Reactivity: Synthesis of the Adduct of Ni²⁺ with Ethylenediamine

• To explore some solution chemistry of nickel
• To enhance your skills of observation
• To learn techniques of synthesis and separation

The passion of the chemist is to make new molecules and explore their physical properties and chemical reactivity. Choosing and carrying out appropriate chemical reactions to accomplish this is a combination of skill and artistry, which must be learned and practiced. This experiment provides an opportunity for you to start with a pure chemical substance and subject it to a series of chemical reactions. Based on what you observe, you will fill in as much information about the nature and products of these reactions as you can.

Several important types of reactions are involved in this experiment. These are called Bronsted acid-base, Lewis acid-base, and precipitation reactions.

Bronsted acid base reactions involve the transfer of hydrogen ions, H⁺, from one substance to another. The substance donating the hydrogen ion is called an acid, and the substance accepting it is called a base. Some examples of acid-base reactions are given in Table 1.

Lewis acid base reactions involve the donation of an electron pair from one species to another. The recipient of the pair is called the acid; the donor of the pair of electrons is called the base. The union of acid and base is called the adduct. Some examples of Lewis acid-base reactions are given in Table 2. Note that Bronsted-Lowry acid base reactions may be interpreted in terms of the more fundamental Lewis ideas.

Precipitation Reactions result in the formation of a solid when two solutions are mixed. The solid is called a precipitate. Some examples of precipitation reactions are given in Table 3.

Nickel is a transition metal that is beautifully colored in many of its compounds. In this experiment, you will begin with a sample of nickel carbonate (NiCO₃) and will carry it through a series of reactions of the types discussed above.

Focus Questions

As you proceed through the experiment, keep the following questions in mind. When you have finished the experiment, provide answers to them.

1. In Part 1, what is the significance of having unreacted NiCO₃ present at the end of reaction?
2. In Part 2, you probably observed some bubbling on addition of HCl. Can you explain why the bubbling was barely noticeable at first but became more and more intense as more HCl was added?
3. What is your explanation of the color changes observed during Part 2?
4. What type of reaction is occurring in Part 1, BL acid-base, precipitation, or Lewis acid-base?
5. In which steps was product lost? How could the procedure be improved to increase the yield?
6. What is the stoichiometry of reaction between Ni²⁺ and ethylenediamine?

Equipment and Materials

• assorted beakers and flasks
• Pasteur pipets
• aspirator trap
• filter paper
• NiCO3
• concentrated HCl (12 M)
• ethylendiamine

Safety goggles must be worn at all times in the laboratory. Concentrated hydrochloric acid and ethylenediamine are corrosive. Wear gloves when handling them. In the event of skin contact, flush the affected area with copious quantities of cold water.

Record all data and observations in your notebook.

Part 1: Weigh 1.30 g of basic nickel carbonate into a small beaker, and add 4-5 mL of water. Swirl to achieve a slurry. With continuous stirring or swirling, add 3.3 mL of 6 M HCl using a Pasteur pipet. When reaction is complete, suction filter the mixture to remove unreacted nickel carbonate. Transfer the filtrate to a clean beaker. Add 2 mL of ethylenediamine dropwise using a Pasteur pipet. When addition is complete, cool the beaker for a few minutes by submerging it in cold tap water until it is again at about room temperature. Add acetone to the solution until the total volume is 50 mL. Suction filter the mixture. Wash the solid twice with small (5-mL) portions of acetone, then suction the product dry.

Weigh the product. Assuming that you obtained 100% yield, what is the approximate molar mass of the product? How many moles of ethylenediamine were needed per mole of nickel?

Part 2: Weigh about 0.1 g of the product from Part 1 into a small beaker, and add 2-3 mL of water. Add 2 M HCl dropwise to the resulting solution, with stirring. Continue until the total volume is at least doubled. Record ALL observations.

Part 3: Purification by Recrystallization Weigh about 0.2 g of product from Part 1 into a small beaker, and add 1-2 mL of water. Warm the mixture gently over a burner flame if necessary to dissolve the solid. Filter the hot solution through a Kimwipe-plugged Pasteur pipet into another small beaker. Place the filtrate beaker in an ice bath to crystallize. Once crystals have formed, isolate them by suction filtration, wash with 2 small portions of acetone, and suction dry.

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

• Clean Pasteur pipets, graduated pipets, and vials by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Place all solutions in the appropriately marked waste bottles. After weighing the product, place it in the appropriate waste bottle.

Preparation Questions