Wednesday, May 4, 2011

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 is in equilibrium with H3O+ + OH



This equilibrium applies to pure water and any aqueous solution.
The value of Kw 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.
Kw = [H3O+][OH-]
At 25oC, the value of Kw 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.
Equations to find [OH] and [H3O} using Kw
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 .


No comments:

Post a Comment