Problem 13 - Chemistry The Central Science In Si Units

Question

Which of these statements about the common-ion effect is most correct? (a) The solubility of a salt MA is decreased in a solution that already contains either \(\mathrm{M}^{+}\) or \(\mathrm{A}^{-}\). (b) Common ions alter the equilibrium constant for the reaction of an ionic solid with water. \((\mathbf{c})\) The common-ion effect does not apply to unusual ions like \(\mathrm{SO}_{3}^{2-}\). (d) The solubility of a salt MA is affected equally by the addition of either \(\mathrm{A}\) - or a noncommon ion.

Step-by-Step Solution

Answer

Statement (a) is most correct regarding the common-ion effect.

Step 1: Understanding the Common-Ion Effect

The common-ion effect refers to the decrease in the solubility of an ionic compound (in this case, salt MA) when a solution already contains one of the ions (either M+ or A-) present in the compound. This occurs because adding more of one ion shifts the equilibrium towards the formation of the solid due to Le Chatelier's principle.

Step 1: Analyzing Statement (a)

Statement (a) claims that the solubility of the salt MA is decreased in a solution that already contains either M+ or A-. This statement aligns with the definition of the common-ion effect, which states that the presence of a common ion will suppress ion dissociation, thus decreasing solubility.

Step 2: Analyzing Statement (b)

Statement (b) suggests that common ions alter the equilibrium constant. While the presence of a common ion affects the position of equilibrium (concentration of ions in solution), it does not change the equilibrium constant itself, which is a fixed value at a given temperature.

Step 3: Analyzing Statement (c)

Statement (c) states that the common-ion effect does not apply to unusual ions like \(\mathrm{SO}_{3}^{2-}\). However, the common-ion effect can apply to all ions, including unusual ones, as long as they participate in equilibrium reactions. Thus, this statement is incorrect.

Step 4: Analyzing Statement (d)

Statement (d) claims that the solubility of salt MA is affected equally by anion A- or a noncommon ion. This is incorrect because noncommon ions will not affect the solubility in the way common ions do. Only ions already present in the equilibrium affect the solubility via the common-ion effect.

Key Concepts

Solubility

Solubility refers to the ability of a substance to dissolve in a solvent, forming a homogeneous solution at a given temperature and pressure. The solubility of a substance is influenced by several factors, such as temperature, pressure, and the presence of other substances in the solution.
In the context of a common-ion effect, solubility pertains to how an ionic compound interacts with other ions present in solution. When a common ion is introduced into the system, solubility can decrease significantly. This is because the equilibrium that governs the dissolution of the ionic compound becomes disturbed.

• This disruption occurs due to increased ion concentration, making it less favorable for the compound to dissolve further.
• Consequently, more solid compound forms, thereby reducing its solubility.

Equilibrium Constant

The equilibrium constant, often denoted as \( K \), quantifies the ratio of concentrations of products to reactants at equilibrium in a reversible reaction. For a simple dissolution reaction of a salt \(MA\), the equilibrium constant expression could be written as:\[ K_{sp} = [M^+][A^-] \]where \([M^+]\) and \([A^-]\) are the molar concentrations of ions at equilibrium.
Despite changes in the concentration of ions due to the common-ion effect, the equilibrium constant itself remains unchanged at a given temperature.

• This constancy allows chemists to predict how much of the ionic compound will dissolve under various conditions.
• The equilibrium position may shift to favor either the dissolved ions or the solid form, but this does not alter \(K\).

Le Chatelier's Principle

Le Chatelier's Principle is fundamental to understanding how equilibrium systems respond to external changes. It states that if a system at equilibrium experiences a change in concentration, temperature, or pressure, the system will adjust to counteract that change and reestablish equilibrium.
In terms of the common-ion effect, when a common ion is added to the solution, Le Chatelier's Principle elucidates why solubility decreases. By adding common ions, equilibrium shifts towards producing more of the precipitate, thereby decreasing solubility.

• This principle helps explain why adding more of an ion that is already in balance pushes the reaction towards the formation of the solid.
• It emphasizes the dynamic nature of chemical equilibria and the tendency of systems to adjust in response to external perturbations.

Ionic Compounds

Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions) held together by electrostatic forces. In solution, they dissociate into their constituent ions, which is crucial for understanding their solubility behavior.

• The presence of a common ion can influence the dissociation of an ionic compound, as described by the common-ion effect.
• Particularly, ionic compounds are susceptible to changes in solubility when exposed to solutions with similar ions.
• This characteristic is essential in many applications, such as determining the extent of solubility in lab settings and understanding how salts behave in biological systems.

Underlying all these concepts is the systematic interaction between ions in solution and how equilibrium principles govern these interactions.