Question

The most soluble of the following solids in \(\mathrm{NH}_{3}(\mathrm{aq})\) is (a) \(\mathrm{Ca}(\mathrm{OH})_{2} ;\) (b) \(\mathrm{Cu}(\mathrm{OH})_{2} ;\) (c) \(\mathrm{BaSO}_{4} ;\) (d) \(\mathrm{MgCO}_{3}\) (e) \(\overline{\mathrm{Fe}_{2} \mathrm{O}_{3}}\).

Short answer

\(\mathrm{Cu}(\mathrm{OH})_{2}\)

Step-by-step solution

Understanding Solubility

Solubility is the ability of a solute to dissolve in a solvent. Ammonia has the ability to form complex ions with certain metal ions, hence increasing the solubility of their salts. In this case, it essentially transforms an otherwise insoluble salt into a soluble complex.

Checking Each Option

We need to check each given compound in the options to see which forms a complex ion with ammonia and hence, will be most soluble in \(\mathrm{NH}_{3}(\mathrm{aq})\). (a) \(\mathrm{Ca}(\mathrm{OH})_{2}\), (c) \(\mathrm{BaSO}_{4}\), (d) \(\mathrm{MgCO}_{3}\) and (e) \(\overline{\mathrm{Fe}_{2} \mathrm{O}_{3}}\) do not form stable complexes with ammonia. However, (b) \(\mathrm{Cu}(\mathrm{OH})_{2}\) forms a deep blue complex ion with ammonia, \(\mathrm{[Cu(NH}_{3}\mathrm{)_4]^{2+}}\) and is therefore, more soluble in ammonia than the other compounds.

Determining the Most Soluble Solid

The most soluble solid in \(\mathrm{NH}_{3}(\mathrm{aq})\), among the given options, is therefore, \(\mathrm{Cu}(\mathrm{OH})_{2}\).

Key concepts

Complex Ions

Complex ions play a significant role in solubility, especially when dealing with metal ions and ligands. A complex ion consists of a central metal ion bonded to one or more molecules or ions called ligands. These ligands donate electrons to the metal to form coordinate bonds, significantly altering the chemical properties of the solute.
In the context of this exercise, ammonia acts as a ligand, and it can form complex ions with certain metal ions. For instance, the copper ion ( Cu^{2+} ) readily interacts with ammonia ( NH_3 ), creating the complex ion [ Cu(NH_3)_4]^{2+} . This particular interaction is notable because it transforms what is typically an insoluble compound, like copper hydroxide ( Cu(OH)_2 ), into a soluble form through complex ion formation.
Complex ion formation is crucial because it varies significantly among different metals. Some metals, like copper, have a strong affinity for complexing with ammonia, while others do not exhibit this behavior. Understanding which metals can form complexes with a given solvent, such as ammonia, can be instrumental in predicting solubility patterns in aqueous solutions.

Ammonia as a Solvent

Ammonia is an interesting and effective solvent due to its ability to act both as a base and as a ligand. In aqueous solutions, ammonia can dissolve compounds that may not typically dissolve in water. This unique characteristic mainly comes from its ability to form complexes with certain metal ions.
When ammonia is present in a solution, it can interact with metal ions like copper to form stable, soluble complex ions. A quintessential example is its reaction with copper hydroxide. Here, ammonia donates a pair of electrons to the copper ion, facilitating the formation of the soluble [ Cu(NH_3)_4]^{2+} complex ion. This process highlights ammonia's dual role as a solvent and a ligand, adding versatility beyond its basic properties.
The ability of ammonia to enhance solubility through complex formation is integral in many laboratory processes and industrial applications. It allows for the manipulation of solubility and paves the way for separating and purifying compounds in a mixed system.

Solubility Rules

Understanding solubility rules is vital for predicting whether a substance will dissolve in a particular solvent. General solubility rules provide guidance, but certain situations, such as the presence of complex ion formation, can alter typical solubility expectations.
In aqueous solutions, many hydroxides like Ca(OH)_2, BaSO_4, and MgCO_3 are only sparingly soluble. This means they do not dissolve well in water under normal conditions. However, when complexation occurs, such as with Cu(OH)_2 and ammonia, the solubility rules are essentially bypassed, resulting in a soluble copper-ammonia complex. This tells us that while standard solubility rules can serve as a useful initial guide, the formation of complex ions should always be considered in real-world scenarios.
By understanding both solubility rules and how complex formation might affect them, you can better predict and control the behavior of substances in different chemical environments. This is particularly important in fields like chemistry and chemical engineering, where knowledge of solubility can influence reaction outcomes and product yields.