Coordination Compound Lab

Preparation and Analysis of Tetraamminecopper(II) sulfate monohydrate Introduction: The class of substances referred to as coordination compounds generally contain a central metal atom to which a fixed number of molecules or ions (called ligands) are coordinately covalently bonded in a characteristic geometry. You will be preparing a coordination compound containing copper(II) and you will perform simple tests to characterize the compound. Background Info: Coordination complex compounds play a vital role in our everyday lives. Fr example, the molecule heme in the oxygen-bearing protein hemoglobin contains coordinated iron atoms. Chlorophyll, the molecule that enables plants to carry on photosynthesis, is a coordination compound of magnesium. The name “tetraamminecopper(II)” indicates that four ammonia molecules are covalently bonded to the copper(II) ion. The formula is written [Cu(NH3)4]2+. The square brackets signify that the four ammonia molecules and the Cu2+ ion act as a group, forming a +2 ion. The ion will form part of the solid crystal [Cu(NH3)4]SO4•H2O, in which there is one molecule of water forming a hydrated species. In aqueous solutions, Cu2+ ions will bond to four water molecules in a square planar geometry. The ion is a light blue color. The water molecules can be displaced by ammonia molecules, which are stronger Lewis bases than water. The appearance of the intense dark blueviolet color of the [Cu(NH3)4]2+ ion indicates that the reaction has occurred. The equation below shows how you will synthesize the compound in this lab: CuSO4•H2O + 4NH3 → [Cu(NH3)4]SO4•H2O + 4H2O Equation 1 Use the following to calculate the percent yield of the compound produced: % yield = actual yield / theoretical yield x 100 Equation 2 Materials: Hydrochloric acid, HCl, 6M Ammonia, NH3, 6M, 5 mL Copper(II) sulfate pentahydrate, CuSO4•5H2O, 3-5 g Ethyl alcohol, CH3CH2OH, 95%, 50 mL Buchner funnel, filter flask, and hose Ice bath (crushed ice in a 250 L beaker with water) Graduated cylinder, 25 mL Rubber/plastic policeman

Weighing dishes Water, distilled Analytical balance Beakers, 100 mL Filter paper (buchner) Stirring rod Hot plate pH paper

Procedure: Part 1: Preparation of the tetraamminecopper(II) sulfate monohydrate crystals 1. Weigh out approximately 1 g of the copper(II) sulfate pentahydrate and record the EXACT mass. 2. Dissolve the copper salt in approximately 10 mL of distilled water in a beaker or flask. Stir thoroughly to make sure that all the copper salt has dissolved before proceeding. Record the color of the solution at this point. Heat the beaker with your solution gently on low/medium heat to speed up the dissolving process. Be sure not to burn the solid onto the bottom of the beaker. Stir often to prevent this. Cool the beaker back to room temperature by placing it in an ice bath (using ice and tap water) before going on to the next step. (note: tetraamminecopper(II) sulfate decomposes in hot water; do not overheat!) 3. Transfer the copper solution to the fume hood and, with constant stirring, slowly add approximately 5 mL of concentrated ammonia solution (USE CAUTION!). The first portion of ammonia added will cause a light blue precipitate of copper(II) hydroxide to form. But upon your adding more ammonia, the precipitate will dissolve as the ammonia complex forms. Record the color of the mixture after all 5 mL of the ammonia has been added (that is, the color of the tetraamminecopper(II) complex) 4. To decrease the solubility of the tetraamminecopper(II) complex, add approximately 10 mL of ethyl alcohol with stirring. A deep blue solid should precipitate.

5. Allow the solid precipitate to stand for 5-10 minutes. While you are waiting, obtain a piece of buchner funnel filter paper and a dry and clean weigh boat and record their combined mass (be sure they are both dry when you mass them together). 6. Set up the buchner funnel (as directed by instructor) and then filter the precipitate under suction. Use a distilled water bottle to rinse all of the solid into the funnel. 7. While it is on the filtering funnel, wash the precipitate with approximately two 5 mL portions of ethyl alcohol, stir, and apply suction to the precipitate until it appears dry (although it will still be damp at this point). 8. Use a plastic policeman to transfer all of your solid and the buchner filter paper to your pre-weighed, dry and clean weigh boat. 9. Allow the precipitate to air dry overnight before weighing it. 10. Weigh the dried precipitate and record the yield. Part 2: Analysis of the tetraamminecopper(II) sulfate monohydrate crystals 11. Dissolve a small amount of the product in a few milliliters of distilled water. In the fume hood, add concentrated HCl dropwise (USE CAUTION!) until a color change is evident. Record the change in color. Ammonia molecules have a stronger affinity (attraction) for protons (H+) than for copper ions. When HCl is added to the copper complex, the ammonia molecules are converted to ammonium ions (NH4+), which no longer have an unshared pair of electrons and which cannot bond to the copper(II) ion. 12. Heat a very small amount of the copper complex in a test tube in a burner flame. Hold a piece of moist pH test paper near the mouth of the test tube while it is being heated. When the copper complex is heated, the ammonia molecules are driven out of it. Record the change in color of the pH test paper. Results: • Using the mass of copper(II) sulfate pentahydrate that you started with (recorded in procedure step #1) and Equation 1, calculate the theoretical yield of the product, tetraamminecopper(II) sulfate monohydrate, [Cu(NH3)4]SO4•H2O. (Note: assume that the ammonia was in excess) • Using Equation 2, calculate the percentage yield of the product, tetraamminecopper(II) sulfate monohydrate. Questions: 1. Explain why the color change of the pH paper that you observed. 2. Which is the limiting reactant in this experiment? Explain. 3. The substance [Cu(NH3)4]SO4•H2O contains both ionic and covalent bonds. Which parts of the compound are held together by each type of bond?

Lab Notebook: Include the following: title, purpose, data Lab report: Include EVERYTHING: title, purpose, materials, procedure, data, results (calculations), conclusions, questions