Question

Explain the difference between these two compounds: \(\mathrm{CrCl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) and \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}\)

Short answer

\( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \) is a hydrated salt, while \([\mathrm{Cr}(\mathrm{H}_2 \mathrm{O})_6] \mathrm{Cl}_3\) is a coordination complex.

Step-by-step solution

Identify the Compounds

The first compound is \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \), which indicates chromium chloride with six water molecules associated with it as waters of crystallization. The second compound is \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3} \), which is a coordination complex with chromium at the center, surrounded by six water molecules, and attached to three chloride ions.

Structural Analysis

In \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \), the water molecules are outside the coordination sphere as hydrates, which means they are loosely associated with the chromium chloride. In \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3} \), the water molecules are coordinated to the chromium ion, forming a coordination bond. The chloride ions are situated outside the primary coordination sphere, meaning they can be dissociated to give three chloride ions in solution.

Chemical Properties

The compound \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \) is regarded as a simple hydrated salt, while \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3} \) functions as a coordination complex. The latter will exhibit typical behaviors of complexes, like potential ligands substitution reactions and unique spectral properties.

Implication in Formation

Formation of \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \) involves simple hydration of the chloride salt, while \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3} \) indicates a well-defined arrangement of chromium ions with coordinating water molecules, which is crucial in determining its reactivity and bonding.

Understanding Solubility and Ionization

Upon dissolving \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \) in water, it will dissociate into Cr\(^{3+}\), Cl\(^{-}\), and non-coordinated water molecules. However, \( \left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3} \) will dissociate into the complex ion \( [\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]^{3+} \) and three Cl\(^{-}\) ions, leaving the water molecules bonded to the chromium.

Key concepts

Coordination Chemistry

Coordination chemistry is a fascinating area of chemistry that explores molecules known as coordination compounds. These compounds feature a central atom or ion, usually a transition metal, bonded to a group of surrounding molecules or ions, called ligands. In the example from our exercise, the compound \([\mathrm{Cr}(\mathrm{H}_{2} \mathrm{O})_{6}] \mathrm{Cl}_{3}\) demonstrates how water molecules (- serve as ligands surrounding the central chromium ion. - form coordination bonds.Such arrangements not only determine the chemical properties but also impact the physical characteristics of these compounds.
Coordination complexes exhibit interesting behaviors such as variable oxidation states and participation in ligand substitution reactions. Understanding these principles can illuminate how various elements form bonds, especially through coordination chemistry, showcasing a rich world beyond simple ionic or covalent bonding.

Hydration in Ionic Compounds

Hydration is an essential concept in understanding ionic compounds like \( \mathrm{CrCl}_{3} \cdot 6\mathrm{H}_{2} \mathrm{O} \). In this type of compound, water molecules, known as waters of crystallization, are integrated into the crystal structure but not part of the coordination sphere. This means:- The water molecules are not directly bonded to the metal ion, in this case, chromium.- They can be removed from the structure, usually upon heating, without disrupting the compound's fundamental framework.In hydrated ionic compounds, water's role is primarily structural, supporting the lattice but being relatively easy to dissociate. Upon dissolving, these waters are released, impacting the solubility and potential reactions involving the compound.

Complex Ion Formation

In the formation of complex ions, transition metals typically play a central role. Consider \[ [\mathrm{Cr}(\mathrm{H}_{2} \mathrm{O})_{6}]^{3+} \mathrm{Cl}_{3} \] from our exercise. Here, each water molecule donates a pair of electrons to create coordination bonds with the chromium ion. - These coordinated water molecules create a defined geometry around the metal center.- The resulting assembly is what we call a 'complex ion'.Unlike simple hydrated salts, complex ions demonstrate distinct properties influenced by the number and type of ligands. These compounds are surprisingly stable and require careful conditions for ligand exchange or replacement. Their formation is a cornerstone of coordination chemistry, revealing the intricate ways atoms can interact.

Coordination Compounds

Coordination compounds are an integral part of chemistry, acting as a bridge between various scientific disciplines. Unlike basic salts, these compounds exhibit unique behaviors due to their structured formation of a central metal atom bonded to ligands. In our example, \[ [\mathrm{Cr}(\mathrm{H}_{2} \mathrm{O})_{6}] \mathrm{Cl}_{3} \] illustrates the detailed nature of these entities.- The structure leads to specific photochemical and spectroscopic properties.- This influences their reaction mechanisms and potential uses in catalysis and material science.Studying coordination compounds unveils complex interactions and is essential for applications in areas like medicine, where such compounds might serve as delivery systems for drugs. Understanding their structure and behavior allows chemists to harness their potential, crafting new materials and solutions to modern challenges.