Last updated 22 nd February 2002 Properties of Aluminum and Its Compounds

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Objectives

• To prepare common alum, KAl(SO₄)₂.12H₂O, from a discarded aluminum beverage can.
• To perform the qualitative analysis of an alum.
• To investigate the acid-base behavior of aluminum compounds.

Grading

You will be assessed on

• the ability to calculate a percent yield for a chemical reaction
• the correctness and thoroughness of your observations
• the ability to deduce reaction products based on observation and your knowledge of chemistry
• the ability to balance chemical equations

Link to the Report Form here. This report is due at the beginning of the next lab period. There is no pre-lab assignment this week but you must read the procedure before coming to the lab and bring your textbook with you. Click here for information on a worked example on the calculation of a percent yield.

Introduction

Aluminum is the most abundant metal in the earth's surface (7.5% by mass). The abundance of aluminum, coupled with its attractive combination of physical and chemical properties, accounts for the fact that it is one of the principal industrial raw materials used by industrialized societies. Production of aluminum from raw materials is an energy intensive process.

Since the metal is not consumed rapidly by corrosion, the amount of scrap aluminum grows rapidly while the available supply of raw materials for the manufacture of aluminum decreases. The average predicted longevity of an aluminum can along the roadside is 100 years.

Environmental problems thus created are typical of those of several different metals. One obvious solution to the problem is to recycle the used fabricated aluminum into other useful metallic objects or into aluminum compounds. Aluminum metal can be recovered from scrap by melting the metal and separating it from solids and volatile impurities. This process uses a large amount of energy.  The energy requirement to prepare an aluminum can from recycling is only 5% of the energy required to produce the can from bauxite ore.

This experiment illustrates a chemical recovery process in which waste aluminum is converted chemically into an aluminum compound, hydrated potassium aluminum sulfate, KAl(SO₄)₂ .12 H₂O, or common alum. Although alum is an important industrial compound, the method of preparation in this experiment is not the way alum is obtained for use in industry. Nevertheless, this experiment will illustrate an interesting example of the reduction of environmental waste. "Alum" is a generic term that describes hydrated double salts of certain metals having the generalized formula, [MM' (SO₄)₂.12 H₂O], in which M (univalent) is commonly Na⁺, K⁺, NH₄⁺, or Rb⁺ and M' (trivalent) is commonly Al³⁺, Ga³⁺, V³⁺, Cr³⁺, Mn³⁺, Fe³⁺, or Co³⁺. True alums crystallize in well-defined octahedral shapes and many are beautifully colored, particularly those containing d-block transition metals. The ancient Egyptians, Greeks and Romans used alum as a mordant in dyeing cloth. A mordant contains metal ions that bind dyes to the fabric. Presently alum is used to harden photographic film, to prepare pickles, as a mordant, and for other purposes.

Experimental

Preparation of an alum

Wear safety goggles. Wear gloves when using concentrated acid and base.

1. Cut 2 x 50-cm square section from a scrap aluminum can. Sand the paint off using steel wool.. You will be doing duplicates on each step. One set will give you the Al(OH)₃ (step 7) that you will need in the final part of the lab when you add ammonia solution and the other set will be used to calculate the yield in step 13.
2. Cut the fragment into small pieces (about 0.5 cm long) and weigh out about 1 g.
3. Place the aluminum in a 400-mL beaker and add 50 mL of 4 M KOH.
4. Place beaker on the gauze on the porcelain tower and heat gently.

   CAUTION - H₂ GAS (VERY EXPLOSIVE) IS PRODUCED.
   ENSURE THAT YOU ONLY HEAT THE BOTTOM OF THE BEAKER AND NOT LET THE FLAMES GET NEAR THE TOP.
    

5. When the bubbles have stopped, remove from the heat, i.e hydrogen is no longer evolved.
6. Carefully rinse your measuring cylinder, pour out 25-30 mL of 9 M H₂SO₄ and then add slowly to the filtrate.

   CAUTION - H₂SO₄ IS A STRONG ACID AND DEHYDRATOR.  SEE TA IMMEDIATELY IF YOU SPILL ANY!
    

7. A white powder of Al(OH)₃ should form. Heat gently while stirring until the solution becomes clear.
8. Add 2 or 3 boiling chips, and boil the solution down to a volume of about 45 mL.
9. While the solution is boiling, prepare an ice bath by filling a bowl half-way with ice and then adding water until the bowl is three-quarters full.
10. After the "boiling off", let the beaker cool to room temperature and then place it in the ice bath. Crystals of alum should form. Allow to cool for 15 minutes.
11. After the 15 minutes are completed, vacuum filter the product and wash with 20 mL of ethanol. (Ask your TA if you need instruction in performing a vacuum filtration.)
12. Allow to dry for a few minutes, remove the boiling chips and weigh.
13. Calculate the percent yield and describe the appearance of the crystals.

 Qualitative Analysis of Alum

1. Use a straw spatula to transfer a few crystals (about 5 mg) to a watch glass. Add 3 drops of water to the crystals. Stir gently until the crystals dissolve.
2. Use a small piece of indicator paper to see whether the solution is acidic, basic, or neutral.
3. Now add 1 drop of 0.5 M BaCl₂ (barium chloride) to the solution. Record your observations.
4. A really good test for the presence of potassium is a flame test. Using the hot grips, hold a stainless spatula in the flame of a Bunsen burner to volatilize impurities from the spatula.
5. When one end of the spatula is red hot, remove it, and quickly touch it to a small cluster of crystals. Several should stick.
6. Slowly bring the spatula (plus crystals) toward the flame and watch carefully. Finally, hold the crystals in the flame for at least 5 seconds (until the solid glows). Record your observations.
7. Remove the spatula and place on a non-asbestos mat.

Acid-Base Properties of Aluminum Compounds

1. Place 50 mL of 0.5 M aluminum nitrate solution in a large beaker.
2. Place 70 mL of 1.0 M sodium hydroxide solution in a second beaker.
3. Place 80 mL of 1.0 M nitric acid in a third beaker.
4. Slowly add about 30 mL of the sodium hydroxide solution to the aluminum nitrate solution and record your observations.
5. Pour about half of the contents of the beaker into another beaker.
6. Continue to add sodium hydroxide solution to the first beaker and record your observations.
7. Add nitric acid to the second beaker and record your observations.
8. Add aluminum hydroxide to a solution of aqueous ammonia and record your observations.