Synthesis of the Lewis Adduct of Nickel with Thiosemicarbazide; Stoichiometry of Reaction

• To improve your skill in observation
• To synthesize a chemical substance and characterize it
• To discover the stoichiometry of a chemical reaction

Chemical synthesis--the art and practice of the design and creation of molecules--is the essence of chemistry. In no other field of science does the practitioner have the ability to create the objects of his/her interest. It is this single fact that sets chemistry apart from physics, geology, and biology, in which the scientist is restricted to examining what s/he finds, rather than what s/he creates. Chemistry has an extra dimension that combines vision and art in the process of creating new substances with potentially new and exciting uses.

This experiment provides you with an opportunity to experience chemical synthesis first hand, to determine the composition of the molecule that you create, and to spectroscopically characterize the molecule (i.e., to discover what types and energies of light the molecule interacts with). The reaction that you will carry out is shown in equation (1).

The reaction will be carried out in ethanol, a commonly-used laboratory solvent. CH₅N₃S is the formula for thiosemicarbazide (click to see the structure). This reaction is yet another example of a class of reaction that permeates chemistry: the donation of an electron pair by a Lewis base to a Lewis acid to form an adduct in which acid and base are joined by a covalent bond. As we have seen and will continue to see, the donation/acceptance of electron pairs is involved in a huge number of chemical reactions ranging from simple laboratory processes to the complex reactions in biological systems.

The adduct of Ni²⁺ with thiosemicarbazide is called a transition metal complex to indicate that it is a complex molecule involving an electron pair donor and a transition metal ion (Ni²⁺).

1. Construct a plot of the height of the column of product versus the volume of nickel solution used in the reaction. Why does the amount of product increase up to a point, then decrease?
2. Construct a plot of the mass of product versus the volume of nickel solution used in the reaction. Does the plot resemble that in 1)? Explain similarities and differences.
3. Which plot more accurately represents the variation in the amount of product with amount of nickel used? Why?
4. Use either of your plots to determine the stoichiometry of the reaction between nickel chloride hexahydrate and sodium diethyldithiocarbamate trihydrate. Write a balanced chemical equation to represent the reaction.
5. Which reagent limited the extent of reaction in test tube 2? In test tube 7? In either case, did you have any visual indication of this?

• 5.75" Pasteur pipets, 12 per group
• 1 Pasteur pipet bulb
• Kimwipes
• 8 small (1 x 6 cm) test tubes
• 2-mL graduated pipet (2)
• syringe pipet pump
• aspirator with trap
• 3 25-mL Erlenmeyer flasks
• Rubber stoppers for Erlenmeyer flasks
• plastic wash bottle containing distilled water
• Buchner funnel
• suction flask
• filter paper
• NiCl₂.6H₂O
• thiosemicarbazide, CN₃H₅S

Safety goggles must be worn at all times in the laboratory.

Record your observations in your notebook.

Synthesis. Obtain the required equipment. If necessary, wash the Erlenmeyer flasks and test tubes using brushes and Alconox detergent, then thoroughly rinse with water to remove all traces of Alconox.

Weigh 3 mmole of thiosemicarbazide (tsc) and transfer it to a 50-mL round bottom flask. Add 30 mL of ethanol, mount a condenser, and heat to reflux with stirring. While heating the tsc solution, weigh 0.12 g of NiCl₂.6H₂O, transfer it to a 25-mL Erlenmeyer flask, and add 10 mL of ethanol. Swirl to dissolve all of the nickel salt.

When the tsc has dissolved completely in ethanol, add the nickel solution dropwise to the refluxing tsc solution through the top of the condenser. Record all observations. Reflux with stirring for 30-60 minutes, then cool the mixture to room temperature and isolate the product by suction filtration. Suction the product dry and transfer it to a weighed vial. Determine the mass of product obtained.

Stoichiometry. Dry the outsides of the test tubes and mark them 1-8 using a Sharpie marker. Place the tubes in order in a test tube rack.

Weigh 474 mg of NiCl₂.6H₂O. Transfer the solid to one of the Erlenmeyer flasks, and add 20 mL of distilled water measured using a 25-mL graduated cylinder. Weigh 182 mg of thiosemicarbazide (hereafter tsc) and transfer it to the other Erlenmeyer flask. Add 20 mL of distilled water. What do you observe about these two solutions?

Using the syringe pump and graduated pipet, transfer, respectively, 0.3, 0.4, 0.5, 0.7, 0.9, 1.2, 1.75 mL, and 1.75 mL of the nickel chloride solution into test tubes 1-8. Rinse the pipet with distilled water and aspirate dry. Then transfer respectively 3.2, 3.1, 3.0, 2.8, 2.6, 2.3, 1.75 mL of thiosemicarbazide solution to test tubes 1-7, and 1.75 mL of distilled water to test tube 8. Mix the contents of each tube by flicking the bottom of the tube with your index finger while holding the top of the tube between your other thumb and index finger. Do all of the tubes look the same? What is similar? What is different? Describe in detail in your notebook what you observe.

In turn, transfer the contents of each test tube to a spectrometer cell and obtain the UV-visible spectrum. Find two wavelengths at which the blue complex absorbs light but aqueous nickel ion does not (test tube 8).

When finished, pour all solutions into the heavy-metal waste jar.

Characterization. If desired, you may characterize your product using infrared, uv-visible, and nmr spectroscopies. See your instructor for assistance.

Clean-up. When you have finished all of your work,

• Clean all glassware with Alconox and water, and shake dry.
• Rinse Pasteur pipets and graduated pipets with distilled water by inserting the nozzle of your wash bottle in the top end and squirting water through. Aspirate dry.
• Return all components to the labkit.
• Return all borrowed equipment to the instructor before leaving lab.

Dispose of dichloromethane/petroleum ether mixtures in the chlorinated solvent waste bottle. Discard bis(thiosemicarbazide)nickel and aqueous nickel chloride in the heavy metal waste. Discard aqueous thiosemicarbazide in the non-halogenated organic waste bottle.

1. The appropriate sections of your textbook.

1. A Job's Method study was carried out for the reaction
   aA + bB ---> dD + fF

   Stock solutions of A and B containing 1 mmole per 10 mL of solution were prepared. These were used to prepare a series of 8 solutions containing varying amounts of A and B, but such that the total moles of A and B was maintained constant. The absorbance of each solution was then measured at a chosen wavelength (the same for all solutions), and the following data were obtained:

   Solution	volume A solution	volume B solution	Volume solvent	A at chosen wavelength
   1	0.3	3.2		0.12
   2	0.4	3.1
   3	0.5	3.0
   4	0.7	2.8
   5	0.9	2.6
   6	1.2	2.3
   7	1.75	1.75		0.12
   8	1.75	0	1.75	0.12

   What is the stoichiometry of the reaction?