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 \(\mathrm{Al}=5137 \mathrm{~kJ} \mathrm{~mol}^{-1} \Delta \mathrm{H}_{\text {hydratian }}\) for \(\mathrm{Al}^{+3}=-4665 \mathrm{~kJ}\) \(\mathrm{mol}^{-1}, \Delta \mathrm{H}_{\text {hydration }}\) for \(\left.\mathrm{Cl}^{-}=-381 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)\) (a) Ionic (b) Covalent (c) Both (a) and (b) (d) None of these

Short answer

(a) Ionic

Step-by-step solution

Calculate the Total Hydration Energy

The total hydration energy includes both the hydration of the aluminum ion and three chloride ions. Thus, we calculate it as follows: \( \Delta H_{\text{hydration total}} = \Delta H_{\text{hydration Al}^{+3}} + 3 \times \Delta H_{\text{hydration Cl}^{-}} \).\[\Delta H_{\text{hydration total}} = -4665 \, \text{kJ/mol} + 3 \times (-381 \, \text{kJ/mol})\] \[\Delta H_{\text{hydration total}} = -4665 \, \text{kJ/mol} - 1143 \, \text{kJ/mol} = -5808 \, \text{kJ/mol}\]

Compare Ionization Energy and Hydration Energy

The ionization energy needed for \( \text{Al}^{+3} \) is \( 5137 \, \text{kJ/mol} \). Since the total hydration energy is \( -5808 \, \text{kJ/mol} \), the hydration energy offsets the ionization energy. The negative value for total hydration energy indicates that the hydration process releases enough energy to overcome the ionization energy.

Analyze the Result

Since the overall process (ionization plus hydration) leads to a release of energy (945808 - 5137 = -671 \, \text{kJ/mol}). This energy release suggests that formation of ionic \( \text{Al}^{3+} \) and \( \text{Cl}^{-} \) ions in solution is favorable energetically.

Conclude the Nature of the Solution

Based on the energy calculations, since the total energy for ionization and hydration is negative, the compound \( \text{AlCl}_3 \) will dissociate into its ionic components in aqueous solution. Therefore, in aqueous solution, \( \text{AlCl}_3 \) acts as an ionic compound.

Key concepts

Ionization Energy

Ionization energy refers to the amount of energy required to remove an electron from an atom or ion in its gaseous state. This energy is crucial because it quantifies how strongly an atom holds onto its electrons, which can affect how it interacts chemically with other substances.
For example, in the case of aluminum in \(\mathrm{AlCl}_{3}\), the ionization energy is given as \(5137 \, \text{kJ/mol}\). This is the energy needed to transform aluminum into \(\text{Al}^{3+}\) by removing three electrons. A high ionization energy suggests that it takes a substantial amount of energy to remove these electrons, indicating strong bonding within the atom itself.

• High ionization energy can prevent an atom from forming ionic bonds easily.
• The requirement of less ionization energy often facilitates the formation of positive ions.

In summary, ionization energy plays a crucial role in determining how likely an atom is to lose electrons and engage in ionic bonding.

Hydration Energy

Hydration energy is the energy change associated with the solvation process, essentially measuring how well ions integrate with water molecules. When ions dissolve in water, they become surrounded by water molecules, leading to stability and a release of energy.
The hydration energy of \(\text{Al}^{3+}\) and \(\text{Cl}^{-}\) are \(-4665 \, \text{kJ/mol}\) and \(-381 \, \text{kJ/mol}\) respectively in the exercise. Total hydration energy is calculated to be \(-5808 \, \text{kJ/mol}\), with the process releasing energy, promoting solvation. This is powerful enough to balance and surpass the ionization energy required.

• Hydration energy depends on the size and charge of the ions; smaller and more charged ions generally have higher hydration energies.
• High hydration energy can offset ionization energy, supporting ionic bond formation when solvation is energetically favorable.

Understanding hydration energy is essential as it helps predict the solubility and stability of compounds in aqueous solutions.

Covalent and Ionic Bonds

Bonds between atoms can be classified as covalent or ionic, based on how they share or transfer electrons.
Covalent bonds involve the sharing of electrons between atoms, aiming to satisfy the electron sharing rule. This typically occurs between non-metals, like Cl in \(\mathrm{AlCl}_{3}\), where electron sharing leads to the formation of stable molecules.
In contrast, ionic bonds result from the transfer of electrons between atoms, usually when there is a significant difference in electronegativity. After transfer, the positive and negative ions attract each other to form a stable structure. For instance, when \(\mathrm{AlCl}_{3}\) becomes ionic in a solution, \(\text{Al}^{3+}\) and \(\text{Cl}^{-}\) ions form due to energy changes.

• Covalent bonds typically form when electron sharing lowers potential energy.
• Ionic bonds are formed when direct electron transfer leads to stable ions with contrasting charges.

The distinction between these bonds is vital in understanding compounds' behavior and reactivity, especially in different environments.

Aqueous Solutions

Aqueous solutions are formed when substances dissolve in water, which is one of the most common solvents due to its polar nature. In aqueous solutions, water molecules interact with solute particles to dissolve them. \(\mathrm{AlCl}_{3}\), for example, when placed in water, dissociates into \(\text{Al}^{3+}\) and \(\text{Cl}^{-}\) ions, illustrating how a substance initially perceived as covalent can become ionic.
Water’s polarity allows it to efficiently stabilize ionic compounds, with positive hydrogen ends attracting anions and negative oxygen ends attracting cations. This stabilizing effect is what allows many substances to readily dissolve in water.

• Polarity of water plays a key role in dissolving ionic compounds.
• Aqueous solutions frequently exhibit different properties from their anhydrous forms due to solute-solvent interactions.

Recognizing the behavior of substances in aqueous solutions helps in comprehending processes like solubility, reactivity, and conductivity in a variety of scientific inquiries.