In this experiment we demonstrate Le Chatelier’s principle in case of copper(II) chloride solution. Le Chatelier's principle in general states that if a chemical dynamic equilibrium is disturbed by changing the conditions (concentration, temperature, volume or pressure), the position of equilibrium moves to counteract the imposed change. So if more reactant is added, the equilibrium shifts to the right in order to consume that extra reactant, which results in more product.
Copper(II) chloride dihydrate is a beautiful green crystalline solid, while its diluted aqueous solution has a pale blue color:
The blue color of CuCl2 aqueous solution is specific for copper ion (Cu2+) coordinated with water molecules. Namely, a complex compound is formed in the reaction of copper(II) ion with six water molecules, which is called hexaaquacopper(II) complex:
Water molecules as ligands surround copper ion in form of an octahedron:
However, when there are not enough water molecules, as in case of more concentrated CuCl2 solution, each copper ion is surrounded with less than six water molecules, but also with one or more chloride ligands. The color of such a solution is green and the exact complex formula depends on the solution concentration (that is on how many water molecules and chloride ions are present in the solution). When the concentration of the CuCl2 solution is very high, water molecules are completely displaced with chloride ions. The structure of the complex in which all water molecules are displaced is shown on the picture bellow:
As it is obvious from the picture, there is a significant structural/geometrical difference between hexaaquacopper(II) and tetrachlorocuprate(II) complexes. These two complexes also differ in complex sphere charges: aqua complex is double positively charged, while chloro complex is double negative. Consequently, the solutions of these two complexes differ in color.
When we summarize all these facts, it can be assumed that diluting a concentrated copper(II) chloride solution with water will results in chloride ions displacement, and consequently in color changing from green to blue:
We say that copper(II) complexes are in an equilibrium that can be altered by adding water or chloride ions. When more water is added hexaaquacopper(II) complex is formed, while excess of chloride ions yields tetrachlorocuprate(II) complex.
Instead of diluting or concentrating the solution of copper(II) chloride, the equilibrium shifting can be easily achieved by successive addition of water and concentrated solution of chloride ions (we used concentrated hydrochloric acid in this experiment). This successive addition will cause alternative changes in solution color.