B. Nickel Ions in Aqueous Solution
Ni^{2+} ions in water covalently bond to water molecules, forming a complex ion, [Ni(H_2O)_6]^{2+}, that gives the solution a green color. If ammonia is then added to the solution, ammonia molecules displace the water molecules bonded to the nickel (II) ion and bond to them in their place. The new complex ion formed is [Ni(NH_3)_6]^{2+}; it gives the solution a pale violet color. In this solution, the following equilibrium exists:
[\text{Ni}(\text{H}_2\text{O})_6]^{2+} (\text{aq}) + 6 \text{NH}_3 (\text{aq}) \leftrightarrow [\text{Ni}(\text{NH}_3)_6]^{2+} (\text{aq}) + 6 \text{H}_2\text{O} (\text{l})
(blue-green to pale violet)
1. Add about 1 mL (20 drops) of 0.1 M NiCl_2 to a small test tube. What is the color of this solution?
2. Add 2 drops of 6M ammonium hydroxide to the test tube. The solution should change to pale violet. Some solid Ni(OH)_2 might initially form.
3. If HCl were added to the test tube, what would you expect to see? Explain your answer using Le Chatelier’s Principle and the chemical natures of HCl and NH_3.
4. Test the prediction you made in #3 by adding 1 drop 6M HCl to the test tube. Record your observations. Were they consistent with your prediction?
5. What would you add to the solution in the test tube to cause its color to turn pale violet again? Explain, using Le Chatelier’s Principle and the chemical nature of the reagent added and of the substances in the solution.
6. Test the prediction you made in #5 by adding 2 drops of 6M reagent that you selected. Record your observations. Was your prediction correct?