Materials Included In Kit

Additional Materials Required

Prelab Preparation

Cut aluminum foil into 18 x 18 cm squares.

Safety Precautions

Sulfuric acid solution is severely corrosive to eyes, skin and other body tissues. Always add acid to water, never the reverse. Potassium hydroxide solution is a corrosive liquid; it is particularly dangerous to eyes and may blister and burn skin. Avoid contac with eyes and skin. Neutralize and clean up all spills immediately. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The leftover sulfuric acid may be disposed of by neutralizing with base and then disposing of down the drain with excess water according to Flinn Suggested Disposal Method #24b. The leftover potassium hydroxide solution may be disposed of by neutralizing with acid and then disposing of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #10. The aluminum potassium sulfate dodecahydrate may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Excess aluminum foil may be recycled or stored for future laboratory use.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students.
• For best results, schedule at least one 50-minute class period and, in addition, at least two hours (overnight is best) to dry the alum crystals. The mass of the alum crystals can be determined the next day or the products may be stored (covered) in a lab drawer until the next regularly scheduled lab (i.e., the following week).
• The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed after the alum crystals have dried and been massed.
• Potassium hydroxide and sulfuric acid solutions are corrosive liquids. We recommend setting out and dispensing these chemicals in a central and supervised location. This is best done in the fume hood. Place the reagent bottles on a demonstration tray or a laboratory spill mat (LabMat^™) to contain any chemical spills. To further reduce spillage, tape a small test tube to the side of each reagent bottle. Store a transfer pipet in the test tube for the students to use.
• When adding the 3 M KOH to the aluminum foil do not use a beaker smaller than a 250 mL because the solution forms hydrogen bubbles and splashing will occur outside of a smaller container.
• Caution the students to be sure to stir the hot mixtures containing solids to prevent “bumping.” Stirring should continue until the solution has cooled significantly.
• Remind students not to use thermometers to stir the solutions.
• Review with students how to properly set up and use a Büchner funnel filtration apparatus.
• Review proper transfer techniques to ensure satisfactory yield.
• Have students compare results and tabulate percent yield data for the class. Typical percent yield results are 48–60%.

Teacher Tips

• For a more advanced class that has already studied balancing redox equations, Equation 1 can be determined by students. Balance this redox equation in base that follows.

  Al(s) + H₂O(l) → Al(OH)₄^–(aq) + H₂(g)
  Al oxidation # = 0 [4OH^–(aq) + Al(s) → Al(OH)₄^–(aq) + 3e^–] x 2
  H in H₂O oxidation # = +1 [2e^– + 2H₂O(l) → H₂(g) + 2OH^– (aq)] x 3
  Al in Al(OH)₄^– oxidation # = +3 8OH^–(aq) + 2Al(s) + 6H₂O(l) + 6e^– →
  2Al(OH)₄^–(aq) + 6e^– + 3H₂(g) + 6OH^–(aq)
  H in H₂ oxidation # = 0 2OH^–(aq) + 2Al(s) + 6H₂O(l) → 2Al(OH)₄^–(aq) + 3H₂(g)

Answers to Prelab Questions

1. What are the safety concerns associated with the reagents used to make aluminum potassium sulfate dodecahydrate?

   Sulfuric acid and potassium hydroxide are corrosive to eyes and skin. Splattering may occur when KOH reacts with aluminum foil.

2. What gas is produced after potassium hydroxide is added to the aluminum foil?

   2Al(s) + 2OH^–(aq) + 6H₂O(l) →2Al(OH)₄^–(aq) + 3H₂(g). Hydrogen gas is formed.

3. Calculate the molar mass of aluminum potassium sulfate dodecahydrate.

   The formula is AlK(SO₄)₂•12H₂O.
   Al 1 x 26.98 = 26.98
   K 1 x 39.10 = 39.10
   S 2 x 32.07 = 64.14
   O 8 x 16.00 = 128.0
   H 24 x 1.008 = 24.19
   O 12 x 16.00 = 192.0
   Molar mass = 474.4 g/mole

4. Determine the maximum or theoretical yield of aluminum potassium sulfate dodecahydrate that can be produced from 1.00 g of aluminum foil?
   {12122_PreLabAnswers_Equation_5}

Sample Data

Answers to Questions

1. Why did the reaction of aluminum foil and potassium hydroxide start out slowly and then proceed more rapidly?

   There is an oxide coating on the aluminum and once this oxide coating is penetrated the reaction proceeds much faster.

