Properties of Systems in Chemical Equilibrium – Le Chatelier’s Principle Complex Ion Equilibria – Color of CoCl2•6H2O Solution in 1.2M HCl Introduction: Color: When concentrated CoCl2•6H2O solution is diluted, the color changes from blue to pink. Objective: To examine the color change that occurs when CoCl2•6H2O solution is diluted. Procedure: 1. Prepare a diluted solution of CoCl2•6H2O. 2. Observe the color change that occurs when the diluted solution is prepared. Discussion: The color change observed when CoCl2•6H2O solution is diluted is due to the shift in equilibrium caused by the addition of water. The reaction can be represented as follows: CoCl2•6H2O (blue) + H2O ⇌ Co(H2O)62+ (pink) The addition of water causes the equilibrium to shift to the right, resulting in the formation of more pink Co(H2O)62+ ions. Conclusion: The color change of CoCl2•6H2O solution when diluted is a result of the shift in equilibrium caused by the addition of water.
The Correct Answer and Explanation is:
Correct Answer:
When CoCl₂·6H₂O is diluted with water, the color of the solution changes from blue to pink. This change is due to a shift in chemical equilibrium according to Le Chatelier’s Principle. The blue color corresponds to the complex ion [CoCl₄]²⁻, while the pink color corresponds to the hexaaquacobalt(II) ion, [Co(H₂O)₆]²⁺.
Explanation
This experiment explores how equilibrium systems respond to changes in concentration, specifically the equilibrium involving cobalt(II) chloride hexahydrate (CoCl₂·6H₂O). When this salt is dissolved in concentrated hydrochloric acid (HCl), the solution appears blue due to the formation of the tetrachlorocobaltate(II) ion, [CoCl₄]²⁻, which dominates under high chloride ion concentration: [Co(H₂O)₆]2+(pink)+4Cl−⇌[CoCl₄]2−(blue)+6H2O\text{[Co(H₂O)₆]}^{2+} (pink) + 4Cl⁻ ⇌ \text{[CoCl₄]}^{2-} (blue) + 6H₂O
However, upon dilution with water, the color of the solution shifts from blue to pink. This color change indicates a shift in the equilibrium toward the formation of the [Co(H₂O)₆]²⁺ complex, the pink species. According to Le Chatelier’s Principle, when the concentration of one of the reactants or products in a system at equilibrium is changed, the system will shift its equilibrium position to counteract that change.
In this case, adding water decreases the chloride ion concentration and increases the concentration of water (a reactant). This disturbs the equilibrium and causes it to shift to the left, favoring the formation of the pink hexaaquacobalt(II) complex.
This color change provides a vivid and practical demonstration of how equilibria respond to changes in concentration. The Co²⁺ ion forms different colored complexes with water and chloride ions, which makes it an ideal system for observing equilibrium shifts visually. This experiment helps students understand the dynamic nature of chemical equilibrium and how observable properties like color can reflect changes at the molecular level.
