AP Chemistry Unit 3

Lab 3-2 Write Up Lab

Coordination Compounds

Skill Set

Use of “Dropwise” titration

Use of Filtration to trap precipitate (ppt)

Production of glassware for labs

Use of oxidizing and reducing flame to anneal glass

Use of Slow oxidation to change valence

Coordination compounds typically refer to a class of compounds that meet the following conditions: 1- they involve transition metals, 2-additional materials (ligands) bond to the transition metals to form a more complex molecule in specifically allowed amounts, 3-the bonding mechanism is always coordinate covalent (bonding electrons come from only 1 atom). These compounds are particularly important to biological applications (hemoglobin in your blood and porphyrin in chlorophyll are 2 examples). In this lab we will attempt to determine what process most easily produces these materials as we prepare some of these substances and observe their properties.

Procedure part 1

Weigh out about 1 gram of copper (II) sulfate. Dissolve in 10 ml water, stirring constantly to insure full solution.

Dropwise and with stirring, add 5 ml of concentrated ammonium hydroxide. Note: ammonium hydroxide is a mucuous membrane irritant. Excessive addition of NH₄OH will create choking fumes. Be Spartan in the addition and handling of this chemical. The first precipitate (ppt) will be a light blue Cu(OH)₂. Addition of more NH₄OH will dissolve this precipitate and eventually produce the tetraammine copper (II) sulfate.

To bring about precipitation add 10 ml of ethanol with stirring. Filter the precipitate, dry and weigh. From this data determine actual yield, theoretical yield, and percent yield.

Procedure part 2

Obtain a 10 cm piece of glass tubing, by notching and breaking a sample from stock. Fire polish each end and allow to cool between each step by placing on a wire mesh. Note: Intense heat from the glass can damage skin and the composite surface of the desks. Exercise caution as you cannot look at a piece of glass and tell if it is hot or not. Roll the glass at the top of an inner blue cone until soft, remove from flame and quickly pull apart into 2 droppers. Allow to cool.

Cobalt is another substance that can form ligand complexes. The complex to be formed in this case is a hexaammine cobalt (III) complex. Weigh 4.0 grams of Co (II) chloride and place in a 250 ml beaker. Add 5 ml of water to dissolve. Weigh out 3.0 grams of ammonium chloride and add to the system. Add a pinch of charcoal to the system.

Slowly add 10 ml of concentrated NH₄OH . The solution should be red at this point. Attach a hose to the air inlet and place a glass dropper in the other end. Bubble slowly for ½ to 1 hr (bubbling the O₂ drive the valence of any Co⁺² up to ⁺³). Collect the yellow brown precipitate by filtration.

Analysis

1. Write balanced equations for each process

2. Determine theoretical yield in each case

3. Determine the % yield in each case

4. Based on a 1.0 gram of CuSO4 being used, what is the theoretical yield of tetraammine copper (II) sulfate possible

5. Why might ligand compounds be more brightly colored than other substances (hint-what electrons are involved in bonding?)

6. Based on a 4.0 gram sample of cobalt (II) chloride being used, what is the theoretical amount of production possible

7. Describe the possible shapes of a tetraammine copper (II) and hexaammine cobalt (III) ligand system.

8. Treating with concentrated NH₄OH is the most common method of obtaining ligand substances but if the systems were instead treated with concentrated HCl, or HCN what might the possible ligands be?

9. Name/give structure to the following
1. K₃Fe(CN)₆
2. K₄Fe(CN)₆
3. Co(H₂O)₆Cl₂
4. Co(NH₃)₆Cl₃
5. Cu₃(OH)₄SO₄
6. Tetraamminecloronitrocobalt(III) chloride
7. Pentaammineclorocobalt (III)chloride
8. Triamminebromoplatinium(II) nitrate
9. Potassium hexaflorocobaltrate(III)
10. Sodium hexahydroxylferrate (III)