If you place an aqueous solutions of CuBr2 (which is then Cu(H2O)4 2+) with solid KBr added to it in a hot water bath what do you observe and why?
DrBob's suggestion is very good. If you do not have easy access to a lab, here is a clue:
• Cu(H2O)4^2+ ions dominate at low CuBr2 concentrations and they are blue.
• CuBr4^2- ion concentrations appear at high Br- ion concentrations and their color is green.
As a follow up to GK's response, think in terms of Le Chatelier's Principle.
When you place an aqueous solution of CuBr2 (which forms Cu(H2O)4 2+) with solid KBr in a hot water bath, you would observe a color change from blue to green and eventually to yellow. This color change occurs due to the formation of different copper complexes.
To understand this observation, let's break it down step by step:
1. Initially, the aqueous solution of CuBr2 dissociates into Cu2+ and 2Br- ions in water.
CuBr2 (aq) → Cu2+ (aq) + 2Br- (aq)
2. The Cu2+ ions in the solution then coordinate with water molecules (H2O) to form hydrated copper(II) complexes, Cu(H2O)4 2+.
Cu2+ (aq) + 4H2O (l) → Cu(H2O)4 2+ (aq)
3. When solid KBr is added to the solution, the K+ ions remain in the solution while the Br- ions will react with the Cu(H2O)4 2+ complex.
Cu(H2O)4 2+ (aq) + 2Br- (aq) → CuBr2(s) + 4H2O (l)
4. The reaction between Cu(H2O)4 2+ and Br- ions leads to the formation of a precipitate of CuBr2(s), which is observed as a blue color.
5. As the temperature increases due to the hot water bath, the solubility of CuBr2 increases, causing the blue precipitate to dissolve. This leads to a change in color from blue to green as the concentration of CuBr2 in the solution increases.
6. If the temperature continues to rise, the concentration of CuBr2 increases even further, resulting in a yellow color as the solution becomes saturated with CuBr2.
In summary, the color change from blue to green to yellow is observed due to the formation of various copper complexes and the solubility change of CuBr2 with temperature.