Synthesis of Alum Lab
This synthesis reaction involves a
redox reaction and the formation of a
complex ion
Balanced Net Ionic Equations
Mole ratios of Al to K to alum are all 1:1.
 Aluminum hydroxide dissolves in a solution with
excess KOH:
Al(OH)3(s) + OH-(aq)  [Al(OH)4]-(aq)
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Aluminum hydroxide dissolving in acid solution:
Al(OH)3(s) + 3H+(aq) + 3H2O(l)  [Al(H2O)63+]-(aq) + 3H2O(l)
Lab Discussion Questions
a)
Aluminum reacts with KOH and water to form
tetrahydroxoaluminate(III) ions, potassium ions
and hydrogen gas:
2Al(s) + 2K+(aq) + 2OH-(aq) + 6H2O(l)  2[Al(OH)4]-(aq) + 2K+(aq) + 3H2(g)
The oxidation and reduction half reactions:
Oxidation:
2Al(s) + 8OH-(aq) + 6H2O(l)  2[Al(OH)4]-(aq) + 6eReduction:
6H2O(l) + 6e-  3H2(g) + 6OH-(aq)
b)
Hydrogen ions from the acid react with
tetrahydroxoaluminate(III) ions to precipitate
aluminum hydroxide:
H+(aq) + [Al(OH)4]-(aq)  Al(OH)3(s) + H2O(l)
c) Aluminum hydroxide reacts with additional hydrogen
ions and water to form hydrated aluminum ions:
Al(OH)3(s) + 3H2O(l) + 3H+(aq)  [Al(H2O)6]3+(aq)
d) Alum forms from potassium ions, hydrated aluminum
ions sulfate ions and water:
K+(aq) + 6H2O(l) + 2SO42-(aq) + [Al(H2O)6]3+(aq) KAl(SO4)2.12H2O(s)
Meanings of These Formulae
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A complex ion consists of a central metal cation
(usually derived from a transition metal) joined by
coordinate covalent bonds to molecules or anions
called ligands. Complex ions can be cations or
anions.
Compounds composed of a complex ion and counter
ions are coordinate compounds
Hydrates are indicated by the dot and number of
water molecules incorporated into the compound.
When finding molar mass of a hydrate, add the
indicated number of water molecules (12H2O = (12 x
18 g/mol).
Relationship between coordinate
compound and complex ion
Coordinate
Compound
Complex Ion
Counter Ion
Cu(H2O)42+
SO42-
Al(SO4)2-
K+
[Al(H2O)6]2(SO4)3
Al(H2O)63+
SO42-
K3[Fe(CN)6]
Fe(CN)63-
K+
[Cu(H2O)4]SO4
K[Al(SO4)2]
Common complex ions
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The number of ligands (things attached to the metal) is
usually double the oxidation number of the metal.
Transition metals with NH3, OH-, or SCN- often form
complexes.
NH3 is added to Cu2+: Cu(NH3)42+ cupric tetraamine
NH3 is added to Ag+: Ag(NH3)2+ silver diamine
Conc. OH- is added to Zn(OH)2: Zn(OH)42+
Fe3+ in thiocyanate (SCN-): Fe(SCN)63- complex
Co2+ with chlorine: CoCl42Al oxidized in base: Al(OH)4-
Counting the Charges
Naming Coordination Complexes
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Cation name precedes anion name
The complex ion:
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Ligands are listed first then central metal ion
Negative ligands are often named by replacing the ion
ending with an –o. (chloro, bromo, nitrito, etc.).
Examples of ligand names are chloro, hydroxo, cyano
Ligand Names Not Ending in –o:
H2O - aqua
NH3 - ammine
CH3NH2 - methylamine
CO - carbonyl
NO – nitrosyl
S2O32--- thiosulfato
WRITING COMPLEX ION
FORMULAS
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write the central ion first
then negative ligands (alphabetical order)
then neutral molecules (alphabetical order)
polyatomics always in parentheses (even if 1)