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 \(\mathrm{Al}=5137 \mathrm{kJmol}^{-1} \Delta \mathrm{H}_{\text {hydrtion }}\) for \(\mathrm{Al}^{+3}=-4665 \mathrm{~kJ}\) \(\mathrm{~mol}^{-1}, \Delta \mathrm{H}_{\text {thydatica }}\) 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

Anhydrous \(\mathrm{AlCl}_3\) becomes ionic in aqueous solution.

Step 1: Understand the Problem

We need to determine if anhydrous \(\mathrm{AlCl}_3\) becomes ionic when dissolved in water. To do this, we will use the given ionization energy of aluminum and the enthalpies of hydration for \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^{-}\).

Step 2: Calculate Hydration Energy

Hydration energy is the energy released when ions are dissolved in water. The data provides the hydration energy for \(\mathrm{Al}^{3+}\) as \(-4665 \mathrm{~kJ/mol}\) and for \(\mathrm{Cl}^{-}\) as \(-381 \mathrm{~kJ/mol}\). Thus, the total hydration energy is computed by summing these values for all ions: \(\Delta H_{\text{hydration}} = -4665 + 3(-381) = -5808 \mathrm{~kJ/mol}\).

Step 3: Compare Ionization Energy and Hydration Energy

The ionization energy required to form \(\mathrm{Al}^{3+}\) from \(\mathrm{Al}\) is given as \(5137 \mathrm{~kJ/mol}\). Compare this with the hydration energy, \(-5808 \mathrm{~kJ/mol}\). If the hydration energy is more negative than the ionization energy, the compound is likely to become ionic as the process is energetically favorable.

Step 4: Draw Conclusion

Since the total hydration energy \(-5808 \mathrm{~kJ/mol}\) is more negative than the ionization energy, \(5137 \mathrm{~kJ/mol}\), it indicates that the energy released upon solvation compensates for the energy required to ionize the aluminum atoms, leading to the compound becoming ionic in an aqueous solution.

Key Concepts

Hydration Energy

When discussing chemistry, hydration energy is a key concept when looking at how compounds dissolve in water. Hydration energy refers to the energy change that occurs when ions are surrounded by water molecules. This is an exothermic process, meaning energy is released. The more negative the energy value, the more energy is released.
Let's think about aluminium chloride (\(\mathrm{AlCl}_{3}\)). When it dissolves in water, the aluminium ion (\(\mathrm{Al}^{3+}\)) and chloride ions (\(\mathrm{Cl}^-\)) separate and are stabilized by the water molecules. The data shows that the hydration energy for \(\mathrm{Al}^{3+}\) is \(-4665\,\mathrm{kJ/mol}\), and for \(\mathrm{Cl}^-\) it is \(-381\,\mathrm{kJ/mol}\).

• The total hydration energy equals the sum of these values for each ion, indicating how much energy is released when the ions dissociate in water.
• In the case of \(\mathrm{AlCl}_{3}\), the total calculated is \(-5808\,\mathrm{kJ/mol}\).

Hydration energy helps us predict whether a compound like \(\mathrm{AlCl}_{3}\) will favor forming ions in solution. If this energy exceeds the ionization energy needed to form ions, the process is energetically favorable.

Ionization Energy

Ionization energy is critical in determining the likelihood of an element forming ions. It is the energy required to remove an electron from an atom to form a cation, which in simpler terms is a positively charged ion.
For \(\mathrm{Al}\), the ionization energy to form \(\mathrm{Al}^{3+}\) is \(5137\,\mathrm{kJ/mol}\). This energy is essential to overcome because unless aluminum can successfully lose electrons to form \(\mathrm{Al}^{3+}\), it won't become an ion in solution.Understanding this helps us see why \(\mathrm{AlCl}_3\) behaves the way it does in water:

• If the energy released during hydration is enough to cover the ionization energy, then forming ions is favored.
• If not, the atoms would prefer to stay as they are.

In the given example, since the \(-5808\,\mathrm{kJ/mol}\) of hydration energy is greater than the \(5137\,\mathrm{kJ/mol}\), it's energetically favorable for \(\mathrm{AlCl}_3\) to ionize.

Covalent and Ionic Compounds

It's important to distinguish between covalent and ionic compounds. These represent two main types of chemical bonding, each with unique properties:

• Covalent compounds are formed when electrons are shared between atoms. This usually happens between non-metals.
• Ionic compounds occur when there is a transfer of electrons from one atom (typically a metal) to another (often a non-metal), creating oppositely charged ions that attract each other.

Initially, anhydrous \(\mathrm{AlCl}_3\) is covalent because aluminum and chlorine share electrons. However, when we add water into the mix, the compound can shift its behavior.Due to the strong hydration energy favoring ionization, \(\mathrm{AlCl}_3\) in water can behave more like an ionic compound:

• The released energy when ions hydrate can outweigh the energy needed to ionize \(\mathrm{Al}\), promoting the formation of \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^-\) ions.
• Hence, in aqueous solution, the compound tends to become ionic rather than staying covalent.