The Synthesis of Alum from Scrap Aluminum

The Synthesis of Alum from Scrap Aluminum

Overview of the Synthesis

The synthesis of alum, KAl(SO4)212H2O, can be accomplished through the following reactions.

Aluminum is first oxidized by potassium hydroxide to form a soluble salt in the chemical reaction.

2 Al(s) + 2 KOH(aq) + 6 H2O(l) 2 KAl(OH)4(aq) + 3 H2(g)

After filtration to remove any residual plastic or paint, the solution is clear and colorless. The H2 is evolved as a gas and mixes with the atmosphere (this is why there is no smoking in the lab). The chemical species in solution are K+, Al(OH)4-, and unreacted KOH. Sulfuric acid is added and two sequential reactions occur. Initially, as the acid is added, a precipitate, Al(OH)3 forms.

2 KAl(OH)4(aq) + H2SO4(aq) 2 Al(OH)3(s) + 2 H2O(l) + K2SO4(aq)

Then as more sulfuric acid is added the precipitate dissolves

2 Al(OH)3(s) + 3 H2SO4(aq) Al2(SO4)3(aq) + 6 H2O(l)

to give aluminum ions, Al3+, in solution. The solution at this point contains Al3+, K+ and SO42-

ions. On cooling these ions form a solid hydrate, KAl(SO4)2 12 H2O (Alum).

cold

Al2(SO4)3(aq) + K2SO4(aq) + 24 H2O(l) 2 KAl(SO4)2 12 H2O(s)

In the experiment the crystallization can be sped up by providing a small seed crystal of alum for new crystals to form upon. Cooling is needed because alum is soluble in room temperature water. Finally the crystals of alum are removed from the solution by vacuum filtration and washed with an alcohol/water mixture. This wash liquid removes any contamination from the crystals and dries quickly because alcohol is relatively volatile.

Bring to lab one aluminum can.

Procedure:

1. Record the producer of the can used. Use scissors to cut out a piece of the can that is 3 cm x 3 cm. Deposit the remaining aluminum can in a recycle box provided. Scour your aluminum with a piece of steel wool on both sides. Wipe the metal clean with a paper towel. Weigh your piece of aluminum. Cut off the edges of your aluminum until you obtain a piece that weighs approximately 0.5 g. If the aluminum is less than 0.5 g cut a new strip from your can and add this to your original piece. Record the final mass of your aluminum.

2. Cut up the weighed piece of aluminum into small pieces, and place in a clean 250 mL beaker (the smaller the pieces the quicker they will dissolve). Use a graduated cylinder to add 50 mL of 1.4 M KOH solution in a hood. (Be careful. KOH is corrosive).

3. Place the beaker on the hot plate in the hood (set the hotplate to "3") to completely dissolve the aluminum. While the aluminum is dissolving, get ready for the gravity filtration by placing the glass funnel into a 250 mL Erlenmeyer flask. Place the filter paper into the funnel by folding the filter paper into a cone. If the aluminum has not dissolved in the KOH solution after 15 minutes of heating add an addition 20 mL of 1.4 M KOH.

4. When the aluminum has dissolved (no more H2 is being produced), filter the solution. Only fill the funnel to within ?" of the top of the paper. The "filtrate" in the Erlenmeyer flask, should be clear and colorless. Allow the flask to cool (about 7-8 min).

5. When the solution is reasonably cool, slowly add 20 mL of 9 M H2SO4 from a graduated cylinder. It is important that the flask be swirled as you add the acid. The solution will get quite hot as the acid is added. Note the gelatinous precipitate of Al(OH)3 which forms as the base is neutralized but then dissolves in the excess acid. If there are any white flecks left in the solution after the addition of the H2SO4, place the flask back on the hot plate and heat with stirring until all the solid material has redissolved.

6. Make an ice bath by putting ice and water into your largest beaker. Allow the Erlenmeyer flask to cool until it is just warm to the touch and then place it in the ice bath. Allow to cool for about 5 minutes. Unless crystals have already started to form, scratch the bottom of your flask with a stirring rod. If necessary add a few seed crystals to begin precipitation. Continue to swirl the flask in the ice bath for 15 minutes to allow complete crystallization.

7. While the solution is cooling, pour 10 mL of a 50% alcohol/water mixture into a large test tube and place the tube in the ice bath to cool.

8. Set-up a vacuum filtration device using the vacuum in the hood. "Seat" the filter paper by pouring some deionized water in the funnel to wet the filter paper and then turn on the vacuum to remove the water. When the filter paper is seated, turn off the vacuum until you are ready to collect the crystals.

9. Remove the flask containing the alum crystals from the ice bath and swirl the solution to get as much of the solid as possible off the sides of the flask. Turn on the vacuum and pour the contents of the flask into the vacuum apparatus. Keep swirling between pours. Remember, the goal is to get as much of the solid out of the flask and onto the filter paper as you can. Use your rubber policeman to get any solid that is on the side of the flask.

10. Once the alum solution has been filtered, try to remove any remaining crystals by rinsing the flask with the cooled ethanol/water solution. Swirl the ethanol/water solution in the flask and poor the contents into the filter flask to collect the remaining crystals. After

pouring the ethanol/water mixture through the filter flask, let the alum crystals dry on the vacuum for 5 minutes.

11. Weigh a clean dry 3" watch glass.

12. Remove the top part of the filter funnel and transfer the crystals onto the watch glass. Use a glass rod to gently dislodge lumps that stick. Try to remove as many crystals as you can, but do not tear the filter paper. This step should make it clear why actual yield hardly ever equals theoretical yield!

13. Weigh the watch glass plus crystals. The crystals are KAl(SO4)2 12 H2O.

14. Turn in your alum in a vial labeled with your name, the date and the contents.

Synthesis of Alum Data Sheet

Name: ____________________________

Brand name of beverage can used

____________________________________

Mass of aluminum used

____________________________________

Mass of alum synthesized

____________________________________

Give the formula for the overall reaction performed in the lab by adding all the chemical steps presented in the lab discussion.

______________________________________________________________________________

Theoretical yield of alum

____________________________________

Percent yield of alum

____________________________________

There are many compounds that are classified as alums. What makes a compound an alum?

What is the general formula of an alum? __________________________________________

If 1.12 g of Al, 50 mL of 1.4 M KOH and 20 mL of 9 M H2SO4 were used in the preparation of alum, what is the limiting reactant? How many grams of alum would be produced?

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