Question

Ionization energy of \(\mathrm{Al}=5137 \mathrm{~kJ} \mathrm{~mole}^{-1}(\Delta \mathrm{H})\) hydra- tion of \(\mathrm{Al}^{3+}=-4665 \mathrm{~kJ} \mathrm{~mole}^{-1} .(\Delta \mathrm{H})_{\text {bydation }}\) for \(\mathrm{Cl}^{-}=\) \- \(381 \mathrm{~kJ} \mathrm{~mole}^{-1}\). Which of the following statement is correct? (a) \(\mathrm{AlCl}_{3}\) would remain covalent in aqueous solution (b) Only at infinite dilution \(\mathrm{AlCl}_{3}\) undergoes ionisation (c) In aqueous solution \(\mathrm{AlCl}_{3}\) becomes ionic (d) None of these

Short answer

(c) In aqueous solution, AlCl₃ becomes ionic.

Step-by-step solution

Understanding Ionization Energy

The ionization energy given is for aluminum and has a value of 5137 kJ/mole. Ionization energy refers to the amount of energy required to remove an electron from an atom or ion. To form ions in a solution, compounds must have favorable energy conditions.

Analyze Hydration Energies

For the hydration of ions in water, the given change in enthalpy (ΔH) for Al³⁺ is -4665 kJ/mole, while for Cl⁻, it is -381 kJ/mole. These values indicate the energy changes when these ions interact with water, with negative values suggesting an exothermic process.

Evaluating AlCl₃ Behavior in Aqueous Solution

When AlCl₃ dissolves in water, it can potentially form Al³⁺ and Cl⁻ ions. The highly negative hydration enthalpy for Al³⁺ suggests that it will interact strongly with water, indicating a tendency to form ions in aqueous solution.

Deduce the Correct Statement

Given the strong tendency for ionization (due to favorable hydration values), AlCl₃ is likely to dissociate into ions in aqueous solution, supporting statement (c). This indicates that in water, it behaves more like an ionic compound.

Key concepts

Hydration Energy

When ions dissolve in water, they interact with water molecules, a process that releases or requires energy. This energy change is known as hydration energy. It measures the energy change when one mole of ions is surrounded by water molecules. Generally, the more negative the hydration energy, the more exothermic the process is, meaning energy is released strongly as the ions become part of the solution.
For example, in the case of \[\mathrm{Al}^{3+},\] the hydration energy is notably negative at \[-4665 \text{ kJ/mol}.\] This suggests that \[\mathrm{Al}^{3+}\] ions release a significant amount of energy as they hydrate, which promotes its dissolution in water. On the other hand, \[\mathrm{Cl}^-\] has a smaller negative hydration energy of \[-381 \text{ kJ/mol}.\] Both indicate exothermic processes, implying a spontaneous affinity towards forming \[an \text{ aqueous solution}.\]

Aqueous Solution

An aqueous solution is formed when a solute, like an ionic compound, dissolves in water, the solvent. Water is an excellent solvent because its molecules have partial charges that can stabilize ions. Within an aqueous solution, ions are surrounded by water molecules, which helps to keep them apart and spreads them throughout the solution. This property is particularly crucial for substances like aluminum chloride (\[\mathrm{AlCl}_3,\] which splits into \[\mathrm{Al}^{3+}\text{ and }\mathrm{Cl}^{-}\] ions when dissolved.
In the water, these ions are stabilized by water's molecule interaction, allowing materials that are solid at room temperature to become more fluid. As compounds dissolve, their individual ions can move freely, which is important for many chemical reactions that occur in biology and chemistry, demonstrating their ionic nature in aqueous conditions.

Ionic Compounds

Ionic compounds are formed when ions of opposite charges bind together due to their electrostatic attractions. This bonding leads to the formation of a solid crystalline lattice. They generally possess high melting and boiling points due to the strong forces holding them together.
When an ionic compound like \[\mathrm{AlCl}_3,\]comes into contact with water, it dissolves and separates into its ions based on the hydration energies discussed. These compounds demonstrate electrolytic properties, meaning they can conduct electricity in aqueous solution because of the presence of free-moving ions. The outcome of their dissociation in water is systematized by hydration energy and enthalpy change, which are measures of how passionate an ionic compound is to separate into constituent ions.

Enthalpy Change

Enthalpy change (\[\Delta H\]) is a measurement of energy change in a system as it undergoes a chemical or physical transformation. It is significant for understanding how energy varies between products and reactants in reactions.
In cases like the dissolution of \[\mathrm{AlCl}_3,\] the hydration process contributes to an overall enthalpy change when the compound splits into ions. The \[\Delta H\] has implications on whether a reaction is endothermic (absorbs energy) or exothermic (releases energy). The negative \[\Delta H\] values for ion hydration point to exothermic processes, which means the process gives off more energy than it consumes. Therefore, a negative enthalpy change for both ions suggests that dissolution in water is favored energetically, often letting the compound behave as ionic under these conditions.