Question

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

Short answer

The presence of a common ion lowers the solubility of an ionic compound due to Le Chatelier's Principle, which states that a system in equilibrium will shift its position to counteract any changes. When an ionic compound dissolves in a solution containing one of its ions (the common ion), the equilibrium will shift towards the side of the solid ionic compound to restore balance. As a result, the solubility of the ionic compound decreases in the solution. For example, when silver chloride (AgCl) dissolves in a solution with sodium chloride (NaCl), the chloride ion (Cl-) is the common ion, and the system shifts to favor the formation of solid silver chloride, thus decreasing its solubility.

Step-by-step solution

Understanding Solubility of Ionic Compounds

Solubility is the maximum amount of a solute that can dissolve in a given solvent at a given temperature to form a homogeneous solution. Ionic compounds are compounds made up of positively charged ions (cations) and negatively charged ions (anions) held together by electrostatic forces. The solubility of an ionic compound can be highly dependent on the presence of other ions in a solution.

Understanding the Common Ion Effect

The common ion effect refers to the effect of a common ion on the solubility of an ionic compound. A common ion is an ion that is present in both the ionic compound and the solution it is dissolving in. For example, if we dissolve silver chloride (AgCl) in a solution that already contains chloride ions (Cl-), the chloride ions from the AgCl will be the common ion.

Understanding Le Chatelier's Principle

To explain the common ion effect, we'll need to understand Le Chatelier's Principle. Le Chatelier's Principle states that if a system in equilibrium is subjected to a change in concentration, temperature, or pressure, the system will shift its equilibrium position to counteract the applied change and restore the equilibrium. In the context of solubility, this means that if an ionic compound is dissolved in a solution containing one of its ions (the common ion), the equilibrium will shift to the side of the solid ionic compound to restore the balance.

Explaining the Lowering of Solubility by the Common Ion

When an ionic compound dissolves in a solution with a common ion, the concentration of that ion in the solution will increase. According to Le Chatelier's Principle, the system will shift its equilibrium position to counteract this change. In this case, the equilibrium position will shift toward the side of the solid ionic compound, ultimately reducing the solubility of the ionic compound in the solution. This means that the presence of a common ion reduces the solubility of an ionic compound in a solution.

Example: Effect of a Common Ion on the Solubility of an Ionic Compound

Let's consider an example to illustrate the common ion effect. Suppose we have a solution that contains silver chloride (AgCl) and sodium chloride (NaCl). The ionic dissociation reaction for silver chloride is: \( AgCl(s) \rightleftharpoons Ag^{+}(aq) + Cl^{-}(aq) \) In this case, the chloride ion (Cl-) is the common ion since it's present in both the silver chloride and sodium chloride. When we add sodium chloride to the solution, the concentration of chloride ions (Cl-) will increase. According to Le Chatelier's Principle, the equilibrium will shift to the left, favoring the formation of solid silver chloride (AgCl). This means that the solubility of silver chloride in the solution will be decreased by the presence of the common ion, chloride.

Key concepts

Solubility of Ionic Compounds

Solubility refers to how much of a substance (the solute) can be dissolved in a given quantity of solvent at a specific temperature to form a saturated solution. When we focus on ionic compounds, these are made of cations and anions, which dissolve in water to become ions in solution.

For example, when table salt (sodium chloride, NaCl) dissolves in water, it dissociates into sodium (Na⁺) and chloride (Cl⁻) ions. The solubility of ionic compounds is not a fixed value; instead, it varies depending on factors such as temperature, pressure, and the presence of other substances in the solution which can influence the dissolution process.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept that offers insight into the response of a system at equilibrium upon disturbance. If a dynamic equilibrium is disrupted by altering conditions such as concentration, temperature, or pressure, the equilibrium will readjust to counteract the change.

This principle can be applied to chemical reactions, including those involving solubility. When a change occurs—like adding more of a common ion—the system seeks to minimize that change, leading to predictable shifts in the equilibrium state. It is Le Chatelier's Principle that illuminates why adding more of a common ion decreases the solubility of an ionic compound.

Equilibrium in Chemistry

Chemical equilibrium is the state where the rates of the forward and reverse reactions in a chemical process are equal, resulting in no net change in the concentrations of the reactants and products. It's important to understand that this is a dynamic condition—molecules constantly react to form products and vice versa, but their concentrations don't change over time.

Equilibrium can be represented visually with a double-headed arrow in a chemical equation, indicating that the reaction is reversible. When discussing solubility, this balance between dissolved ions and undissolved solute illustrates the dynamic nature of the dissolution and crystallization processes.

Ionic Dissociation

Ionic dissociation is the process whereby an ionic compound separates into its constituent ions when it dissolves in a solvent like water. This is a reversible reaction, as solid compounds can re-form from their ions. The dissociation of an ionic compound in water is governed by the equilibrium between the solid and the dissolved ions.

For instance, when silver chloride (AgCl) is placed in water, it dissociates into silver (Ag⁺) and chloride (Cl⁻) ions to a certain extent, depending on its solubility product. Understanding ionic dissociation is crucial for grasping concepts like the common ion effect, as it directly influences the solubility of ionic compounds in a solution.