Question

the following equilibrium of a cobalt ion in solution:

Co(H2O)62+ (aq) + 4 Cl− (aq) ⇌ CoCl42- (aq) + 6 H2O (l)

A. Both an ice bath and a boiling water bath were prepared for use in the following steps.
B. In a test tube ~1 mL of Co2+ solution was added followed by drops of 12M HCl until a color change was the seen.

C. What is happening at the molecular level as HCl is added to the solution? How is the equilibrium of the solution affected by the addition?

D. Water was then added to the solution until a color change was observed.

E. How does the reaction respond to the addition of water?

F. In a new test tube half filled with fresh Co2+ solution, drops of 12M HCl were added until a color somewhere between pink and blue was observed. This solution was then placed in boiling water. Which way did the reaction shift when heat was added?

G. The test tube was removed from the boiling water and placed it in an ice water bath. Which way did the reaction shift when heat was removed?

H. Do the results from steps G & H indicate that the forward reaction is endothermic or exothermic? Explain.

Answer 1

The given equilibrium reaction,

(Pink) Co(H₂O)₆²⁺ (aq) + 4 Cl⁻ (aq) (Blue) CoCl₄^2- (aq) + 6 H₂O (l)

This equilibrium can be disturbed by changing the concentrations of Cl^- ions or H₂O or by altering the temperature according to Le-Chatelier's principle.

C. The HCl dissociates to produce Cl^- and H⁺ ions. Upon adding 12 M HCl to a Co²⁺ solution the Cl^- concentration in solution increases. An increase in the concentration of reactants cause the equilibrium to shift towards the right side i.e towards the products thus increasing CoCl₄^2- concentration causing the colour of solution to change to blue.

E. Upon addition of H₂O the overall concentration of H₂O increases on the product side which cause the equilibrium to shift towards the left side i.e towards the reactants thus increasing [Co(H₂O)₆]²⁺ concentration causing the colour of solution to change to pink.

F. Upon placing the reaction mixture in boiling water bath, i.e on addition of heat it was seen that the solution turned blue i.e the concentration of CoCl₄^2- ions increased in solution. We can say that the equilibrium has shifted towards the right i.e towards the products on providing heat.

G. Upon placing the reaction mixture in ice water bath i.e on removal of heat it was seen that the solution turned pink i.e the concentration of [Co(H₂O)₆]²⁺ ions increased in solution. We can say that the equilibrium has shifted towards the left i.e towards the reactants on removing heat.

H. The results from F and G indicate that the reaction is endothermic. Since the reaction is endothermic hence on adding heat, the equilibrium shifts in the forward direction i.e towards the products thus increasing the concentration of CoCl₄^2- and turning the solution blue.

Also removal of heat shifts an endothermic reaction in backward direction, increasing the concentration of [Co(H₂O)₆]²⁺ and turning the solution pink.