Chemistry 103: Synthesis of Alum

In this experiment, scrap aluminum metal will be converted to the compound called alum. The name alum commonly refers to
potassium aluminum sulfate dodecahydrate, KAl(SO4)2·12H2O, a white, crystalline solid. Among the many uses for alum are
water purification, leather tanning, mordant dyeing, as an astringent, and in baking powder.

A synthesis may involve one chemical reaction or a series of chemical reactions. In today’s experimental procedure, the four
following equations describe the series of reactions involved in the synthesis of alum.

        2 Al(s) + 2 KOH(aq) + 6 H2O(l) ---> 2 K[Al(OH)4](aq) + 3 H2(g)

        2 K[Al(OH)4](aq) + H2SO4(aq) ---> 2 Al(OH)3(s) + K2SO4(aq) + 2 H2O(l)

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

        K2SO4(aq) + Al2(SO4)3(aq) + 12 H2O(l) ---> 2 KAl(SO4)2·12H2O (s)

The lab procedures involved in this lab include filtering, purification, crystallization, and melting point determination.

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Part 1.     Synthesis of Alum
1.    Weigh out about 1g (±0.01 g) of aluminum metal and place the aluminum into a 250 mL Erlenmeyer flask.   Record the
        mass of the aluminum.

2.   CAREFULLY add 50 mL of 20% potassium hydroxide, KOH, to the flask.
 
CAUTION: KOH is a caustic material and is harmful to skin!   Immediately rinse affected area with plenty of water if the skin comes in contact with KOH.

3.    Asdemonstrated by the lab assistant, place the flask on a wire screen supported by a ring and ring stand and use a bunsen
       burner to GENTLY warm the flask so as to initiate the reaction.  The reaction is exothermic and a small bunsen burner
       flame should  be sufficient to sustain a slow reaction rate.  When no further reaction of aluminum is evident, turn off the
       bunsen burner.
 
CAUTION: Hydrogen gas is evolved in this reaction, so make sure that the ring stand is place directly under the exhaust hood.

4.    As demonstrated by the lab assistant, use a long stem funnel, and a piece of #4 filter paper, to gravity filter the warm
      reaction solution into a 250 mL beaker. This will remove any insoluble impurities.  Using irons tongs, remove the filter
      paper and place in the trash can. Allow the clear solution to cool.
 

Use CAUTION  because when filtering the solution.   Both the solution and flask may be hot!

5.    With constant stirring, slowly add 30 mL of 6 M H2SO4 to the cool solution.  Solid aluminum hydroxide may form.

6.    If needed, GENTLY  heat the reaction mixture with stirring until all of the aluminum hydroxide dissolves.

7.    If there are no solid impurities present, proceed to the next step.  If solid impurities are present, gravity filter the solution
      again using a clean piece of filter paper.

8.    Cool the beaker and solution in an ice bath.  Alum crystals should form within 20 minutes. If crystals do not form, gently
       heat to reduce the volume by about one-half and then cool in the ice bath.

9.    Using a Buchner funnel, isolate the alum crystals and wash the crystals by pouring about 20 mL of 50% ethanol over them
       in the filter while air is being drawn over the crystals.  After washing, continue drawing air over the crystals for about 3-5
       minutes.   Dispose of the liquid filtrate in the Buchner flask in the waste bucket under the hood.

10.    Remove the crystals from the Buchner funnel and further dry them by pressing them between the folds of paper towel.
         Dispose of the filter paper in the trash can.

11.    Weigh and record the mass of alum isolated.

Part 2.    Melting Point Determination
1.    Place a small amount of the alum into the bottom of a melting point capillary tube.

2.    Secure the capillary tube to the bulb of a thermometer with a rubber band and set up a melting point apparatus as
      demonstrated by the lab assistant.

3.    Slowly heat the water so that the temperature increases at a rate of about 3o per min.

4.    Watch the solid in the capillary tube and record the temperature when the alum first begins to melt and the temperature
       when the last crystal of alum melts. The literature value for the melting point of alum is 92.5oC.

5.    Dispose of the capillary tube in the broken glass container.

6.    Place the remaining alum in the glass jar labeled "Student Prep Alum."

Chemistry 103: Synthesis of Alum

Name___________________________________________________________ Hood No.__________ Date ___________

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mass of aluminum _______________g
mass of alum isolated _______________g
melting point of alum _______________oC