Question

Preparation of Ammine Complexes of Cobalt(III)

1) Give at least two differences between the two preparations that are carried out in this lab.

2) Base on the structures of complex ions shown in this lab, draw a pictures od what you think the complex ion [Fe(NH3)6]+2 would look like.

3) Suggest another small molecule that, like water and ammonia, might make a good ligand. Remember that ligands are electron pair donors, which means that one atom must have a lone pair of electrons on it. Draw the Lewis dot structure of the molecule you think would make a good ligand.

4) How would your percent yield be affected if you didn't dry your product completely? Explain your answer.

5) Why do you think both reaction mixtures were cooled before the products were collected by filtration?

6) What was the purpose of the hydrogen peroxide used in this lab?

7) What might have happened if the temperature of the oven were set to 210 Celsius instead of 110 Celsius called for in the procedure?

Answer 1

Solution.

Part A Pentammine chlorocobalt(III) chloride-

[Co(H₂O)₆]²⁺ +5NH₃ = [Co(NH₃)₅(H₂O)]²⁺ + 5H₂O

2[Co(NH₃)₅(H₂O)]²⁺ + H₂O₂ --> 2[Co(NH₃)₅(OH)]²⁺ +2H₂O

[Co(NH₃)₅(OH)]²⁺ +3Cl^- --> [Co(NH₃)₅Cl]Cl₂(s) + 2OH^-

Part B Hexammine cobalt(III) chloride-

[Co(H2O)₆]²⁺ + 6NH₃ --> [Co(NH₃)₆]²⁺ + 6H₂O

2[Co(NH₃)₆]²⁺ + 2H₂O₂ --> 2[Co(NH₃)₆]³⁺ + 2OH^-

[Co(NH₃)₆]³⁺ +3Cl^- --> [Co(NH₃)₆]Cl₃(s)

1) The first difference - formation of pure ammonia complex in the part B ( [Co(NH₃)₆]²⁺), whereas the complex with different linands formed in part A ([Co(NH₃)₅(H₂O)]²⁺).

The second difference - in the third reaction chloride-ion acts as a ligand in part A, but it is only a counterion in part B.

2) The structure of the complex   [Fe(NH₃)₆]²⁺ is given below:

3) Other good ligands are ethylenediamine, thiocyanate (given below):

4) In this case, the percent yield will be exaggerated, because the mass will be greater.

5) Cooling enhances crystallization process.

6) Hydrogen peroxide was used as an oxidation agent, changing the oxidation state of a central atom.

7) Higher temperatures can cause complex decomposition.