Common Ion Effect – Definition, Explanation, Examples

Common ion effect is a phenomenon in chemistry that describes the decrease in solubility of a compound when it is dissolved in a solution that already contains one of its constituent ions. This effect occurs due to the principle of chemical equilibrium and Le Chatelier’s principle.

In this article, we will learn in detail about the common ion effect, its impact on solubility, and its validity in the case of acids, bases, and salts. We will also learn its limitations and FAQs based on the common ion effect.

What is Common Ion Effect?

Common ion effect is a chemical phenomenon that causes a decrease in the solubility of an ionic precipitate by the addition of a soluble compound with an ion in common with the precipitate. It is based on the principle of chemical equilibrium, which states that if an equilibrium becomes unbalanced, the reaction will shift to restore the balance. The common ion effect is a result of Le Chatelier’s principle.

Explanation of Common Ion Effect

To understand the common ion effect, let us first understand the common ion. Common ion is when addition of an ion in the solution that comes from two different sources. Imagine there are two solutions, NaCl and AgCl, where Cl is common. Hence, it becomes a common ion. So the common ion effect is when the addition of an ion, already present in the solution disrupts the equilibrium of the solution.

We know that when AgCl dissolves in water, it forms Ag⁺ (silver ions) and Cl^– (chloride ions).

Now imagine in the NaCl solution we put some undissolved AgCl.

As the concentration of the common ion Cl^– increases in the solution, according to Le Chatelier’s principle, the equilibrium will shift to manage these increasing Cl^– ions. As a result, AgCl will precipitate out of the solution and reduce the solubility of AgCl in solution due to presence of dissolved ions of NaCl.

Impact of Common Ion Effect on Solubility

If you observe in the above example, solubility of AgCl was affected due to common ion effect. The other impacts of common ion effect on solubility is as described below,

Decreased Solubility due to Common Ions

As shown in NaCl and AgCl example, the equilibrium shifts due to excess chloride ions and reduces the solubility of AgCl. According to Le Chatelier’s principle, the solution precipitate out AgCl and reduces its solubility in NaCl solution due to the presence of common chloride ions.

Common Ion Effect on pH

Common ions may result in effects of disassociation of weak acid and bases in order to maintain equilibrium in the solution. It Influences the degree of dissociation of weak acids and bases by shifting the equilibrium in a particular direction that reduces the concentration of common ion and hence disrupts the pH level of solution. Let us see Common Ion Effect in both strong and weak acids and bases.

Common Ion Effect in Weak Acids and Bases

Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.

Common Ion Effect in Weak Bases

Let us consider one weak base ammonia(NH₃), we will add ammonium chloride which will decrease the dissociation of ammonia and disrupts the impact of common ion on ammonia, which will reduce basicity i.e. pH decreases of the solution.

Dissociation of ammonia in water

NH₃ + H₂O → NH₄ + OH^–

When we add ammonium chloride (NH₄Cl)

NH₄Cl → NH₄ + Cl^–

When Ammonium Chloride is added to Dissolved ammonia solution, the extra NH₄ and the common ion effect results in a decrease of OH^– and hence reduces basicity of Ammonia in water.

Common Ion Effect in Weak Acids

When acetic acid(weak acid) and sodium acetate(strong electrolyte) generate acetate ions, where sodium acetate is dissolved completely but acetic acid only partly ionizes, since common ion effect results in reduced dissociation of acetic acid, pH increases and will be less acidic.

Dissociation of acetic acid in water

CH₃COOH(acetic acid) + H₂O(water) → CH₃COO^–(acetate ion) + H₃O⁺(Hydronium ion)

When we add sodium acetate

CH₃COONa → CH3COO^– + Na⁺

which decreases the concentration of H₃O⁺ and makes the solution less acidic.

Common Ion Effect in Strong Acids and Bases

Solutions Containing Strong Acids and Bases, its common ion effect is minimal or negligible as these elements completely dissociate in water. For example, take one strong acid like HCl which dissociates in H⁺ and Cl^–. When we add NaCl on this, it will have negligible effect as H⁺ ions is already completely ionized.

Common Ion Effect in Salts

Common Ion Effect in Salts leads to reduced solubility of salts when it already contains one of the ions. The effect is seen in the form of solubility and precipitation.

Common Ion Effect in Solubility of Salts

Solubility of salts reduces in presence of common ion. Consider NaCl, when it is dissolved in water it dissociates in sodium and chloride ions and when some more chloride ions from any other source like AgCl , reduces the solubility of the salt and may result in precipitation.

Common Ion Effect in Precipitation Reactions

In Precipitation Reactions, the common ion effect reduces the solubility and results in precipitation. The increase in common ion shifts the equilibrium and decreases the solubility.

Applications of Common Ion Effect

The applications of common ion effect is mentioned below:

Control of Precipitation: The common ion effect is utilized to control the precipitation of sparingly soluble salts. For Example, Common Ion Effect is used to extract calcium carbonate out of drinking water by adding sodium carbonate. Sodium Carbonate is highly soluble while calcium carbonate is not(sparingly soluble).This helps in treatment of water and reduce calcium carbonate in water.

Buffer Solution: The common ion effect is essential in the functioning of buffer solutions. Buffer solutions are aqueous solutions that resist changes in pH when small amounts of acid or base are added to them.

Industrial Process: Common Ion effect can also be observed in salting out process of soap manufacture. The soaps are precipitated out by adding sodium chloride to the soap solution in order to reduce its solubility.

Limitations of Common Ion Effect

Common ion effect is useful in understanding equilibrium in solutions but its affects can depend on various factors like temperature and other external factors. Some of the limitations of Common ion effect are listed below,

• Common ion effect only works with strong electrolytes(where complete dissociation occurs) When weak electrolytes, affects are negligible.
• Common ion effect needs relatively high concentration of ions, as a result in dilute solutions, the changes are negligible.
• Sometimes, In Precipitation reactions, if the ratio of common ion and solution is not appropriate, precipitation will occur without common ion effect.
• Changes in temperature affect ionization and solubility equilibrium.

Conclusion

Common ion effect gives an appropriate idea of equilibrium in any solution which we have to handle carefully(take care of temperature, the presence of ions and strong electrolytes). The effect on its solubility, common ion effects in salt and common ion effects in acid or bases is discussed thoroughly in the article.

Common Ion Effect FAQs

What is meaning of common ion effect?

Common ion effect means when a compound is dissolved in a solution containing one of its component ions, the equilibrium shifts to the left, reducing the amount of dissolved compound and causing precipitation or decreased solubility.

What is a real world application of common ion effect?

The real life application of common ion effect is in Manufacturing of Soap, baking soda and water treatment are some of the real world applications.

What is the Le Chatelier principle of the common ion effect?

According to Le Chatelier’s principle, the system (the equilibrium between dissolved and undissolved ions) will adjust to counteract the addition of the common ion and restore equilibrium.

Where common ion effect is not valid?

If the reaction has no common ion, it can not undergo common ion effect. It is also not valid in case of strong electrolytes, highly diluted solutions etc.