Question

Anhydrous \(\mathrm{AlCl}_{3}\) is covalent. From the data given below, predict whether it would remain covalent or become ionic in aqueous solution (ionisation energy of \(\mathrm{Al}=5137 \mathrm{~kJ} \mathrm{~mol}^{-1}, \Delta \mathrm{H}_{\text {indration }}\) for \(\mathrm{Al}^{+3}=-4665 \mathrm{~kJ}\) \(\mathrm{mol}^{-1}, \mathrm{AH}_{\text {hydation }}\) for \(\left.\mathrm{C}^{-}=-381 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)\) (a) Ionic (b) Covalent (c) Both (d) None

Short answer

The solution is (a) Ionic.

Step-by-step solution

Understanding the Problem

We need to determine whether anhydrous \( \mathrm{AlCl}_{3} \), which is covalent, remains covalent or becomes ionic when dissolved in water. We are given both the ionization energy of aluminum and the enthalpy of hydration for \( Al^{3+} \) and \( Cl^- \).

Calculating Ionic Energy Change

Calculate the change in energy when \( \mathrm{AlCl}_{3} \) dissolves in water and becomes ionic. The energy change is the ionization energy of \( \mathrm{Al} \) plus the combined hydration energies. Thus, the formula is: \[ E_{ionic} = 5137 \text{ kJ/mol} + (-4665 \text{ kJ/mol}) + 3(-381 \text{ kJ/mol}). \]

Performing the Calculation

Substitute the given values into the formula: \[ E_{ionic} = 5137 - 4665 - 1143 = -671 \text{ kJ/mol}. \]This calculation shows an energy release of 671 kJ/mol, indicating a conversion from covalent to ionic due to the negative energy change.

Interpreting Results

Since the energy change \( E_{ionic} \) is negative (-671 kJ/mol), it implies a favorable process for \( AlCl_3 \) to become ionic in solution. A negative energy change suggests that the dissolution and subsequent ionic state are energetically favored over the initial covalent state.

Key concepts

Ionization Energy

Ionization energy is key when understanding chemical bonding, especially in determining the nature of compounds. It refers to the amount of energy required to remove an electron from an atom to form an ion. For example, aluminum (Al) requires an ionization energy of 5137 kJ/mol to release electrons and form a stable ion.

In the context of \(\mathrm{AlCl}_{3}\) dissolving in water, examining ionization energy helps us predict the possibility of forming ionic bonds. When the energy required to form ions is less than the energy released during the next steps, such as hydration, the compound tends to become ionic. Understanding ionization energy allows chemists to gauge how readily an element will participate in ionic reactions, which is crucial for predicting reactivity and compound formation.

• Indicates how strongly an atom holds onto its electrons.
• High ionization energy implies a stable electron configuration.
• In \(\mathrm{AlCl}_3\) : High ionization energy was overcome by subsequent processes.

Enthalpy of Hydration

The enthalpy of hydration is another fundamental concept that plays a critical role in the transition of a compound from a covalent to an ionic state. It describes the change in energy when ions dissolve in water and become surrounded by water molecules, forming a solution.

For \(\mathrm{Al}^{3+}\), the enthalpy of hydration is -4665 kJ/mol, and for \( Cl^-\), it is -381 kJ/mol. These negative values indicate that energy is released during hydration—a sign that this process is exothermic and energetically favorable.

• Measures the energy change as ions interact with water molecules.
• Negative values indicate energy release that drives the transition from covalent to ionic.
• Enthalpy of hydration contributed significantly to making the dissolution of \(\mathrm{AlCl}_3\) energetically favorable.

In the case of \( \mathrm{AlCl}_3 \), the energy released during hydration helps overcome the energy needed for ionization, facilitating the compound's transformation into an ionic form in aqueous solution.

Energetics of Reactions

Understanding the energetics of reactions involves looking at total energy changes during a reaction or a process. Energetics hinge upon the balance of various energy inputs and outputs—including ionization energy and enthalpy of hydration. Summing these energies gives insight into whether a process will naturally occur or need external energy.

For \( \mathrm{AlCl}_3 \), calculating the total energy change means adding ionization energy and subtracting the enthalpy of hydration sums. The negative value of -671 kJ/mol indicates that the energy released during hydration offsets the energy required for ionization and subsequent processes.

• Total energy change helps determine reaction feasibility.
• Negative values suggest that a reaction will proceed favorably, i.e., without needing additional energy input.
• In the solution, \( \mathrm{AlCl}_3 \) becomes ionic due to the net energy release.

The energetics allow predictions regarding whether \( \mathrm{AlCl}_3 \) remains covalent or becomes ionic. The observed negative energy change during dissolution supports the transition to an ionic state, affirming that the reaction is exothermic and thermodynamically favorable.