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2.2 Day 1 Procedure

  • Page ID
    212055
  • You will first prepare two cobalt compounds, \([Co(NH_3)_4(CO_3)]NO_3\) and \([Co(NH_3)_5Cl]Cl-2\). These reactions may be carried out in air. However, they should be performed in a fume hood. Each member of a pair of researchers will individually synthesize \([Co(NH_3)_4(CO_3)]NO_3\). As part of the pre-lab calculate the molar quantities of each reactant required for the two syntheses. For each synthesis specify the limiting reagent. 

    Day 1 - Synthesis of \([Co(NH_3)_4(CO_3)]NO_3\)

    In a 50 or 100mL beaker containing a half-inch magnetic stir bar set on a magnetic stir/hot plate in a fume hood dissolve 4.0 g of \([Co(H_2O)_6](NO_3)_2\) in 10 mL of water (Helpful hint: note how this volume appears in the beaker, see below).

    In a small beaker dissolve 5 g of \((NH_4)_2CO_3\) in 10 mL of water and add 15 mL of concentrated ammonia in water. Add the \((NH_4)_2CO_3\) solution slowly with stirring to the cobalt solution. Observe the color change. While stirring, add 2 mL of 30% \(H_2O_2\) dropwise over a period of about 1 minute.

    While heating and stirring, concentrate the solution by allowing the temperature of the
    solution (measure it with a thermometer) to reach but go no higher than 75-80°C. Add a total of 1 g of (\((NH_4)_2CO_3\) in several small portions during the process of reducing the volume to ~10 mL. (A small final volume ensures that more of the product will crystallize out.) Let stand undisturbed for 15 minutes, then place the beaker in an ice bath for 15 minutes. (Disturbing the solution will cause rapid formation of fine crystals that are not as easy to filter off.)

    Filter off the purple-pink crystals using a 30-50 mL medium glass frit and wash once with 3 mL of ice water, then twice with 3 mL of ethanol. (Helpful hint: turn off the vacuum to add the wash solution, and then turn the vacuum back on) Dry in vacuo (if possible) by placing a rubber stopper over the top of the funnel and pulling a vacuum through the end of the funnel. Release by shutting off the vacuum, holding the stopper, and removing the tube to the vacuum. Record the weight, repeating the drying process until the weight no longer changes (Helpful hint: use two cork rings stacked to support the glass frit on the balance). Store the product in your desiccators. (Helpful hint: be sure the desiccant is dry i.e., free-flowing) Alternatively you may simply let the product air dry until the next class period. Calculate your theoretical yield (based on the actual quantity of limiting reagent used) and your percent yield, which should be in the range 35- 65%. Clean the glass frit by running dilute hydrochloric acid followed by water through the frit.

    Obtain IR spectra, following the procedure demonstrated by your TA, of the product, \([Co(NH_3)_4(CO_3)]NO_3\), \(CaCO3\) (chalk) and \([Co(H_2O)_6](NO_3)_2\). Be sure to also obtain spectra of the materials used to prepare the sample for analysis. Compare the spectra obtained. Identify the absorptions above 1300 \(cm^{-1}\) that can be assigned to ammonia, carbonate, water, and nitrate.

     


    QuestionYou can readily identify the peak due to free or uncoordinated carbonate in the \(CaCO3\) spectrum. Without consulting the literature, how can you determine which peak is due to nitrateʼs \(NO\) stretch and which are due to carbonateʼs \(CO\) stretches in \([Co(NH_3)_4(CO_3)]NO_3\)? Justify your answer.

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