Problem 41 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of water molecule(s) directly bonded to the metal centre in \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) is

Step-by-Step Solution

Answer

Four water molecules are directly bonded to the metal center in \( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \).

Step 1: Identify the Compound Structure

The compound given is \( CuSO_4 \, \cdot \, 5H_2O \), which is copper (II) sulfate pentahydrate. In this compound, 5 water molecules are associated with each \( CuSO_4 \) unit.

Step 2: Understand Hydrate Composition

In general, hydrates like \( CuSO_4 \, \cdot \, 5H_2O \) have a specific number of water molecules as part of their crystalline structure but not all of them are chemically bonded directly to the metal.

Step 3: Determine Directly Bonded Water Molecules

In \( CuSO_4 \, \cdot \, 5H_2O \), only a subset of water molecules are coordinated directly to the central metal ion (copper). Typically, in copper (II) sulfate pentahydrate, 4 of these water molecules are coordinated directly to the central copper ion, forming a complex with the \( Cu^{2+} \) ion.

Key Concepts

Water of Crystallization

Water of crystallization refers to the water molecules that are part of the crystalline structure of a compound. These water molecules are not just loosely trapped between the crystals but are integral to maintaining the shape and stability of the crystal lattice.
For example, in the compound \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\), which is copper (II) sulfate pentahydrate, there are 5 water molecules for every copper sulfate unit. This water is included in the chemical formula and is essential for the crystal to retain its shape and properties.
During heating, these water molecules can be removed, which might result in a change in the color and structure of the compound as it loses its crystallinity.

Hydrates

Hydrates are compounds that incorporate water molecules into their solid structure. The water is not just absorbed on the surface, but firmly held within the crystal lattice.
Chemical formulas for hydrates often show this association by including ".nH_2O" at the end, indicating the number of water molecules per formula unit.

• For example, \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) is a hydrate with 5 water molecules.
• Hydrates can be either stoichiometric, having a fixed and consistent number of water molecules, or non-stoichiometric, where water content can vary.

These water molecules can contribute to the compound's color, solubility, and other physical properties. When the water is removed, typically through heating, the resulting dehydrate may be substantially different in appearance and behavior.

Metal-ligand Bonding

Metal-ligand bonding is a cornerstone of coordination chemistry. It explains how metal ions bond with other molecules or ions, known as ligands. In coordination complexes, ligands donate electron pairs to metal ions, forming coordinate covalent bonds.
In copper (II) sulfate pentahydrate \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\), metal-ligand bonding involves the interaction of water molecules with the central copper ion \(Cu^{2+}\). Here, typically four of the five water molecules act as ligands and coordinate directly with the copper ion.

• Ligands can be charged or neutral and commonly include ions or molecules like water, ammonia, or chloride ions.
• The number of coordinate bonds formed is called the coordination number, which in this case is four for the copper ion's interaction with water.

This type of bonding significantly affects the chemical reactivity and properties of the metal complex, influencing factors such as solubility, color, and magnetic properties.