Study The Shift In Equilibrium Between [Co(H2O)6]2+ And Chloride Ions By Changing The Concentration Of Either Of The Ions

Let us study the equilibrium reaction between [Co(H2O)6]2+ and chloride ions through the change in the concentration of the ions. Following displacement, takes place in the chemical reaction between [Co(H2O)6]2+ and chloride.

Study The Shift In Equilibrium-1

The equilibrium constant is given by the formula

Study The Shift In Equilibrium-2

Finding equilibrium constant for the reaction between [Co(H2O)6]2+ and chloride ions is given by

Study The Shift In Equilibrium-3

This reaction takes place in an aqueous medium 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 color.


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.

Materials Required:

60mL Acetone/alcohol, 30 mL Concentrated hydrochloric acid, Cobalt chloride 0.6000 g, 100 mL Conical flask, 100 mL Beakers – 3, 3 Burettes, 6 Test tubes, Test tube stand, Glass rod


  1. In a 100 mL conical flask take 60 mL of acetone and dissolve 0.6000 g of CoCl2 to get a blue solution.
  2. Make sure you have 5 test tubes of the same volume. Mark them as A, B, C, D, and E.
  3. Now add 3.0 mL of cobalt chloride solution into each of the test tubes.
  4. Also add 1.0 mL, 0.8 mL, 0.6 mL, 0.4 mL and 0.2 mL of acetone into the test tubes A, B, C, D, and E.
  5. Add 0.2 mL, 0.4 mL, 0.6 mL and 0.8 mL of water to test tubes B, C, D and E respectively, so that the total volume of solution in each of the test tubes is 4.0 mL.
  6. As the quantity of water increases, the mixture turns from blue to pink. Note the changes.
  7. Take 10 mL cobalt chloride solution in acetone prepared above and add 5 mL distilled water to it. A solution of pink color will be obtained.
  8. Take 1.5 mL of pink solution in five different test tubes labeled as A′ B′, C′, D′, and E′.
  9. Add 2.0 mL, 1.5 mL, 1.0 mL and 0.5 mL of water for the test tubes labelled from A′ to D′ and 0.5 mL, 1.0 mL, 1.5 mL, 2.0 mL and 2.5 mL concentrated HCl respectively in the test tubes A′ to E′ so that total volume of solution in the test tubes is 4 mL.
  10. The color gradually changes from pink to light blue with increasing amounts of hydrochloric acid. Record observations in tables given below.

Precautions to be taken during the experiment:

  • Use only distilled water.
  • Use burette or graduated pipette for adding water or solutions.
  • Handle Acetone and alcohol with care, since it is flammable


Study of equilibrium shift when adding water

Test Tube The volume of acetone added The volume of CoCl2 solution added The volume of water added Colour of mixture
A 1.0 3.0 0.0
B 0.8 3.0 0.2
C 0.6 3.0 0.4
D 0.4 3.0 0.6
E 0.2 3.0 0.8

Study of Equilibrium shift on adding Cl– ions

Test Tube The volume of concentrated HCl added The volume of aquo complex solution added The volume of water added Colour of mixture
A 0.5 1.5 2.0
B 1.0 1.5 1.5
C 1.5 1.5 1.0
D 2.0 1.5 0.5
E 2.5 1.5 0.0

Viva Voice:

  1. What happens when the temperature of the reaction mixture is increased at equilibriumIt reduces the effect of change in the reaction.
  2. Is it correct to replace aqueous solution of sodium chloride to concentrated HCl?No since Adding concentrated hydrochloric added raises the chloride ion concentration, causing the equilibrium to move to the right.
  3. Why is it necessary to keep the total volume of the solution equivalent in each test tube?To study the shift in the equilibrium and to know the exact measurement of the reactants added the test tubes are filled with the same quantity of solution.
  4. Give an example of ligand displacement reaction?
    Study The Shift In Equilibrium-4
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