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Home » Class 11 » Chemistry » Ionic Equilibrium » Common Ion Effect

Common Ion Effect

Last Updated on March 20, 2019 By Mrs Shilpi Nagpal 4 Comments

If to an ionic equilibrium, ABsign of equilibrium reaction A+ + B‾ , a salt containing a common ion is added, the equilibrium shifts in the backward direction. This is called common Ion effect.

Acetic acid being a weak acid, ionizes to a small extent as:

CH3COOH sign of equilibrium reaction CH3COO‾ + H+        

To this solution , suppose the salt of this weak acid with a strong base is added. It ionizes almost completely in the solution as follows and provides the common acetate ions:

CH3COONa ————> CH3COO‾ + Na+

the concentration of CH3COO‾  ions increases and by Le Chatelier’s principle, the dissociation equilibrium shift backwards i.e. dissociation of acetic acid is further suppressed.

2) To the solution of the weak base , NH4OH , NH4Cl is added which provides the common NH4+ ions ,dissociation of NH4OH is suppressed:

NH4OH sign of equilibrium reaction NH4+ +OH‾

NH4Cl ———->NH4+ +Cl‾

3) To the solution of silver chloride in water, if NaCl is added which provide the common Cl‾ ions, the solubility AgCl decreases :

AgCl sign of equilibrium reactionAg+ + Cl‾

NaCl ——–> Na+ + Cl‾

Increase in the concentration of Cl‾ ions shifts the equilibrium in the backward direction i.e. some solid AgCl separates out.

If to the solution of a weak electrolyte, which ionises to a small extent ,a strong electrolyte having a common ion is added which ionizes almost completely, the ionization of the weak electrolyte is further suppressed .

If to the solution of a sparingly soluble salt, if a soluble salt having a common ion is added ,the solubility of the sparingly soluble salt further decreases.

Filed Under: Chemistry, Class 11, Ionic Equilibrium Tagged With: common ion effect, examples of common ion effect

About Mrs Shilpi Nagpal

Author of this website, Mrs Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading Free Education to everyone.

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  1. Mahender says

    November 13, 2017 at 9:13 pm

    Thanxs

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    • Mrs Shilpi Nagpal says

      November 14, 2017 at 11:05 am

      Welcome

      Reply
  2. Sanjeevani Baluni says

    December 11, 2018 at 9:49 pm

    Thank u so much

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  3. EMMANUEL NDEZURE says

    June 21, 2019 at 9:43 pm

    good work done,keep it up

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