The common-ion effect | Equilibrium | AP Chemistry | Khan Academy

TL;DR

Adding a common ion to a saturated solution can decrease the solubility of a slightly soluble salt.

Transcript

• [Instructor] The presence of a common ion can affect a solubility equilibrium. For example, let's say we have a saturated solution of lead II chloride. Lead II chloride is a white solid, so here's the white solid on the bottom of the beaker. And the solid's at equilibrium with the ions in solution. So that would be Pb2+ and Cl-. Notice how the mo... Read More

Key Insights

• 🙂 The presence of a common ion in a solubility equilibrium can decrease the solubility of a slightly soluble salt.
• 😑 Le Chatelier's principle predicts that the system will shift to counteract the increase in concentration of the common ion.
• ❓ The reaction quotient (Qsp) is used to determine whether the system is at equilibrium or not.
• 😑 The molar solubility of a slightly soluble salt in the presence of a common ion is lower than its molar solubility without the common ion.
• 😑 The decrease in solubility due to the common ion effect can be quantitatively predicted using calculations with an ICE table.
• 🙂 Adding a common ion can decrease the solubility of a slightly soluble salt by a factor of 10 or more.
• 😑 The common ion effect is observed in various chemical systems, not just in the solubility of salts.

Questions & Answers

Q: What is the common ion effect?

The common ion effect refers to the decrease in solubility of a slightly soluble salt that occurs when a solution already contains an ion that is also present in the salt.

Q: How does Le Chatelier's principle explain the common ion effect?

Le Chatelier's principle states that a system at equilibrium will shift to counteract any changes or stresses imposed upon it. In the case of the common ion effect, if the concentration of the common ion is increased, the system will shift in the direction of decreasing the concentration of the common ion.

Q: How does the addition of a common ion affect the solubility of a slightly soluble salt?

The addition of a common ion decreases the solubility of a slightly soluble salt by shifting the equilibrium towards the formation of more solid precipitate.

Q: What is the relationship between the reaction quotient (Q) and the common ion effect?

When a common ion is added, the value of the reaction quotient (Qsp) increases, indicating that the system is not at equilibrium. The system will then shift to the left to decrease the value of Qsp and re-establish equilibrium.

Summary & Key Takeaways

• The presence of a common ion in a solubility equilibrium can affect the solubility of a slightly soluble salt.

• When a common ion is added to a saturated solution, the system is no longer at equilibrium and will shift to decrease the concentration of the common ion.

• The common ion effect can be explained using Le Chatelier's principle and the reaction quotient (Q).

• The molar solubility of lead II chloride in the presence of a common ion is lower than its molar solubility without the common ion.