Common Ion Effect

Definition

The common ion effect is a principle in chemistry that describes the influence of a common ion on the solubility of a salt or the position of a chemical equilibrium. It is a fundamental concept that underlies various equilibrium processes in chemistry, including equilibrium calculations, hydrolysis of salts, buffer solutions, precipitation and dissolution, and coupled equilibria.

5 Must Know Facts For Your Next Test

1. The common ion effect decreases the solubility of a salt by suppressing the dissociation of the salt in the presence of a common ion.
2. In the hydrolysis of salts, the common ion effect can shift the equilibrium towards the reactants, reducing the extent of hydrolysis.
3. The common ion effect is a key principle in the preparation and use of buffer solutions, as it helps maintain a relatively constant pH.
4. The common ion effect influences the solubility of salts and can lead to the precipitation or dissolution of a solid phase in a solution.
5. The common ion effect is a crucial consideration in coupled equilibria, where the presence of a common ion affects the position of multiple equilibria simultaneously.

Review Questions

• Explain how the common ion effect can be used to decrease the solubility of a salt in a solution.
  • The common ion effect states that the presence of a common ion in a solution will decrease the solubility of a salt. This is because the common ion suppresses the dissociation of the salt, shifting the solubility equilibrium towards the undissociated salt. By adding a compound that shares an ion with the salt of interest, the concentration of that ion in the solution increases, causing the salt to become less soluble and potentially precipitate out of the solution.
• Describe the role of the common ion effect in the hydrolysis of salts and the maintenance of buffer solutions.
  • In the hydrolysis of salts, the common ion effect can shift the equilibrium towards the reactants, reducing the extent of hydrolysis. For example, the addition of a common ion, such as the conjugate base of a weak acid, can suppress the hydrolysis of the salt's conjugate base, leading to a decrease in the solution's pH. Similarly, the common ion effect is a key principle in the preparation and use of buffer solutions, as it helps maintain a relatively constant pH by counteracting changes in the concentration of hydrogen ions.
• Analyze how the common ion effect can influence the precipitation and dissolution of solids in a solution, and how this concept is related to coupled equilibria.
  • The common ion effect can significantly impact the precipitation and dissolution of solids in a solution. By adding a compound that shares a common ion with a salt, the solubility of that salt can be decreased, leading to precipitation. Conversely, the addition of a common ion can also increase the solubility of a salt, causing it to dissolve. This principle is particularly relevant in the study of coupled equilibria, where the presence of a common ion affects the position of multiple equilibria simultaneously. Understanding the common ion effect is crucial in predicting and manipulating the solubility and precipitation of salts, as well as in analyzing the behavior of complex, coupled equilibrium systems.

Related terms

Solubility Equilibrium: The equilibrium between a solid salt and its dissolved ions in a saturated solution.

Le Chatelier's Principle: A principle stating that a system at equilibrium will shift to counteract any change in conditions in order to reach a new equilibrium state.

Ionic Strength: A measure of the concentration of ions in a solution, which affects the activity coefficients of the ions and the solubility of salts.