Study the Shift in Equilibrium between [Co(H₂O)₆] ²⁺ and Cl⁻ ions


To understand the process of shift in equilibrium between [Co (H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.


The Theory

What is Chemical equilibrium?

Chemical equilibrium refers to the state of a system in which the concentration of the reactant and the concentration of the products do not change with time and the system does not display any further change in properties.

Let's learn about the shift in equilibrium between [Co (H2O)6] 2+ and Cl ions 

The equilibrium reaction occurs between hexa aqua cobalt (II) complex ion [Co (H2O)6]2+ and chloride Cl ions through the change in the concentration to create the formation of tetra aqua cobalt (II) chloride complex ion, [CoCl4]2-. The following equation represents the reverse reaction and dissociation between tetra aqua cobalt (II) chloride complex and hexa aqua cobalt (II) complex ion and chloride ion with a colour change.  

[Co(H2O)6]2+ + 4Cl- ⇌ [[CoCl4] 2-] + 6H2O

Pink                              Blue              



Common factors influencing the equilibrium of the reverse reaction and dissociation between [CoCl4]2- and [Co (H2O)6] 2+ and Cl  ions 

Concentration of reactants and products: 

According to the Le Chatelier’s principle, the increasing amount of the concentration of hexa aqua cobalt (II) complex [Co (H2O)6]2+ and chloride Clion, reflects the equilibrium to the right to form more tetra aqua cobalt (II) chloride complex [CoCl4]2- and water., Similarly, with the increasing amount of concentration of [CoCl₄]²⁻ or water, the equilibrium will shift to the left to consume [Co (H2O)6]2+ and chloride Cl ions. 

Complex formation constants:  

The equilibrium constant (K) is influenced by the stability of the complex ions involved throughout the reaction.  

Consider a general reaction for the equilibrium constant A + B → C + D as,

The expression for the equilibrium constant for the reaction between [Co(H2O)6]2+ and chloride ions is

Hence, the reaction takes place in an aqueous medium where the concentration of H2O remains constant. 

At equilibrium either [Co(H2O)6] 2+ ion or Cl – ions concentration is increased, and this would result in an increase in [CoCl4] 2– ion concentration thus, maintaining the value of K as constant. There is a shift in the equilibrium in the forward direction along with the change in colour. 


Learning Outcomes

  • Students understand the term ‘chemical equilibrium’ through the experiment. 
  • Students understand the effect of change in concentration on the equilibrium of a reaction. 
  • Students understand the shift in equilibrium position with respect to the concentration change. 
  • Students are better able to perform the experiment in the real lab after experiencing it in the virtual lab.