Autoionozation of water for Grade 10

The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H₃O⁺ and hydroxide, OH⁻.




Chemically pure water has an electrical conductivity of 0.055 µS·cm⁻¹. According to the theories of Svante Arrhenius, this must be due to the presence of ions. The ions are produced by the self-ionization reaction

H₂O + H₂O H₃O⁺ + OH⁻

This equilibrium applies to pure water and any aqueous solution.

The value of K_w decreases as temperature increases and it decreases with increasing pressure.

The equilibirum expression for the above reaction is written below and is treated mathematically like all equilibrium expressions.

K_w = [H₃O⁺][OH^-]
At 25^oC, the value of K_w has been determined to be 1 x 10^-14. 

If one knows the concentration of either the hydronium ions or of the hydroxide ions in a water solution,. the other ion concentration can be determined.

Example:  What is the hydronium ion concentration in a water solution that is 0.050 M NaOH?
The NaOH is a strong base and will 100% dissociate into its component ions.  Therefore, the concentration of the hydroxide ions will be 0.050 M.  The hydronium ion concentration is then calculated.

[H₃O⁺] = (1 x 10^-14)/(0.050) = 2.0 x 10^-13 M
Example:  What is the hydroxide ion concentration in a water solution that is 4.0 x 10^-5 M H₃O⁺?

[OH^-] = (1 x 10^-14)/(4.0 x 10^-5) = 2.5 x 10^-10 M .