- The equilibrium \(H_2SO_4(aq) \Leftrightarrow 2H^+(aq) + SO_4^{2-}(aq)\) involves the common ions \(H^+\) and \(SO_4^{2-}\).
- By dissociating each compound, we find that \(MgSO_4\) produces \(SO_4^{2-}\) ions.
- Adding \(MgSO_4\) increases the concentration of sulfate ions, shifting the equilibrium toward the reactants according to Le Chatelier's principle.
- The correct choice is \(\boxed{d}\).
Explanation
- Analysis of the Equilibrium System
The given equilibrium equation for the dissociation of sulfuric acid in an aqueous solution is:
\[H_2SO_4(aq) \Leftrightarrow 2H^+(aq) + SO_4^{2-}(aq)\]
In this equilibrium, the species present are the undissociated \(H_2SO_4\) molecule, the hydrogen ion (\(H^+\)), and the sulfate ion (\(SO_4^{2-}\)). The common ion effect occurs when a substance that contains an ion already present in the equilibrium is added to the system, causing the equilibrium position to shift in accordance with Le Chatelier's principle. For this equilibrium, we are looking for a compound that, when dissolved in water, releases either \(H^+\) or \(SO_4^{2-}\) ions.
- Evaluation of Compounds
We evaluate the dissociation of each potential compound in an aqueous solution to see if it contributes a common ion:
A\(FeCl_3 \rightarrow Fe^{3+}(aq) + 3Cl^-(aq)\): This compound provides iron(III) ions and chloride ions. Neither of these is a common ion in our system.
B\(KNO_3 \rightarrow K^+(aq) + NO_3^-(aq)\): This compound provides potassium ions and nitrate ions. Neither of these is a common ion in our system.
C\(NaCl \rightarrow Na^+(aq) + Cl^-(aq)\): This compound provides sodium ions and chloride ions. Neither of these is a common ion in our system.
D\(MgSO_4 \rightarrow Mg^{2+}(aq) + SO_4^{2-}(aq)\): This compound provides magnesium ions and sulfate ions. Since the sulfate ion (\(SO_4^{2-}\)) is already present in our initial equilibrium, this is a common ion.
- Conclusion
Because \(MgSO_4\) dissociates to produce sulfate ions (\(SO_4^{2-}\)), adding it to the equilibrium mixture increases the concentration of one of the products. According to Le Chatelier's principle, if the concentration of a product is increased, the equilibrium will shift to the left to counteract this change, consuming some of the added \(SO_4^{2-}\) and more \(H^+\) to form more \(H_2SO_4\). Therefore, \(MgSO_4\) is the compound that causes a shift in equilibrium due to the common ion effect. The correct choice is d.
Examples
The common ion effect is a fundamental concept in chemistry used to control the solubility of salts and the acidity of solutions. For example, in analytical chemistry, one might add a source of sulfate ions to a solution containing metal sulfates to force the precipitation of the metal salt, effectively cleaning the solution or recovering the material. This technique is vital in industrial wastewater treatment to remove heavy metal contaminants by ensuring they precipitate out of the liquid phase.