Some metal hydroxides are amphoteric, or capable of acting as either
an acid or base.
LEARNING OBJECTIVE [ edit ]
Identify the conditions under which amphoteric hydroxides act as acids.
KEY POINTS [ edit ]
Amphoteric molecules can act as either acids or bases.
Hydroxides with highly­charged metal cations are often amphoteric.
Amphoteric hydroxides act as either Bronsted­Lowry bases (accepting protons) or Lewis
acids (accepting an electronpair), depending on reaction conditions.
TERMS [ edit ]
Lewis acid
an electron pair acceptor
base
a proton acceptor or an electron pair donor
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In our discussion of acids and bases so far, hydroxides have always been synonymous with
bases. Now we will examine amphoteric hydroxides—that is, hydroxide salts that can act as
either an acid or a base, depending on reaction conditions. We will begin with the familiar
case of a hydroxide acting as a base.
Reaction of Amphoteric Hydroxides in AcidicSolution
One of the most common and familiar examples of an amphoteric hydroxide
is aluminum hydroxide, Al(OH)3. From our solubility rules, we know that Al(OH)3 is
largelyinsoluble in neutral water; however, in a strongly acidic solution, the situation
changes. For example, consider the reaction of Al(OH)3 with HCl:
HCl(aq) + Al(OH ) 3 (aq) → AlC l3 (aq) + 3H 2 O(l)
This is a classic acid­base neutralization reaction: the HCl completely protonates all three
hydroxides per mole of Al(OH)3, yielding pure water and the salt AlCl3. From what we know
about the basic properties of hydroxides so far, this is exactly as we would expect—so how
can a hydroxide act as an acid?
Reaction of Amphoteric Hydroxides in Basic Solution
We will now consider aluminum hydroxide's reaction in a strongly basic solution:
−
Al(OH ) 3 (aq) + OH − (aq) → Al(OH ) 4 (aq)
Here, aluminum hydroxide picks up an hydroxide ion out of solution, thereby acting as a
Lewis acid. How is this possible? Consider the Lewis structure for Al(OH)3.
Aluminum hydroxide
Aluminum hydroxide can act as either a Bronsted­Lowry base, by accepting protons from an acidic
solution, or as a Lewis acid, by accepting an electron pair from hydroxide ions in a basic solution.
The central aluminum atom is electron deficient—it forms only three bonds, and the octet
rule is not fulfilled; thus, Al3+is quite happy to accept a pair of electrons and form another
bond, given the right conditions. In a basic solution, it forms a bond with an OH­ ion, pulling
it out of solution and lowering the solution's pH.
Conclusion
Metal hydroxides with a highly­charged central metal atom can be amphoteric. In addition to
aluminum, metals such as zinc, tin, lead, and beryllium can also form amphoteric oxidesor
hydroxides. Whether such hydroxides behave as acids or bases depends on
the surrounding solution's pH.