Can H2O act as a lewis acid, lewis base, or both?

1 Answer

Water can act as both a Lewis acid and a Lewis base.


Water is amphoteric: this means that it can act both as an acid and as a base. The reason for this is that water is self-ionising. Observe the following equation:

#color(red)(2)"H"_2"O" rightleftharpoons "H"_3"O"^+ + "OH"^-#

Source: Wikipedia

This reaction is reversible, because the hydronium ion (#"H"_3"O"^+#) is a strong acid and the hydroxide ion (#"OH"^-#) is a strong base, they will react together in the opposite direction to form water. These reactions occur according to the ionisation constant of water, #"P"_w#, which is #1.0 * 10^(-14)# at room temperature.

As mentioned above, since the hydronium and hydroxide ions are quite strong acids and bases, respectively, the self-ionisation of water allows it to function both as a Lewis acid and as a Lewis base via protonation.