Chemistry 111 Lab Synthesis of a Copper Complex Page H 1 SYNTHESIS AND ANALYSIS OF A COORDINATION COMPOUND OF COPPER In this experiment you will synthesize a compound by adding NH3 to a concentrated aqueous solution of copper sulfate The blue CuSO4 solution will turn a still deeper blue and a mass of small deep blue to violet crystals will form as ethyl alcohol is added On the basis of the analysis of this solid for Cu2 SO42 and NH3 you will be able to propose a formula for the compound THEORY Four principal species are present initially in the reaction mixture copper II ions actually Cu H2O 62 ions ammonia molecules NH3 sulfate ions SO42 and water The product of the synthesis is therefore presumed to be formed by the reaction of two or more of these species Ethanol is also present but it is an indirect participant in the reaction In aqueous solutions ethanol which is miscible with water but of lower dielectric constant decreases the solubility of ionic compounds The marked color change that occurs in the reaction is an important clue to the nature of the product The product is analyzed for copper II ions sulfate ions and ammonia molecules Water is determined as the mass of a sample of the compound that is not accounted for as one of these three species The analyses to be performed in this experiment are quantitative and are of three types gravimetric volumetric and spectrophotometric The gravimetric analysis is for sulfate ions the volumetric analysis is for ammonia molecules and the spectrophotometric analysis is for copper ions With careful attention to detail and techniques one can obtain excellent results for each part of the analysis Cu H2O 62 Revised June 2003 Co NH3 63 The general formula of the unknown compound is Cux NH3 y SO4 z a H2O You performed gravimetric volumetric and spectrophotometric analyses in earlier experiments Complex ions containing bound water and ammonia The structure of Cu2 ion in water is on the left Note that the water molecules are bound through oxygen atoms to the copper ion In this experiment you will make a compound that has NH3 molecules bound to the copper II ion through N atoms similar to the complex ion Co NH3 6 3 Page H 2 Chemistry 111 Lab Synthesis of a Copper Complex PROCEDURE A Synthesis of the Compound 1 Weigh out 10 0 grams of copper sulfate pentahydrate CuSO4 5H2O on a triple beam balance and place the crystals in a 250 mL beaker 2 Add 10 15 mL of water to the solid and then in a hood add 20 mL of 15 M NH3 concentrated ammonia Stir to dissolve the crystals as ascertained by holding the beaker up to the light 3 Over a period of 1 minute slowly add 20 mL of 95 ethanol ethyl alcohol to the solution stir and cool to room temperature 4 Meanwhile prepare 30 mL of a solution from 15 mL each of concentrated ammonia and 95 ethanol Cover all your solutions containing ammonia with watch glasses to prevent the fumes of NH3 from saturating the lab atmosphere and return to your desk 5 Set up an apparatus for vacuum filtration Moisten the filter paper and turn on the aspirator Carefully filter the slurry of crystals that has formed in the copper containing solution and suck off all the solution In the event that a significant amount of crystalline product remains in the beaker you should use the filtrate from the filter flask to wash the crystals onto the filter paper To prevent backup of tap water into the filter flask which would render the filtrate useless as a wash pull the hose off the aspirator while the water is still running Remove the filter funnel and pour the filtrate from the filter flask back into the beaker that contains the crystals Reassemble the filter apparatus and collect the remaining product on the filter paper CAUTION acetone is flammable Turn off the aspirator and carefully pour 10 mL of the ammonia ethanol solution onto the crystals Break up all lumps of solid to permit the liquid to penetrate the mass completely and then turn on the aspirator to suck off the liquid Repeat the washing procedure twice with additional 10 mL portions of ammonia ethanol Next wash twice with 10 mL portions of 95 ethanol and finally with two 10 mL portions of acetone breaking up the mass of crystals with a spatula in each step before turning on the aspirator At this point your crystals should appear nearly dry that is not moistened with liquid to any great extent If you feel they could be dried more add additional amounts of acetone as described above To remove the last traces of moisture and other solvents from your solid draw air through the crystals for at least 5 minutes using your spatula to break up any remaining lumps 6 Put the crystals in a large test tube or small beaker lightly cover and leave them in your desk to dry thoroughly until the next laboratory period 7 When you come to the laboratory period following the one in which the crystals were prepared inspect them to insure they are dry and then weigh the entire sample to the nearest 0 1 gram Record the mass on the report form and then go on to the next portion of the experiment Revised June 2005 Chemistry 111 Lab Synthesis of a Copper Complex Page H 3 ANALYSIS OF THE COMPOUND A Gravimetric Analysis for Sulfate Ion SO421 Weigh to the nearest milligram about 0 9 1 1 gram of your compound after making absolutely sure it is dry 2 Dissolve the solid compound in 10 mL of 6 M nitric acid HNO3 The reaction occurring at this point is Cux NH3 y SO4 z a H2O y H x Cu2 y NH4 z SO42 a H2O 3 Obtain 5 6 mL of 1 M lead acetate Pb C2H3O2 2 and add it dropwise to the acid solution until precipitation is complete The reaction now occurring is Pb2 aq SO42 aq PbSO4 s Use the following procedure to check for completeness of precipitation First allow all the white precipitate to settle to the bottom of the beaker Using your eyedropper allow one drop of lead acetate solution to run down the side of the beaker into the light blue solution If precipitation of the SO42 ion is not yet complete you will see a white precipitate of PbSO4 form just as the drop slides down the beaker wall and into the solution If you do not observe a white solid forming at this point then precipitation is complete and you can proceed 4 Once you have decided that precipitation of the PbSO4 is complete weigh a piece of filter paper to the nearest milligram Assemble the apparatus for gravity filtration After properly folding the paper into a cone place it in the funnel moisten it with a little water and adjust it so that it fits the