Solutions and precipitates of different color are obtained when blue solution is mixed with colorless solutions.
name: Copper ions combine easily with different ligands to form coordination complexes. Precipitates of different color are produced in reactions of copper sulfate with sodium salts.
[Cu(H2O)6]2+ + 4NH3 → [Cu(H2O)2(NH3)4]2+ + 4H2O (blue)
[Cu(H2O)6]2+ + 2C2H8N2 → [Cu(H2O)2(C2H8N2)2]2+ + 4H2O (dark blue)
[Cu(H2O)6]2+ + C10H12N2O84- → [Cu(C10H12N2O8)]2- + 6H2O (bright blue)
[Cu(H2O)6]2+ + 4Br- → [Cu(Br)4]2- + 6H2O (violet)
[Cu(H2O)6]2+ + 4Cl- → [Cu(Cl)4]2- + 6H2O (green)
CuSO4 + 2NaOH → Na2SO4 + Cu(OH)2 (blue precipitate)
CuSO4 + Na2S → Na2SO4 + CuS2 (dark-brown precipitate)
CuSO4 + Na2CO3 → Na2SO4 + CuCO3 (light-blue precipitate)
CuSO4 + 2C7H5NaO2 → Na2SO4 + Cu(C7H5O2)2 (milky-blue precipitate)
Cu(OH)2 + NaClO + NaCl → CuO + NaCl + H2O (black precipitate)
The first part of this experiment is based on a very important phenomenon: the ability of metals to form chemical structures called coordination compounds. In such structures the central metal atom is bounded with one or more ligands. A ligand is an atom or a molecule which contains one or more electron pairs that can be shared with a metal ion. Copper ions are well known to combine easily with different ligands to form coordination complexes. These compounds have very symmetrical, octahedral geometry. Depending on the ligand type and coordination number (the number of neighbors near a metal) complexes have different colors. The water solutions in the beakers in the considered experiment contain various ligand ions which form well defined colored complexes.
Glass 1. Cooper sulfate dissociates in the water solution and gives hydrated Cu2+ ions (Cu(H2O)2+) and SO42- ions:
CuSO4 + 6H2O → Cu(H2O)62+ + SO42-(light blue)
When it comes into contact with solutions in the glasses, ligand ions (molecules) replace water molecules from the Cu coordination shell.
Stability of complex compounds depends on the number of bonds between the metal ion and ligand. The more chelating ligands give more stable complexes. In the row NH3 → C2H8N2 → C10H16N2O8 the chelating ability increases and so each next substance added replaces previously bounded molecules.
The second part of this experiment is devoted to the ability of cooper ions to form precipitates. It easily reacts with NaOH, Na2S, Na2CO3, C7H5NaO2, NaOH mixed with NaClO producing differently colored insoluble substances.