Problem 68 - Chemistry Matter and Change

Question

Explain why a common ion lowers the solubility of an ionic compound.

Step-by-Step Solution

Answer

A common ion decreases solubility by shifting equilibrium to favor the formation of the undissolved solid, thus reducing the amount of the solute ions in solution.

Step 1: Understand the concept of solubility

Solubility refers to the maximum amount of a solute that can dissolve in a solvent at a given temperature. For ionic compounds, solubility is determined by the equilibrium between the solid compound and its ions in solution.

Step 2: Define the common ion effect

The common ion effect occurs when the addition of an ion (that is common to an equilibrium process) suppresses the ionization of a weak acid or a weak base. In the context of solubility, it reduces the solubility of an ionic compound when a compound containing one of its ions is added to the solution.

Step 3: Apply Le Chatelier's Principle

According to Le Chatelier's Principle, if an additional substance (a common ion) is added to a solution at equilibrium, the system will adjust to counteract this change. For ionic solids, adding a common ion shifts the equilibrium position to decrease the concentration of the ions in solution.

Step 4: Illustrate with an example

Consider a sparingly soluble salt, like silver chloride (AgCl), which dissociates in water as \[ AgCl_{(s)} \rightleftharpoons Ag^{+}_{(aq)} + Cl^{-}_{(aq)} \]. Adding sodium chloride (NaCl), which also provides \(Cl^-\) ions, increases the chloride ion concentration in solution. According to Le Chatelier's principle, the equilibrium shifts left, reducing AgCl's solubility.

Key Concepts

Understanding Solubility

Solubility refers to how much of a solute can dissolve in a solvent at a given temperature. It's a measure of the "dissolve-ability" of a compound. For ionic compounds, this involves the balance between the compound as a solid and its ions once they dissolve into the solution.
In essence, solubility tells us the maximum amount of ionic compound that can be dissolved before the solution becomes saturated.
This creates an equilibrium state, where the rate of dissolving equals the rate of precipitation, meaning the ions going into solution equal the ions coming out and forming a solid again.

Le Chatelier's Principle and Its Role

Le Chatelier's Principle helps us understand how a chemical system at equilibrium responds to changes. When something external affects the system, such as adding or removing a component, the system shifts to counteract this change and re-establish equilibrium.
For ionic compounds, adding a common ion pushes the equilibrium position, often decreasing the concentration of the compound's ions in a solution.
Thus, the principle shows us that the system will respond by reducing the solubility of the compound, maintaining stability in ion concentration levels.

Ionic Compounds in Detail

Ionic compounds form when positive and negative ions combine due to electrostatic forces. They typically form crystal lattice structures when in solid form, which is stable due to the alternating charges.
However, when an ionic compound dissolves in water, it dissociates into its constitutive ions, disrupting the stable lattice.

• Example: Silver chloride (AgCl) dissociating into \(Ag^{+}\) and \(Cl^{-}\) ions in water.

The solubility of such compounds depends largely on the strengths of the intermolecular forces in the solid versus the forces between ions and solvent molecules.

Equilibrium Adjustment with Common Ions

The introduction of a common ion into a solution alters the solubility equilibrium. This is typically referred to as the "common ion effect."
When a solution already contains an ion from another source, the equilibrium will shift to decrease the solubility of the original ionic compound, as it attempts to balance the increased ion concentration.

• Example: Adding sodium chloride (NaCl) to a silver chloride (AgCl) solution increases \(Cl^{-}\) ions, causing the equilibrium to move left and reduce AgCl solubility.

Again, Le Chatelier's Principle explains this adjustment as a natural response to maintain equilibrium.