Amphoterism
Home Up Proton Transfer Reactions Conjugate Pairs Amphoterism

 

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 rtarrow.gif (850 bytes) NH4+ + OH-

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 rtarrow.gif (850 bytes) Cl- + H3O+

 

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 rtarrow.gif (850 bytes) OH- + H3O+

 

Bicarbonate ion is also amphoteric. It can act either as an acid or a base as shown in these reactions.
HCO3- + OH- rtarrow.gif (850 bytes) CO32- + H2O
HCO3- + H3O+ rtarrow.gif (850 bytes)   H2CO3 + H2O

 

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-).

Pb(OH)2

In this reaction Pb(OH)2 is reacting as a base.

Pb(OH)2(s) + 2 H+ rtarrow.gif (850 bytes) Pb2+(aq) + 2 H2O

In this reaction Pb(OH)2 is reacting as an acid.

Pb(OH)2(s) + 2 OH- rtarrow.gif (850 bytes) Pb(OH)42-(aq)

We can make use of this property of some metal hydroxides to help identify them later in this lesson.

 

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