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Amphoterism
Hopefully you noticed in previous examples that water was sometimes an
acid and sometimes a base. The ability of some chemicals to act either as
an acid or a base is called amphoterism. Whether an
amphoteric chemical acts as an acid or a base depends on what other chemicals happen to be
around.
| If a base ( like NH3) is present, water can act as an
acid and react by donating a proton to that base. In doing so, water is changed
into its conjugate base, hydroxide ion. |
H2O + NH3  NH4+ + OH- |
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| If an acid (like HCl) is present, water can act as a base
and react by accepting a proton from that acid. In doing so, water is changed into its
conjugate acid, hydronium ion. |
H2O + HCl Cl- + H3O+ |
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| The self-ionization of water is another example of water
being able to react either as an acid or a base. The molecules in pure water continuously
collide and react with one another. In that reaction, one water molecule can transfer a
proton to another water molecule. One water molecule acts as an acid and the other acts as
a base. The solution is neutral because equal quantities of H3O+ and
OH- are made. |
H2O + H2O OH- + H3O+ |
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| Bicarbonate ion is also amphoteric. It can act either as an acid or a base
as shown in these reactions. |
| HCO3- + OH- CO32- + H2O
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| HCO3- + H3O+
H2CO3
+ H2O |
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| Some metal hydroxides have amphoteric properties, such as lead(II)
hydroxide, shown here. This insoluble compound can be made to dissolve by reacting with
either acid (H+) or base (OH-). |
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| In this reaction Pb(OH)2 is reacting as a base. |
Pb(OH)2(s) + 2 H+ Pb2+(aq) + 2 H2O |
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| In this reaction Pb(OH)2 is reacting as an acid. |
Pb(OH)2(s) + 2 OH- Pb(OH)42-(aq) |
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| We can make use of this property of some metal hydroxides to help identify
them later in this lesson. |
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Eden Francis
Clackamas Community College
©1998, 2003 Clackamas Community College, Hal Bender
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