Auto-Ionization of Water
\(K_{w}\)
Water ionizes to a very small extent:
\(\text{H}_{2}\text{O}(\text{l}) + \text{H}_{2}\text{O}(\text{l})\) \(\rightleftharpoons\) \(\text{H}_{3}\text{O}^{+}(\text{aq}) + \text{OH}^{-}(\text{aq})\)
This type of reaction (the transfer of a proton between identical molecules) is known as auto-protolysis. This reaction is also known as the auto-ionisation of water and the ions formed are a conjugate acid and base pair of water:
Definition: Auto-protolysis and auto-ionisation of water
Auto-protolysis is the transfer of a proton between two of the same molecules. The auto-ionisation of water is one example of auto-protolysis.
\(\text{K}_{\text{w}}\) is the equilibrium constant for this process:
\(\text{K}_{\text{w}}\) = \([\text{H}_{3}\text{O}^{+}][\text{OH}^{-}]\)
At \(\text{25}\) \(\text{℃}\): \([\text{H}_{3}\text{O}^{+}]\) = [\(\text{H}^{+}\)] = \(\text{1} \times \text{10}^{-\text{7}}\), therefore:
\(\text{K}_{\text{w}}\) = \(\text{1} \times \text{10}^{-\text{14}}\) at \(\text{25}\) \(\text{℃}\)
This lesson is part of:
Acid-Base and Redox Reactions