2. What is the theoretical yield of alum assuming all the aluminum metal reacted?
   {12122_Answers_Equation_6}
3. What is the percent yield obtained after running this laboratory procedure?
   {12122_Answers_Equation_7}
4. If 500 g of laboratory grade aluminum potassium sulfate cost $8.25, is it cost effective to make the aluminum potassium sulfate yourself? Consider the following data and the percent yield when determining the answer.

   Al foil costs $1.69 for a 20-sq-foot roll and 1 square foot weighs 3.75 g. Sulfuric acid solution, 3 M costs $9.80 for 500 mL.
   Potassium hydroxide solution, 3 M costs $12.80 for 250 mL.
   When calculating the answer, only consider the amount of chemicals needed and not the capital cost of the chemicals, equipment or labor time.
   
   Part A: What is the cost of the chemicals used to obtain the alum crystals in this experiment?

   {12122_Answers_Equation_8}

   $0.0169 + $0.49 + $0.77 = $1.28 per actual yield
   Sample Data: $1.28 is the cost per 6.96 g
   
   Part B: Using the conversion factor of chemical cost (Part A) in relation to specific yield, calculate the cost of making 500.0 g of aluminum potassium sulfate.

   {12122_Answers_Equation_9}
   Part C: Is this laboratory method cost effective? Why or why not?

   This method is not cost effective. It would cost more than 11 times as much to produce the alum crystals from aluminum foil as it would cost to buy them directly.

References

Gillette, M. L., Neidig H. A. Synthesizing Alum Chemical Education Resources, Inc. SYNT 451.

Introduction

Alum is the common name for the chemical aluminum potassium sulfate dodecahydrate. Alum is used for many different purposes—from pickling foods to aftershave for men! In this activity, alum crystals will be produced by recycling aluminum foil.

Concepts

• Chemical reactions
• Synthesis
• Crystallization
• Percent yield

Background

Aluminum potassium sulfate dodecahydrate, AlK(SO₄)₂•12H₂O, is used for many applications, including as a food preservative, astringent (to stop the bleeding of a minor cut), sizing agent in the papermaking industry, and as a natural deodorant. It is even sold as a spice in many grocery stores. Alum is commonly used in the papermaking process to fill in the pores of the paper. Filling the pores with alum prevents ink that is applied to the paper from spreading out or bleeding. Alum also reacts with the cellulose and releases aluminum ions. These aluminum ions attract the hydroxide (OH^–) part of the water molecule, releasing the H⁺ or H₃O⁺ ion creating an acidic medium. This acid will break down the paper over time. Papers used for archiving and photo scrapbooking do not use alum as a sizing agent and are labeled as “acid-free.”

Alum is produced in industry by chemical synthesis. A synthesis reaction occurs when two or more substances combine to produce a more complex new substance. In industry, the goal is to optimize chemical processes to give the highest percent yield with the lowest use of resources (cost).

In the laboratory synthesis of alum, aluminum foil is the source of aluminum, potassium hydroxide is the potassium source, and sulfuric acid is the sulfate source. The process has two main steps—dissolving aluminum metal and the combination of components to produce alum. Aluminum is dissolved into solution so the ions are more readily available to react. Potassium hydroxide reacts with the aluminum to form Al(OH)₄^– complex ions and hydrogen gas (Equation 1).

Experiment Overview

In this experiment, a quantitative synthesis reaction is performed to produce aluminum potassium sulfate dodecahydrate crystals. Alum will be created working with aluminum foil, potassium hydroxide, sulfuric acid, water and using techniques, such as dissolution, crystallization and filtering. The percent yield of alum will be determined.

