Pre-Lab: Obtain an empty, rinsed aluminum soft drink can and bring it to lab
Introduction
Aluminum is the most abundant metal in the earth's crust, 8.1% by mass. The abundance of aluminum
along with its useful combination of physical and chemical properties makes it one of the world's most
widely used raw materials in industry. Aluminum's self-protective film of aluminum oxide (alumina)
makes it rather resistant to corrosion under many conditions. Its strength, particularly when alloyed with
other metals, makes it a useful structural material.
The mass of a single aluminum can is quite small, but a huge number of such cans are produced each year,
using a large tonnage of the metal. Since large amounts of electrical energy are required to produce virgin
aluminum, recycling the metal or converting it into other useful materials is desirable, both
environmentally and economically.
Here we will convert scrap aluminum into an important compound whose common name is alum
(potassium aluminum sulfate twelve hydrate). It is used as a mordant to harden photographic film, to
prepare pickles, and to bind dyes to fibers. Using it in dyeing was first introduced by the ancient
Egyptians, Greeks, and Romans.
Alum is also a generic term to describe a series of compounds with the general formula
MM(SO4)212H2O. Here M+ is commonly Na+, K+, NH4+, or Rb+, and M3+ is commonly Al3+, Ga3+, V3+,
Cr3+, Mn3+, Fe3+, or Co3+. Alums form well-defined crystals and those containing transition metals as M3+
may have beautiful colors.
One significant fact about aluminum is that it can be dissolved by a strong acid or by a strong base:
(1) 2 Al(s) + 6 H3O+(aq) = 2 Al3+(aq) + 3 H2(g) + 6 H2O(l), and
(2) 2 Al(s) + 2 OH-(aq) + 6 H2O(l) = 2 Al(OH)4-(aq) + 3 H2(g)
The fact that aluminum oxide is amphoteric is used in separating Al from iron impurities found in bauxite,
the common ore of aluminum. The ore is dissolved in NaOH(aq), leaving the iron behind as the insoluble
hydrated oxide.
In this experiment we will dissolve the scrap aluminum with KOH as in equation (2). Since there is
excess KOH, sulfuric acid is used to neutralize this excess. The sulfate ions are also necessary to convert
the aluminum to the complex sulfate product, KAl(SO4)212H2O.
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Conversion of aluminum scrap into a useful substance
Procedure
(Remember to record your observations of ALL of the processes as they occur in the Report section. )
A. Cut out a piece of the aluminum can about 7 cm by 5 cm. Use sandpaper to remove the paint and
protective coating from BOTH sides. Now cut the piece of aluminum into smaller pieces to increase
the surface area. (Why should one want to increase the surface area?) Weigh about a gram of clean
scrap aluminum (±0.001 g) in a clean, dry 250-mL beaker.
B. To the small pieces in the beaker, add 20 mL of deionized water and 15 mL of 6 M KOH(aq).
CAUTION: KOH is very corrosive to skin, clothing, hair, ...
C. In a HOOD, heat the beaker on a hot plate to get the reaction started. Do NOT boil the solution. Then
heat just enough to keep the reaction going at a moderate rate. NO FLAMES can be used where a
flammable substance is involved. Continue warming until the evo-lution of gas ceases (about 10 to 15
minutes). If the liquid level drops below one-fourth of the original level, add deionized water to bring
the level back to one-half the original level. (It is advan-tageous later if the volume is not more than
30-35 mL at this time.)
D. When hydrogen evolution has stopped, use beaker tongs to remove the beaker from the hot plate; take
it to your bench, and allow it to cool to room temperature, stirring occasionally. If the solution is clear
and contains no solid particles, no filtration is needed at this time.
E. If any solid particles remain, prepare a filter by forcing a small plug of glass wool (irritant) into the
bottom of a funnel. Pouring the solution through the glass wool should trap any solid particles. If you
need to filter the solution, collect the clear filtrate in a clean 150-mL or 250-mL beaker. Use a small
amount of water from your plastic wash bottle to rinse all of the solution into this beaker.
