In this experiment, the main goal is to study the formation and properties of a coordination compound. A coordination compound is composed of a central atom bound to multiple groups called ligands. There is a wide variety of possible ligands that can bond directly to the central atom, but only two common ones, NH3 and CO3, are to be studied in this particular experiment. In the formation of carbonatotetraamminecobalt (III) nitrate, denoted [Co(NH3)4CO3]NO3, the transition metal which serves as the central atom is cobalt.
Its first coordination sphere, or everything directly bonded to the cobalt, is four nitrogen atoms and two oxygen atoms. This coordination sphere is represented in the chemical formula with brackets. Because cobalt is involved in six bonds, its coordination number is six, and the structure around the cobalt is octahedral. It is apparent where the four nitrogen atoms come from, since there are three ammonia molecules in the formed complex. However, there is only one carbonate molecule.
This ligand is called a bidentate ligand because two of its atoms were bonded to the metal.
Thus, the ammonia ligands are all monodentate ligands because only one of their atoms was bonded to the metal. Because the first coordination sphere does not break apart in aqueous solution, it is considered a separate ion like NO3. The charge of the cobalt octahedron, called a complex ion, is +1 and the charge of the nitrate is -1. It can therefore be determined that cobalt had a charge of +3. Since the cobalt nitrate that will be used in the synthesis of the coordination compound has a charge of +2, the cobalt loses an electron and is oxidized.
Hydrogen peroxide is used as the oxidizing agent, so it is reduced to water. The formation of the coordination compound is a redox reaction, and the equation can be balanced using half-equations. The balanced equation is shown below: 2Co(NO3)2 + 6NH3 + 2(NH4)2CO3 + H2O2 –> 2[Co(NH3)4CO3]NO3 + 2NH4NO3 + 2H2O Theoretically, carbonatotetraamminecobalt (III) nitrate is formed, but further analysis of the coordination compound will be necessary to validate this assertion. The complex will be characterized in terms of absorbance, conductivity, magnetic susceptibility, and infrared spectroscopy. The Beer’s Law equation A = ?
lC can be used to calculate the mass of carbonatotetraamminecobalt (III) nitrate needed to prepare a solution of which the absorbance reading will be 0. Once the absorbance spectrum is obtained, Beer’s Law can also be used to calculate the molar absorptivity or extinction coefficient by substituting lambda max for A. In this equation, A is absorbance, ? describes how well the compound absorbs light, l is the path length, and C is the concentration. Measuring the conductivity of the cobalt coordination compound is necessary to determine how many ions are present in one formula unit of the complex.
The expected conclusion is that two ions will dissociate, resulting in a conductance of 118-131 i?? seimens. Determining the magnetic susceptibility of the cobalt complex is important in deciding whether it is paramagnetic or diamagnetic. If the complex is paramagnetic, this means that the reading will be positive and that unpaired electrons are present. If the complex is diamagnetic, the reading will be negative and no unpaired electrons are present. An infrared spectrum is a plot of the absorption intensities of different wavelengths of infrared light.
Each compound has unique bonds, types of bonds, molecular weight, and other factors giving it a “fingerprint. ” The infrared spectra of sodium nitrate and pentaamminechlorocobalt (II) chloride are given, as well as the spectrum for carbonatotetraamminecobalt (III) nitrate. The peaks of the carbonatotetraamminecobalt (III) nitrate spectrum should differ from those from sodium nitrate and pentaamminechlorocobalt (II) chloride, signifying that the synthesized cobalt coordination compound was created correctly. Experimental A. Synthesizing Carbonatotetraamminecobalt (III) nitrate: [Co(NH3)4CO3]NO3.
Crush (NH4)2CO3 into a fine powder and weigh out 10 g. 2. Dissolve 10 g (NH4)2CO3 in 30 ml H2O and add 30 ml of concentrated aqueous NH3. While stirring, pour into a solution containing 7. 5 g [Co(OH2)6](NO3)2 in 15 ml H2O. Slowly add 4 ml of a 30% H2O2 solution. Mark the 50 mL point on the outside of an evaporating dish. Pour solution into an evaporating dish and concentrate over a gas burner in a hood to 50 ml. Pour the solution into a graduated cylinder to check the volume. Continue evaporating until the level is below 50 mL. 8. During evaporation, add 2.
5 g (NH4)2CO3 in small portions. 9. Suction filter the hot solution and cool the filtrate in an ice water bath. 10. Under suction, filter off the red product crystals. Stir the crystals occasionally with a glass rod. 11. Wash the [Co(NH3)4CO3]NO3 in the filtration apparatus with a few ml of ice cold H2O and then with a few ml of ethanol. 12. Wait until the crystals are dry, and record their mass. 13. Calculate the percent yield of [Co(NH3)4CO3]NO3. B. Measuring Absorbance Spectroscopy Prepare a . 006 M solution of the cobalt coordination compound by dissolving 0.
149 grams of the crystals in 100 mL of water in a volumetric flask. Fill a cuvette with the . 006 M solution. Wipe the cuvette with a kimwipe. Press F5 for spectroscopy at the MeasureNet station. Press F2 for Absorbance. Set the y-max to 2, the y-min to 0, the x-max to 650, and the x-min to 350. Press Display to begin recording data. Using the MeasureNet Workstation, measure the electronic absorption spectrum of [Co(NH3)4CO3]NO3 from 350 nm to 650 nm. At the Spectrometer, enter the station number of the MeasureNet. 6. Insert the light block into the machine and press zero.
Insert a blank cuvette filled with distilled water into the machine and press reference. Insert the cuvette containing the cobalt solution into the machine and press sample. After the data has been recorded, press File Options on the MeasureNet station. Press F3 to save. 10. Find lambda max of each absorption band and calculate the extinction coefficient at each lambda max. C. Measuring Conductivity 1. In a 100 mL Erlenmeyer flask, dissolve 0. 025 g [Co(NH3)4CO3]NO3 in 100 mL H2O. Make sure the bottom of the meniscus is aligned with the 100 mL line on the flask.
Pour the 0. 001 M [Co(NH3)4CO3]NO3 solution into a beaker. Fill approximately half of a second beaker with ethanol. Fill approximately half of a third beaker with 0. 01 M KCl solution. Rinse the conductivity probe in the ethanol and dry with a kimwipe. Dip the conductivity probe in the 0. 01 M KCl solution. Wait until the reading stabilizes and record the value. Calculate k, the correction factor for calculating molar conductance. 8. Again, rinse the conductivity probe in the ethanol and dry with a kimwipe.
Dip the conductivity probe in 0.001 M [Co(NH3)4CO3]NO3 solution. Wait until the reading stabilizes and record the value. 10. Calculate the molar conductivity of the aqueous solution of [Co(NH3)4CO3]NO3 and determine how many ions each formula unit contains. D. Measuring Magnetic Susceptibility Place an empty sample tube into the Magnetic Susceptibility Balance (MSB). Take a mental average of the fluctuating reading. Record the mass of the empty sample tube. Fill the tube with crystals to approximately 3 cm. The crystals should be tightly packed into the tube. Tap the tube gently on a kimwipe.
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