Common-ion effect
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2008-05-07T02:52:31Z
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/* Examples */
The '''common-ion effect''' is a term used to describe the effect on a solution of two dissolved solutes that contain the same ion.
The presence of a common ion suppresses the [[ionization]] of a [[weak acid]] or a [[weak base]].
==Examples==
*If both [[sodium acetate]] and [[acetic acid]] are dissolved in the same solution they both dissociate and ionize to produce [[acetate ion]]s. Sodium acetate is a [[strong electrolyte]] so it dissociates completely in solution. Acetic acid is a [[weak acid]] so it only ionizes slightly. According to [[Le Chatelier's principle]], the addition of acetate ions from sodium acetate will suppress the ionization of acetic acid and shift its [[chemical equilibrium|equilibrium]] to the left. Thus the percent dissociation of the acetic acid will decrease and the pH of the solution will increase.
:NaC<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sub>(s)</sub> → Na<sup>+</sup><sub>(aq)</sub> + C<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sup>-</sup><sub>(aq)</sub>
:HC<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sub>(l)</sub> ↔ H<sup>+</sup><sub>(aq)</sub> + C<sub>2</sub>H<sub>3</sub>O<sub>2</sub><sup>-</sup><sub>(aq)</sub>
This will decrease the hydrogen ion concentration and thus the common-ion solution will be less acidic than a solution containing only acetic acid.
"A salt will be less soluble if one of its constituent ions is already present in the solution"
== See also ==
*[[Buffer solution]]
*[[Solubility]]
*[[Solubility equilibrium]]
[[Category:Ions]]
[[Category:Physical chemistry]]
[[it:Effetto ione comune]]
[[zh:同离子效应]]