Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Anhydrous \(\mathrm{AlCl}_{3}\) is covalent. From the data given below, predict whether it would remain covalent or become ionic in aqueous solution (ionization energy of \(\mathrm{Al}=5137 \mathrm{kJmol}^{-1} \Delta \mathrm{H}_{\text {hyuntion }}\) for \(\mathrm{Al}^{+3}=-4665 \mathrm{~kJ}\) \(\mathrm{mol}^{-1}, \Delta \mathrm{H}_{\text {hydation }}\) for \(\left.\mathrm{Cl}^{-}=-381 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)\) (a) ionic (b) covalent (c) both (a) and (b) (d) none of these

Step-by-Step Solution

Answer

(a) ionic

Step 1: Determine the Energy Change for Ionization

To determine whether \(\mathrm{AlCl}_3\) remains covalent or becomes ionic in aqueous solution, we need to consider the energy changes involved in ionization and hydration. The ionization energy of aluminum is \(5137 \text{ kJ/mol}\). This represents the energy required to remove three electrons to form \(\mathrm{Al}^{3+}\).

Step 2: Calculate the Total Hydration Energy

The hydration energy for converting \(\mathrm{Al}^{3+}\) in solution is given as \(\Delta \mathrm{H}_{\text{hydration}} = -4665 \text{ kJ/mol}\). The hydration energy for each chloride ion \(\mathrm{Cl}^-\) is \(-381 \text{ kJ/mol}\), resulting in a total chloride ion hydration energy of \(3 \times -381 = -1143 \text{ kJ/mol}\).

Step 3: Compute Overall Energy Balance

Calculate the overall energy change by summing the ionization energy and the hydration energies. The total energy required to ionize \(\mathrm{Al}\) is the ionization energy \(5137 \text{ kJ/mol}\), while the energy released during hydration is the sum of \(-4665 \text{ kJ/mol}\) for \(\mathrm{Al}^{3+}\) and \(-1143 \text{ kJ/mol}\) for \(3 \mathrm{Cl}^-\). Thus, the net energy change is: \[\Delta E = 5137 - 4665 - 1143 = -671 \text{ kJ/mol}.\]

Step 4: Analyze the Energy Balance

Since the energy balance \(\Delta E = -671 \text{ kJ/mol}\) turns negative, the process of formation of ionic \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^-\) ions in aqueous solution is energetically favorable. This implies that the compound is likely to become ionic in solution.

Key Concepts

Covalent and Ionic Bonds

Chemical bonding is the way atoms connect to form molecules and compounds. There are different types of bonds, with covalent and ionic bonds being two of the main ones.

• Covalent Bonds: This type of bond occurs when atoms share electrons. It usually happens between non-metal atoms. For instance, in \( ext{AlCl}_3\), aluminum and chlorine share electrons in a covalent manner. The sharing allows each atom to achieve a stable electron arrangement.
• Ionic Bonds: In contrast, ionic bonds form when one atom gives up one or more electrons to another atom. This transfer creates ions—charged particles. Typically, metals transfer electrons to non-metals in ionic bonding. The result is an attraction between the positively charged ion (cation) and the negatively charged ion (anion), forming a stable ionic compound.

In the exercise, anhydrous \( ext{AlCl}_3\) is covalent when not in solution. However, when dissolved in water, it can form ions because water molecules can break the covalent bonds and lead to an ionic state. This change involves energy processes such as ionization and hydration.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom or ion. It’s a crucial concept in understanding how elements react. Here are some key points:

• Definition: It represents the minimum energy needed to remove the outermost, or "valence," electron of an atom in the gaseous state. A high ionization energy indicates that an atom holds its electrons tightly.
• Multiple Electron Removal: The ionization energy increases significantly as more electrons are removed. For aluminum, the given ionization energy of \(5137 \text{kJ/mol}\) indicates the energy required to remove three electrons to form \(\text{Al}^{3+}\).

In the context of the exercise, this energy is substantial and must be overcome for \(\text{AlCl}_3\) to convert into its ionic form when dissolved in water. The net process is favorable if the hydration energy released compensates for this ionization energy.

Hydration Energy

Hydration energy refers to the energy change when ions dissolve in water and become surrounded by water molecules. This process stabilizes the ions in solution.

• Role of Water: Water molecules are polar, meaning they have a partial negative charge on one side and a partial positive charge on the other. This allows them to interact with ions effectively.
• Energy Release: When ions dissolve, the interaction between water molecules and ions releases energy. For \(\text{Al}^{3+}\), the release is significant at \(-4665 \text{kJ/mol}\), while for \(\text{Cl}^-\), it totals to \(-1143 \text{kJ/mol}\) overall for three ions.

In our scenario, the total hydration energy is enough to balance and outweigh the ionization energy needed to form \(\text{Al}^{3+}\) ions. This energetically favorable condition suggests that \(\text{AlCl}_3\) becomes ionic in solution. Understanding these energy relationships helps predict and explain the behavior of chemical compounds in different environments.