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Chuck W. · Stefan V.
Jan 15, 2015

When an acid reacts with water, it forms a hydronium ion plus the conjugate base of the original acid. Likewise, when a base reacts with water, it forms hydroxide ion plus the conjugate acid of the original base.

Using generic symbol HA for a weak acid, we can write the following balanced proton transfer reaction with water:

#HA + H_2O harr A^(-) + H_3O^+#

Because #A^(-)# differs from #HA# only by the proton that was lost to water, we say that #A^(_)# is the conjugate base of the acid #HA#. Likewise, HA is the conjugate acid of the base #A^(-)#. In other words, #HA# and #A^(-)# form a conjugate acid-base pair.

In the same way, we can write a balanced proton transfer reaction of a base, #B#, with water:

#B + H_2O harr BH^+ + OH^(-)#

For the same reasons as above, we say that #BH^+# and #B# form a conjugate acid-base pair.

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