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 \(\mathrm{Al}=5137 \mathrm{kJmol}^{-1} \Delta \mathrm{H}_{\text {bytaribo }}\) for \(\mathrm{Al}^{+3}=-4665 \mathrm{~kJ}\) \(\mathrm{mol}^{-1}, \Delta \mathrm{H}_{\text {lydation }}\) for \(\mathrm{Cl}^{-}=-381 \mathrm{~kJ} \mathrm{~mol}^{-1}\) ) (a) ionic (b) covalent (c) both (a) and (b) (d) none of these

Step-by-Step Solution

Answer

In aqueous solution, \(\mathrm{AlCl}_3\) becomes ionic (Option a).

Step 1: Understand the Question

We need to predict if anhydrous \(\mathrm{AlCl}_3\) remains covalent or becomes ionic in aqueous solution. We're given ionization energy for \(\mathrm{Al}\) and hydration energies for \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^-\).

Step 2: Calculate Energy Required for Ionization

The ionization energy for \(\mathrm{Al}\) to form \(\mathrm{Al}^{3+}\) is given as \(5137\, \mathrm{kJ\,mol}^{-1}\). This is the energy needed to remove three electrons from aluminum and form the \(\mathrm{Al}^{3+}\) ion.

Step 3: Find Total Energy from Hydration

The energies released from hydration are \(\Delta \mathrm{H}_{\text{hydration}}\) of \(-4665\, \mathrm{kJ\,mol}^{-1}\) for \(\mathrm{Al}^{3+}\) and \(-381\, \mathrm{kJ\,mol}^{-1}\) for \(\mathrm{Cl}^-\). Since \(\mathrm{AlCl}_3\) produces three \(\mathrm{Cl}^-\) ions, we calculate the total hydration energy as \(-4665 + (3 \times -381) = -5808\, \mathrm{kJ\,mol}^{-1}\).

Step 4: Determine Net Energy Change

Calculate the net energy change by adding the ionization energy and hydration energies: \(5137\, \mathrm{kJ\,mol}^{-1} - 5808\, \mathrm{kJ\,mol}^{-1} = -671\, \mathrm{kJ\,mol}^{-1}\). A negative value indicates that the process is exothermic.

Step 5: Conclusion

Since the net energy change is negative, \(\mathrm{AlCl}_3\) becomes more stable in the ionic form due to the energy released during hydration outweighing the energy required for ionization.

Key Concepts

Ionization Energy

Ionization energy is a critical concept in understanding how atoms interact with each other. It refers to the energy required to remove an electron from a gaseous atom or ion. This energy is crucial because it determines how an element like aluminum (Al) behaves when forming compounds. In the case of aluminum, transforming this metal into its ion (\(\mathrm{Al}^{3+}\)) requires removing three electrons, which is a significant energy investment, specifically (\( 5137 \mathrm{kJ/mol}^{-1} \)).

The concept of ionization energy is particularly important in differentiating ionic bonding from covalent bonding. A higher ionization energy suggests that an atom strongly holds onto its electrons, which often results in sharing electrons with another atom, forming a covalent bond. Conversely, when the ionization energy is overcome, as seen in this exercise with aluminum, the atom can lose its electrons, resulting in an ionic character in a bond.

Understanding ionization energy helps explain why (\( \mathrm{AlCl}_3 \)) might become ionic in aqueous solutions; the energy required for ionization is compensated by the release of hydration energy.

Hydration Energy

Hydration energy plays a pivotal role in the transformation of compounds from covalent to ionic states when dissolved in water. It is the energy released when ions interact with water molecules, stabilizing them in solution. In the case of (\( \mathrm{AlCl}_3 \)), even though the compound is initially covalent in nature, the hydration process can greatly influence its state once dissolved.

When aluminum ions (\( \mathrm{Al}^{3+} \)) and chloride ions (\( \mathrm{Cl}^- \)) are surrounded by water molecules, hydration energies of (\( -4665 \mathrm{kJ/mol}^{-1} \)) for aluminum and (\( -381 \mathrm{kJ/mol}^{-1} \)) for each chloride ion come into play. This significant release of energy during the ion hydration process makes the transformation energetically favorable, contributing to the overall stability of the ionic configuration.

In summary, hydration energy serves as a driving force for the transition from covalent to ionic forms. It suggests that the energy released by the interaction with water can outweigh the energy required for ionization, leading (\( \mathrm{AlCl}_3 \)) to become more stable in an ionic form in aqueous solutions.

Exothermic Process

Exothermic processes are chemical reactions that release energy, usually in the form of heat, to the surroundings. In the context of this exercise with (\( \mathrm{AlCl}_3 \)), the transformation to an ionic compound upon hydration is an exothermic process.

The net energy change calculated in the solution shows (\( -671 \mathrm{kJ/mol}^{-1} \)), indicating the process results in an overall release of energy. This negative value arises because the energy liberated from hydration, (\( -5808 \mathrm{kJ/mol}^{-1} \)), is greater than the energy used for ionization, (\( 5137 \mathrm{kJ/mol}^{-1} \)).

An exothermic process is significant because it denotes stability in chemical reactions. The energy released during hydration promotes the stability of ionic (\( \mathrm{AlCl}_3 \)), suggesting an energetically favorable condition wherein the compound is more stable in the ionic form in water than in the covalent form. This principle helps us understand and predict the behavior of compounds in aqueous solutions.