Question

During hydrate formation from aqueous solution, water can be associated in different forms. Indicate the wrong combination. (i) Coordinated water - \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+} 3 \mathrm{Cl}^{-}\) (ii) Interstitial water - \(\mathrm{BaCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (iii) Hydrogen bonded water \(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+} \mathrm{SO}_{4}^{2-} \cdot \mathrm{H}_{2} \mathrm{O}\) (a) (i) (b) (ii) (c) (iii) (d) None of these.

Short answer

(b) (ii)

Step-by-step solution

Understand Types of Water in Hydrates

Hydrates can contain water in different ways: as coordinated water, where water is directly bonded to a metal cation; interstitial water, where water molecules are present in the spaces of the crystal lattice; and hydrogen-bonded water, where water molecules are not directly bonded to a metal ion but are part of the crystal structure through hydrogen bonding.

Analyze the Given Combinations

Examine each given combination to determine if the type of water association is correct. (i) Coordinated water with a complex cation is correctly shown by the hexaaquachromium(III) ion. (ii) Interstitial water should be present in the lattice, so we need to determine if BaCl2 has a crystal structure that could include water in its interstices. (iii) Hydrogen-bonded water is shown as a water molecule outside the coordination sphere, potentially involved in hydrogen bonding with the sulfate anion.

Determine the Incorrect Combination

Combination (i) properly represents coordinated water within a complex ion. In combination (iii), one additional water molecule is hydrogen-bonded to the coordination compound, which seems plausible. However, combination (ii) does not represent interstitial water properly; it suggests that water is a part of the crystalline structure of BaCl2 but not sitting in the gaps or interstices of the crystal lattice, which is characteristic of water of crystallization. BaCl2 typically forms a hydrate BaCl2·2H2O where water molecules are coordinated, not interstitial.

Key concepts

Coordinated Water in Chemistry

Coordinated water refers to water molecules that are directly bonded to metal ions within a complex. When water molecules act as ligands, they interact with a central metal atom to form coordination compounds, which are distinctive types of hydrates. These water molecules are crucial for forming the structure of the compound and determining its chemical properties.

For instance, in the complex \(\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{3+} 3 \mathrm{Cl}^{-}\), the water molecules are directly attached to the chromium ion, coordinating with it to create a stable structure. The coordination number, which is the number of ligand-atom bonds to the central metal atom, is six in this example, indicating that six water molecules are coordinated to a single chromium ion.

Interstitial Water

Interstitial water is found within the interstitial spaces of a crystal lattice, which are the voids or 'gaps' between the atoms or ions in the solid structure. These water molecules are not coordinated to any specific ion, nor are they hydrogen-bonded, but are instead 'trapped' within the lattice during crystal formation.

When assessing a salt like \(\mathrm{BaCl}_{2}\), which is often found in a hydrated form, the water molecules that would be considered interstitial would be located in the spaces of the crystalline array. However, in \(\mathrm{BaCl}_{2} \cdot 2 \mathrm{H}_{2}\mathrm{O}\), the water molecules are actually part of the crystalline structure, bonded in a definite stoichiometric ratio and not simply occupying interstitial spaces. Thus, indicating that \(\mathrm{BaCl}_{2} \cdot 2 \mathrm{H}_{2}\mathrm{O}\) has interstitial water is incorrect as these water molecules should be referenced as water of crystallization instead.

Hydrogen Bonded Water

Hydrogen-bonded water is characterized by water molecules that are incorporated into a hydrate's crystal lattice not by direct coordination to a central ion, but through hydrogen bonding. These bonds are known for their role in giving hydrates their structure and properties.

In the compound \(\left[\mathrm{Cu}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}\right]^{2+} \mathrm{SO}_{4}^{2-} \cdot \mathrm{H}_{2}\mathrm{O}\), the water of hydration is not directly bonded to the copper ion. Instead, the water molecule is hydrogen bonded to the coordination complex, typically to other water molecules or to sulfate anion. This type of water is essential for stabilizing the structure of many hydrates and is a prime example of how hydrogen bonding contributes to the overall assembly of complex crystals.

Chemical Hydrates Classification

Chemical hydrates are classified based on how water molecules are associated with the substance. The main categories include coordinated water, where water is a ligand in complex ions; interstitial water, which fits into the crystal lattice; and hydrogen-bonded water, associated through hydrogen bonds.

The classification of hydrates is important for understanding their chemical behavior and properties. For example, the water in a hydrate can influence its melting point, solubility, and reactivity. By studying the types and arrangements of water molecules in hydrates, scientists and students can predict the physical and chemical characteristics of these compounds, making such knowledge invaluable for applications in chemistry and materials science.