A U-shaped glass tube shown on the video contains aqueous solution of cobalt(II) chloride that is pink in color at room temperature. When one arm of the glass tube is heated, the solution turns blue. The pink color can be reestablished by cooling down the solution and this phenomenon is explained by Le Chatelier’s principle.
Aqueous cobalt(II) chloride solution has a pink color, because six water molecules surround one cobalt(II) ion, thus forming octahedral hexaaquacobalt(II) complex, [Co(H2O)6]2+:
This [CoCl4]2- complex is doubly negatively charged and four chloride ions are arranged around cobalt(II) ion in form of a regular tetrahedron:
These two complexes have different colors because they have different geometries, as well as different ligands (water vs. chloride ions) around cobalt ion.
The above equilibrium reaction is weakly endothermic, DrHo > 0 which means that heat is absorbed during this substitution process. According to the Le Chatelier’s principle, 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. If we apply the Le Chatelier’s principle to this particular case, we can conclude that heating the pink hexaaquacobalt(II) complex will shift the equilibrium to the right, in order to absorb this energy. During this equilibrium shifting blue tetrachlorocobaltate(II) complex is formed. Contrary, cooling down the hot solution leads to the blue complex decomposition and formation of pink hexaaquacobalt(II) complex – the reaction is reversible.