Question

Tetrachlorocobalt (II) ion reacts with water to form hexaaquacobalt (II) ion according to the following net ionic equation:
[CoCl₄]^2-(aq, blue) + 6 H₂O(l) <=> [Co(H₂O)₆]²⁺(aq, pink) + 4 Cl^-(aq)

(A) A student doing this experiment placed a test tube containing a blue equilibrium mixture in an ice-water bath. The solution turned pink. When the student removed the test tube from the ice-water bath and placed it in the hot water bath, the solution turned blue. Is the forward reaction exothermic or endothermic? Give complete explanation.

(B) What change do you observe when you add concentrated hydrochloric acid, HCl, solution. HCl undergoes dissociation according the following equation: Give complete explanation.

HCl(aq) → H⁺(aq) + Cl^-(aq)

(C) What experimental evidence do you have that the equilibrium is affected by the addition of water. Give complete explanation.

Answer 1

(A) The forward reaction is favored when the test tube is cooled. So, when Co²⁺and water combine
to form pink [Co (6H2O)]²⁺ ion, heat will be released (since heat is released during cooling process)
Hence it will be exothermic reaction.

(B) Adding HCl raises the Cl^- ion concentration, causing the equilibrium to move towards left according
to Le Chatelier's principle. So the color change would be reversible i.e) pink will turn into blue.

(C)  If you add water it will lower the Cl^- ion concentration, hence the equilibrium will move towards
   opposite direction. So, instead the equilibrium will head towards right and color will change from
blue to pink.