{(aq)}$

Reaction of Copper(II) with concentrated hydrochloric acid

If you add concentrated hydrochloric acid to a solution containing hexaaquacopper(II) ions, the four of the water molecules are replaced by four chloride ions.

Because the reaction is reversible, you get a mixture of colors due to both of the complex ions. The color of the tetrachlorocuprate(II) ion may also be described as olive-green or yellow. If you add water to the green solution, it returns to the blue color.

$[Cu(H_2O)_6]^{2+}_{(aq)} + Cl^-_{(aq)} \leftrightharpoons [CuCl_4(H_2O)_2]^{2-}_{(aq)} + 4H_2O_{(l)}$