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:

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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 \(\text{25}\) \(\text{℃}\)

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This is a lesson from the tutorial, Acid-Base and Redox Reactions and you are encouraged to log in or register, so that you can track your progress.

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