Cobalt chloride equilibrium and colour changes
A student investigates the equilibrium between pink hydrated cobalt(II) ions and blue tetrachlorocobaltate(II) ions: [Co(H2O)6]2+(aq) + 4Cl−(aq) ⇌ CoCl4^2−(aq) + 6H2O(l). A pink solution is made by dissolving a small amount of cobalt(II) chloride in water. When concentrated hydrochloric acid is added dropwise, the solution turns blue. The student divides the blue mixture into two test tubes. One is placed in a hot water bath at about 70 °C and becomes a deeper blue; the other is cooled in an ice bath and turns pink. The student then adds 10 mL of distilled water to a fresh portion of the blue solution, and it turns pink again. The student is puzzled because water is the solvent, and they expected dilution to have little effect on an equilibrium.
1. Write the equilibrium expression for Kc and state what the temperature experiment shows about the enthalpy change of the forward reaction. Justify your answer.
Read the worked answer
Kc = [CoCl4^2−]/([Co(H2O)6^2+][Cl−]^4). Water is the solvent, so its concentration is effectively constant and is omitted from Kc. On heating, the mixture becomes deeper blue, meaning the equilibrium shifts to the right (products). Increasing temperature favours the endothermic direction, so the forward reaction is endothermic (ΔH > 0). Cooling shifts the equilibrium in the exothermic direction, back to pink.
2. Use Le Chatelier to explain the effect of adding concentrated HCl and then adding water.
Read the worked answer
Adding concentrated HCl increases [Cl−]. The system shifts to remove some of the added chloride, so it shifts to the right, forming more blue CoCl4^2−. Adding water dilutes all dissolved ions. Although water is not in Kc, the concentration of Cl− falls sharply, and the other ions are also diluted. The equilibrium shifts towards the side with more dissolved particles in solution. The left side has five aqueous particles (one [Co(H2O)6]2+ and four Cl−), while the right side has one aqueous particle (CoCl4^2−). The shift is therefore to the left, producing pink [Co(H2O)6]2+ and consuming blue CoCl4^2−.
3. Predict and explain what happens if a few drops of silver nitrate solution are added to the blue equilibrium mixture. Ag+ reacts with Cl− to form AgCl(s).
Read the worked answer
Adding Ag+ removes Cl− from the equilibrium: Ag+(aq) + Cl−(aq) → AgCl(s). Removing a reactant causes the equilibrium to shift towards the left to replace some Cl−. This consumes blue CoCl4^2− and forms pink [Co(H2O)6]2+, so the mixture turns pink. A pale AgCl precipitate may also be seen.