Sodium hydroxide reacts with aluminum and water to produce hydrogen gas:
2 Al(s) + 2 NaOH(aq) + 6 H20(1) - 2 NaAl(OH)4(aq) + 3 H2()
What mass of hydrogen gas would be formed from a reaction of 1.24 g Al and 4 .50 g NaOH in water?
Answers
Answer:
Synthesis of Alum from Aluminum
OBJECTIVES
• Become more familiar with single-replacement redox reactions
• Practice mass and volume measurement techniques
• Calculate the theoretical yield and percent yield of a synthesis reaction
DISCUSSION
Alums are ionic compounds that crystallize from solutions containing sulfate ion, a trivalent
cation such as Al3+, Cr3+, or Fe3+ and a monovalent cation such as K+
, Na+
, or NH4
+
. Six of the
water molecules bind tightly to the trivalent metal ion; the remaining six molecules bind more
loosely to the monovalent cation and the sulfate anion.
In this experiment you will prepare an alum–KAl(SO4)2·12H2O [potassium aluminum sulfate
dodecahydrate]–from scrap aluminum. This compound is widely used in dyeing fabrics, making
pickles, making paper and purifying water.
Although aluminum metal sits well above hydrogen in the activity series, it reacts only slowly
with dilute acids because a thin coating of aluminum oxide protects the metal surface. Aluminum
reacts with alkaline solutions to produce hydrogen because the excess hydroxide ion first attacks
the tough Al2O3 layer so the metal can react. Aluminum converts to the tetrahydroxoaluminate
ion Al(OH)4
–
. Slow addition of acid to a solution of this ion causes the precipitation of solid
Al(OH)3 followed by the dissolving of the precipitate to form the aluminum ion Al3+. The solid
Al(OH)3 will also dissolve in excess base due to formation of Al(OH)4
–
. We call a hydroxide that
can react with either acids or bases amphoteric.
Al(aq)
3+
add OH–
⎯ ⎯ ⎯ →
add H ← ⎯ ⎯ +⎯ Al(OH)3
conc. OH–
⎯ ⎯ ⎯ ⎯ →
add H ← ⎯ ⎯ +⎯ Al(OH)4
–
Except for metals in Group 1, Ca2+, Sr2+, and Ba2+, all metal ions form insoluble hydroxides
These insoluble hydroxides dissolve in acid solution. Amphoteric hydroxides also dissolve in
strong base solutions (concentrated solutions of hydroxide ion made by dissolving solid NaOH
or KOH in water).
The equations given below show the complete sequence of reactions involved in our experiment.
1. Reaction of aluminum with KOH (the dissolution step)
2 Al(s) + 2 KOH(aq) + 6 H2O(l) →2 KAl(OH)4 (aq) + 3 H2 (g)
2. Initial addition of sulfuric acid (precipitation of Al(OH)3)
2 KAl(OH)4 (aq) + H2
SO4 (aq) →2 Al(OH)3 (s) + K2
SO4 (aq) + 2 H2O(l)
3. Further addition of sulfuric acid (dissolving of Al(OH)3)
2 Al(OH)3 (s) + H2
SO4 (aq) → Al2 (SO4 )3 (aq) + 6 H2O(l)
4. Precipitation of alum on cooling
K2
SO4 (aq) + Al2 (SO4 )3 (aq) + 24 H2O(l) →2 KAl(SO4 )2 ⋅12 H2O(s)
It is possible to write an overall net ionic equation for the precipitation process: