Problem 32 - General Chemistry: Principles and Modern Applications

Question

Explain the following observations in terms of complex-ion formation. (a) \(\mathrm{CoCl}_{3}\) is unstable in aqueous solution, being reduced to \(\mathrm{CoCl}_{2}\) and liberating \(\mathrm{O}_{2}(\mathrm{g}) .\) Yet, \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) can be easily maintained in aqueous solution. (b) AgI is insoluble in water and in dilute \(\mathrm{NH}_{3}(\mathrm{aq})\) but AgI will dissolve in an aqueous solution of sodium thiosulfate.

Step-by-Step Solution

Answer

The stability and solubility of CoCl3, [Co(NH3)6]Cl3, and AgI are due to the formation and behavior of complex ions in solutions. CoCl3 is unstable because it doesn't form a stable complex ion in water, while [Co(NH3)6]Cl3 is stable because ammonia forms a stable complex ion with Cobalt. AgI is insoluble in water and dilute ammonia because no complex ion formation occurs, but in sodium thiosulfate solution it forms a soluble complex ion, making it soluble.

Step 1: Explain CoCl3 stability

Analyze the CoCl3 instability in aqueous solution. When CoCl3 dissolves in water, it gets reduced to CoCl2, releasing Oxygen. This happens because CoCl3 is not a stable complex ion in water.

Step 2: Explain [Co(NH3)6]Cl3 stability

Now let's analyze why [Co(NH3)6]Cl3 can be maintained easily in aqueous solution. Ammonia (NH3) forms more stable complex ions with Cobalt (Co). In this case, [Co(NH3)6]3+ is formed, which is a very stable complex ion, hence can easily be maintained in aqueous solution without breaking down.

Step 3: Explain AgI solubility in water and dilute NH3

Let's analyze the case of AgI. Silver iodide (AgI) is insoluble in water and in dilute ammonia solution because no complex ion formation occurs that could increase the solubility of AgI in these solutions.

Step 4: Explain AgI solubility in sodium thiosulfate

Silver iodide (AgI) is soluble in sodium thiosulfate solution because it forms a complex ion with thiosulfate ion (S2O3). The reaction is as shown: \[ \text{AgI} + 2 \text{S}_2\text{O}_3^{2-} \rightarrow [\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-} + \text{I}^- \] This complex ion is soluble in water, making AgI soluble in the sodium thiosulfate solution.

Key Concepts

Stability of Complex Ions

Complex ions are a cornerstone of coordination chemistry, showcasing how central atoms, often metals, can bond with molecules or ions called ligands to form stable structures with various applications. The stability of complex ions is key in understanding many chemical phenomena.

Stability is influenced by several factors, including the nature of the central metal and the ligands. In the example of cobalt complexes, cobalt can form bonds with different ligands, resulting in distinct levels of stability. Generally, ligands that donate electron density more effectively to the central metal create stronger metal-ligand bonds, thereby producing a more stable complex. The stability of a complex has profound implications, such as in the medicinal use of metal complexes in chemotherapy or magnetic resonance imaging (MRI) contrast agents.

Here, chelation effect—where ligands form multiple bonds with a single metal ion—also plays a crucial role. Chelating ligands wrap around the metal ion, dramatically enhancing the complex's stability and solubility. Such stability is highly relevant, for instance, in water treatment or metal recovery from ores.

Cobalt Ammonia Complex

When diving into the specifics of cobalt ammonia complexes, these systems exemplify how coordination chemistry can drastically alter the properties of a metal ion. Ammonia acts as a ligand, coordinating to cobalt ions to form a cobalt-ammine complex, typically with a formula of \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]\mathrm{Cl}_{3} \).

The ammonia ligands offer a strong field, which means they are good at donating electron density to cobalt. This results in a very stable octahedral complex where the cobalt is at the center surrounded by six nitrogen atoms from ammonia molecules. This robust architecture contrasts the unstable nature of cobalt chloride in water. Given its stability, the cobalt-ammine complex finds utility in various chemical sensors and catalysts, as well as in teaching laboratories to explore the principles of coordination chemistry.

Silver Iodide Solubility

The solubility of substances is a vital concept within various scientific disciplines, including environmental science, pharmacology, and chemical engineering. Silver iodide (AgI) is a compelling subject of study due to its poor solubility in water. This property has important implications, particularly in photography and cloud seeding.

Silver iodide is insoluble in water and dilute ammonia because it lacks a suitable ligand to stabilize it in solution. Nonetheless, when introduced to sodium thiosulfate, a transformation occurs. The thiosulfate ions act as ligands, forming a complex with silver ions, which dramatically increases AgI's solubility. The chemical reaction \[ \text{AgI} + 2 \text{S}_2\text{O}_3^{2-} \rightarrow [\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-} + \text{I}^- \] illustrates the complex ion that results in the water solubility of otherwise insoluble AgI. This knowledge can be applied in areas ranging from photographic film development to the detoxification of heavy metal pollutants.