Materials

Prelab Questions

1. What are the safety concerns associated with the reagents used to make alum?
2. What gas is produced after potassium hydroxide is added to the aluminum foil?
3. Calculate the molar mass of aluminum potassium sulfate dodecahydrate.
4. Determine the maximum or theoretical yield of aluminum potassium sulfate dodecahydrate that can be produced from 1.00 g of aluminum foil?.

Safety Precautions

Sulfuric acid solution is severely corrosive to eyes, skin and other body tissues. Always add acid to water, never the reverse. Potassium hydroxide solution is a corrosive liquid; it is particularly dangerous to eyes and may blister and burn skin. Avoid contact of all chemicals with eyes and skin. Notify the instructor and clean up all spills immediately. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines

Procedure

1. Obtain a piece of aluminum foil approximately 18 cm x 18 cm. Cut or tear it into very small pieces, 1 cm x 1 cm or smaller.
2. Place a weighing dish on a balance. Tare the balance.
3. Transfer aluminum foil pieces to the weighing dish until the mass of the aluminum foil is near 0.75 g. Record the exact mass in the data table on the Recycling Aluminum Worksheet.
4. Transfer the 0.75 g of aluminum foil pieces into a 250-mL or larger beaker. Note: The beaker should not be smaller than 250 mL due to the vigorous reaction of aluminum foil and the potassium hydroxide.
5. Add 15 mL of 3 M potassium hydroxide solution to the beaker and stir the mixture with a glass stirring rod until the reaction is well underway. Caution: Hydrogen gas and heat are produced in this reaction. At first the reaction is slow because the aluminum oxide coating must dissolve, but once the aluminum is exposed to the potassium hydroxide, the reaction is vigorous.
6. Once the foil has reacted completely, record all observations on the worksheet for the reaction and the resulting solution.
7. While stirring, carefully add 25 mL of 3 M sulfuric acid 5 mL at a time to the beaker. Record all observations on the worksheet.
8. White crystals should now be visible. Place the beaker on a hot plate at a low setting and stir until all white crystals dissolve. Note: Black specs may be disregarded and the solution may be off-white to light brown in color. Once all the white crystals are dissolved the solution is ready to be filtered.
9. Set up a gravity filtration apparatus by placing a clean filtering funnel in an appropriately sized ring clamp attached to a ring stand, as shown in Figure 1.
   {12122_Procedure_Figure_1}
10. Fold filter paper as shown in Figure 2. Fold the filter paper in half and in half again. Carefully rip off one corner of the filter paper. This will create a better seal when filtering.
    {12122_Procedure_Figure_2}
11. Moisten the filter paper with distilled water and press the edges of the filter paper against the funnel so there is a good seal between the filter paper and the glass filter funnel. A good seal will prevent the loss of product and increase the filter rate. Replace the funnel in the ring clamp and place the stem inside a 250-mL beaker.
12. Transfer the solution from step 8 into the moistened filter paper from step 10 and filter the solution into a clean 250-mL beaker. Do not overfill—the solution level should not exceed the top 1 cm of the filter paper. The filtered solution should be clear and colorless.
13. Create an ice bath by placing ice into an 800-mL beaker until it is half full.
14. Place the 250-mL beaker containing the filtrate into the ice bath to cool (see Figure 3).
    {12122_Procedure_Figure_3}
15. Frequently stir the cooling filtrate with a glass stirring rod.
16. Cool the filtrate to less than 6 °C.
17. Set up a vacuum filtration apparatus as shown in Figure 4.
    {12122_Procedure_Figure_4}
18. When crystallization is complete, turn on the water flow to the aspirator and filter the alum crystals through the Büchner funnel.
19. Transfer all of the crystals onto the filter paper in the Büchner funnel using the rubber policeman on the end of the stirring rod. (Optional) Wash the crystals with two 5 mL portions of ethyl alcohol.
20. If time allows, continue running the aspirator vacuum until the crystals are very dry—this will take 10–15 minutes.
21. Mass a clean, dry weighing dish or beaker and transfer the crystals into the container. If the crystals were dried to completion in step 20, record the mass of the crystals. If step 20 was not completed, cover the crystals and store them until the next laboratory session. Measure and record the mass of the dry crystals.
22. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12122_Student1.pdf