Question

Both \(\mathrm{Mg}^{2+}\) and \(\mathrm{Cu}^{2+}\) are present in the same aqueous solution. Which of the following reagents would work best in separating these ions, precipitating one and leaving the other in solution: \(\mathrm{NaOH}(\mathrm{aq}), \mathrm{HCl}(\mathrm{aq})\), \(\mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq}),\) or \(\mathrm{NH}_{3}(\mathrm{aq}) ?\) Explain your choice.

Short answer

The best reagent to separate Mg²⁺ and Cu²⁺ ions is \(\mathrm{NH}_{3}(\mathrm{aq})\). It precipitates the Cu²⁺ ions, leaving the Mg²⁺ ions in solution.

Step-by-step solution

Study the behaviour of Mg²⁺ and Cu²⁺ with NaOH(aq)

Sodium hydroxide (NaOH) reacts with both Mg²⁺ and Cu²⁺ to form precipitates. Therefore, it will not be suitable as it will precipitate both ions.

Study the behaviour of Mg²⁺ and Cu²⁺ with HCl(aq)

Hydrochloric acid (HCl) does not react with either Mg²⁺ or Cu²⁺ in a way that would precipitate one and not the other. Hence, it's not suitable here.

Study the behaviour of Mg²⁺ and Cu²⁺ with NH₄Cl(aq)

Ammonium chloride (NH₄Cl) does not have the ability to precipitate either of the two ions. It will, thus, not serve the purpose.

Study the behaviour of Mg²⁺ and Cu²⁺ with NH₃(aq)

Ammonia (NH₃) forms a precipitate with Cu²⁺ ions, but not with Mg²⁺ ions. Therefore, by adding NH₃(aq), we can selectively precipitate the Cu²⁺ ions while leaving the Mg²⁺ ions in the solution.

Key concepts

Precipitation Reactions

Precipitation reactions are fascinating chemical processes where two dissolved substances react to form an insoluble compound. This compound, known as a precipitate, separates out of the mixture, often appearing as tiny solid particles that can be seen settling or floating within the liquid. Imagine two dancers, \(\mathrm{Mg}^{2+}\) and \(\mathrm{Cu}^{2+}\), swirling in a dance hall of water. When the right music—or in chemical terms, the right reagent—plays, one of these ions seizes the moment and pairs with the reagent to take a solid form, leaving the other still in the dance of solubility. In the given exercise, ammonia (\(\mathrm{NH}_3(\mathrm{aq})\)) is the tune that pulls \(\mathrm{Cu}^{2+}\) out of the aqueous waltz to form a precipitate, while \(\mathrm{Mg}^{2+}\) continues to dance in solution.

By understanding and applying this concept, we harness the chemical properties of ions to separate them. For instance, in wastewater treatment, precipitation reactions help remove harmful metals by turning them into solid waste that can be collected and disposed of properly.

Solubility Rules

To predict which reagent will force one of our aqueous dancers to leave the floor, we refer to solubility rules. These rules are like a codebook that tells us which compounds dissolve in water and which form precipitates. The solubility rules state that all sodium, potassium, and ammonium compounds are soluble, which informs us why neither \(\mathrm{NaOH}\) nor \(\mathrm{NH}_4\mathrm{Cl}\) can separate \(\mathrm{Mg}^{2+}\) and \(\mathrm{Cu}^{2+}\).

However, there's always an exception to the rule: \(\mathrm{Cu}^{2+}\) ions form a precipitate with hydroxide ions in the presence of ammonia due to complex ion formation, which brings us back to our solution, opting for ammonia to precipitate \(\mathrm{Cu}^{2+}\) ions. Knowing these rules is essential for students and chemists alike to predict outcomes in chemical reactions and prevent trial and error in both laboratories and real-world applications.

Chemical Properties of Ions

Each ion in a solution holds a unique set of chemical properties, much like every person has a different set of fingerprints. In our problem, understanding the behaviors of \(\mathrm{Mg}^{2+}\) and \(\mathrm{Cu}^{2+}\) ions in the presence of various reagents helped us reach the solution. \(\mathrm{Cu}^{2+}\) ions with ammonia \(\mathrm{NH}_3\) not only create a precipitate but also a deep blue complex ion, \(\mathrm{[Cu(NH_3)_4]^{2+}}\), which is responsible for the precipitation seen.

Ions react differently depending on their electron configuration, charge density, and the presence of other ions or molecules. This individuality governs their solubility, ability to form complexes, and overall reactivity. As such, when we're faced with a mixture containing multiple ions, like we were in this exercise, it's their unique chemical properties that guide us to selectively precipitate and separate them, as we do with \(\mathrm{Cu}^{2+}\) using \(\mathrm{NH}_3\).