Question

Two test tubes are filled with 3 mL of 1 M aqueous colbalt(II) chloride solution. 12 M HCl is added to each test tube until the solutions turn purple. (The purple color indicates an equilibrium mixture of Co(OH₂)₆²⁺ and CoCl₄^2- ions).

Test tube 1 is placed in an ice bath, where it turns pink. Test tube 2 is places in a boiling water bath, where it turns blue.

1. Based on these observations, is this reaction endothermic or exothermic? Justify your answer with an explanaton.

Answer 1

The element cobalt can form compounds in two different oxidation states, +2 and +3. The +2 state is more common. The ion Co2+ (aq) is pink. Other compounds of cobalt(II), which include both anhydrous Co2+ and complex ions, are commonly blue.

If an aqueous solution contains both cobalt(II) and chloride ions, the blue ion CoCl42- forms, in equilibrium with the pink Co2+ (aq) ion.

CoCl42- (aq) <===========> Co2+ (aq) + 4Cl1-(aq)
Blue Pink

At relatively low concentrations of chloride, the equilibrium lies far to the right, and the solution is pink. If there is a large concentration of excess chloride, the equilibrium tends to the left, and the solution tends to be blue.

The equilibrium is sensitive to temperature as well as to concentration of solutes. At lower temperatures, the equilibrium tends to lie to the right, that is, to be more pink; at higher temperatures, it lies to the left and appears more blue.

Therefore in test tube 1 is exothermic

In test tube 2 is endothermic