Chemical Concepts Demonstrated: Complex formation and solubility chemistry of copper
Demonstration:
The following solutions are added to a beaker containing 0.5M Cu(NO3)2 solution.
| Solution | Chemical Equation | Observations |
| NaBr | Cu 2+ (aq) + 4 Br -(aq) <=> CuBr4 2-(aq) | An emerald green solution of the CuBr4 2- complex ion is produced. |
| H2C2O4 | CuBr4 2-(aq) + H2C2O4 (aq) <=> CuC2O4 (s) + 2 H+ (aq) + 4 Br -(aq) | A light blue copper (II) oxalate precipitate is formed. |
| Na2CO3 | CuC2O4 (s) + CO3 2- (aq) <=> CuCO3 (s) + C2O4 2- (aq) | A dark blue gelatinous precipitate is produced. |
| NH3 | CuCO3 (s) + 4 NH3 (aq) <=> Cu(NH3)4 2+ (aq) + CO3 2-(aq) | A deep royal blue solution is formed. |
| HOAc | Cu(NH3)4 2+ (aq) + 4 CH3CO2H (aq) <=> Cu(CH3CO2)2 (s) + 4 NH4 +(aq) + 2 CH3CO2 - (aq) |
A teal blue precipitate is formed. |
| NaOH | Cu(CH3CO2)2 (s) + 2 OH -(aq) <=> Cu(OH)2 (s) + 2 CH3CO2 - (aq) | The precipitate changes color from teal to light blue. |
| HCl | Cu(OH)2 (s) + 2 H + (aq) + 4 Cl -(aq) <=> CuCl4 2- (aq) + 2 H2O (l) | A light green solution is formed. |
| K4Fe(CN)6 | 2 CuCl4 2- (aq) + Fe(CN)6 4-(aq) <=> Cu2[Fe(CN)6] (s) + 8 Cl -(aq) | A red-brown precipitate is formed. |
| Na2EDTA in NaOH | Cu2[Fe(CN)6] (s) + 2 EDTA 4-(aq) <=> 2 Cu(EDTA) 2-(aq) + Fe(CN)6 4-(aq) | A black precipitate is produced that ultimately dissolves to give a midnight blue solution. |
| Na2S | Cu(EDTA) 2-(aq) + S 2- (aq) <=> CuS + EDTA 4-(aq) | A black precipitate is formed. |