Question

Explain why the addition of NH₃ or HNO₃ to a saturated solution of Cu(OH)₂ in contact with solid Cu(OH)₂ increases the solubility of the solid.

a) NH₃ and HNO₃ react with Cu(OH)₂ to form a more soluble complex.
b) NH₃ and HNO₃ decrease the temperature, promoting dissolution.
c) NH₃ and HNO₃ react with Cu(OH)₂ to form an insoluble precipitate.
d) NH₃ and HNO₃ increase the pressure, promoting dissolution.

Answer 1

NH₃ and HNO₃ increase the solubility of Cu(OH)₂ by forming more soluble compounds, [Cu(NH₃)₄]²⁺ and Cu(NO₃)₂, respectively, shifting the equilibrium to favor dissolution.

Step-by-step explanation:

The addition of NH₃ or HNO₃ to a saturated solution of Cu(OH)₂ in contact with solid Cu(OH)₂ increases the solubility of the solid because both substances interact with the Cu(OH)₂ to form more soluble products. For NH₃, it forms a complex ion with Cu²⁺, namely [Cu(NH₃)₄]²⁺(aq), which is highly soluble in water.

This reaction pulls the equilibrium towards the right, allowing more Cu(OH)₂ to dissolve in order to form more of the complex ion. In the case of HNO₃, it reacts with Cu(OH)₂ to form soluble copper(II) nitrate, Cu(NO₃)₂, thereby also increasing the solubility of Cu(OH)₂ by converting it into the more soluble salt.