Cobalt(II) ions, Co^2+, can form an aquo complex Co(H_2O)_6^2+ that is pink in c
ID: 525261 • Letter: C
Question
Cobalt(II) ions, Co^2+, can form an aquo complex Co(H_2O)_6^2+ that is pink in color, and a chloro complex CoCl_4^2- that is blue. Co(H_2O)_6^2+(aq) + 4Cl^-(aq) + heat doublesidearrow CoCL_4^2+(aq) + 6 H_2O(l) pink blue Prepare the aquo complex by dissolving about 0.26 g of cobalt chloride crystals, Co(H_2O)_6Cl_2 in 10 mL water. (a) Put 4mL of the aquo complex in a 25 mL beaker. Slowly add 8 mL of concentrated HCl. Keep this solution - you will use it in the next two steps. b1ue (b) Put 2 mL of the solution from step a in a test tube. Add 2 mL of water. light Pink (c) Put another 2 mL of solution from step a in a second test tube. Add 0.1M silver nitrate solution (0.17g AgNO3 in 10 mL water) dropwise. (d) Put 4 mL of the initial pink aquo complex in a 25 mL beaker. Slowly add 4 mL of concentrated HCl. Prepare three test tubes each containing 2 mL of this solution. Heat one tube in a hot water bath, cool the second tube in an ice bath, and leave the third tube at room temperature for a standard. What is the effect of the addition of hydrochloric acid in step a? Can you rationalize the changes caused by the addition of water and silver nitrate in steps b and c? How does the heating/cooling shift the equilibrium in step d?Explanation / Answer
The following equilibrium is taking place :
Pink cobalt species + chloride ions <---> Blue cobalt species + water
When concentration of chloride ions is decreased significantly, the solution turns pink. When conc is increased significantly, color turns blue. At a middle conc. the color is purple.
(1)
Adding HCl in good amount leads to the formation of blue complex.
(2)
When water is added, the Cl- conc decreases which again produces a pink color.
When AgNO3 is added, the Cl- decreases and also Cobalt nitrate forms which is pinkish red in color,
(3)
Increasing the temp shifts the reaction towards right. So at high temp the color is blue because of complex of cobalt with Cl- ions. At low temp the color is pink because of aquo complex. At room temp eqb doesnt change and color remains purple.