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 ___________
Put on your CHEMICAL SPLASH-PROOF SAFETY GOGGLES!
mass of aluminum | _______________g |
mass of alum isolated | _______________g |
melting point of alum | _______________oC |