F. After the solution has cooled to room temperature, slowly add 30 mL of 6 M H2SO4, stirring
continuously. Large lumps of aluminum hydroxide should form.
G. At your bench, use a burner to heat the mixture gently (NO boiling), with continuous stirring, for
about 10 minutes, or until all the aluminum hydroxide dissolves.
H. When you have a clear solution, remove it from the heat. If any solids are present, filter the hot
solution as in C above. Cool the beaker with your solution in an ICE bath for about 20 minutes.
Alum crystals should form in the beaker. (If no crystals appear after 10 minutes, add a minute
(smaller than a rice grain) amount of powdered alum from the bottle in the lab to "seed" the solution.)
I. Suction filter the crystals using a B_chner funnel, filter paper, and a filter flask. (The filter flask must
be clamped securely so that it does not fall over.) Griswold, pp 16 & 17.
J. note: If time permits, you can recover more alum crystals. Remove the filtrate from the filter flask.
Evaporate the solvent to about 1/2 of its initial volume. Cool the concentrated solution in an ice bath
and collect the second crop of crystals. If you do this, indicate the mass of each crop and the total
mass of alum.
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Conversion of aluminum scrap into a useful substance
K. Detach the hose momentarily and pour 20 mL of a 50-50 ethanol-water mixture onto the crystals.
Reattach the hose and draw the liquid off the crystals. Once dry, detach the hose before turning off the
water so that water is NOT drawn back into the flask. Transfer the dried crystals to a pre-weighed
beaker and determine the mass of the dried crystals. Show the crystals to your lab instructor, who will
initial Table 1.
L. Determine the melting point of your alum sample. Use a mortar and pestle to grind some of the alum
to a fine powder. Pack some of this fine powder in the bottom of a melting point capillary tube to a
depth of about 0.5 cm. Use a small rubber band to attach the melting point tube near the bottom of
your 110oC thermometer so the closed end of the capillary is even with the thermometer bulb. Place
the thermometer with the attached capillary tube in a 400-mL beaker containing enough water to cover
the thermometer to the immersion line, making sure that the open end of the capillary is above the
water level. Heat the water slowly so that the water temperature increases no faster than 3oC per
minute. Carefully watch the solid in the capillary tube. At the moment the solid melts, note the
temperature. (If the solid starts to melt and continues for a time, note both the starting and ending
temperatures for the melting process.)
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Conversion of aluminum scrap into a useful substance
REPORT
Conversion of aluminum scrap into a useful substance
Name(s) ____________________________________________________________________________
Section ____________________ Instructor ________________________Date ___________________
Purpose:
Table 1
mass of aluminum
mass of alum
melting point
crystals (instructor's initials)
Observations:
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Conversion of aluminum scrap into a useful substance
Analysis of data (show all calculations neatly)
A.
Equation (2) above represents the dissolving of the aluminum metal. Write an equation that
represents the formation of the product from the aluminum-hydroxide complex ion.
B.
What is the theoretical yield of alum, based on the mass of aluminum used?
C.
What is the actual yield of alum?
D.
What is the per cent yield?
E.
Why were you instructed to dissolve the aluminum in the hood?
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Conversion of aluminum scrap into a useful substance
F.
What is the function of the seed crystal? (Answer this question even if you did not use any.)
G.
How might the observed melting point be affected if you heated the bath rapidly, say 10oC per
minute?
H.
With the apparatus you used, what is the maximum melting point that could be determined?
Explain.
I.
Why is(was) it possible to obtain more alum crystals from the filtrate?
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Conversion of aluminum scrap into a useful substance
J.
State your conclusions.
Name ________________________________________________ Percent Contribution _________
Name ________________________________________________ Percent Contribution _________
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Conversion of aluminum scrap into a useful substance