Problem 55 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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 for \(\mathrm{Al}=5137 \mathrm{~kJ} \mathrm{~mol}^{-1} ; \Delta H_{\text {hydration for }}\) \(\mathrm{Al}^{3+}=-4665 \mathrm{~kJ} \mathrm{~mol}^{-1} ; \Delta H_{\text {hydration }}\) for \(\left.\mathrm{Cl}^{-}=-381 \mathrm{~kJ} \mathrm{~mol}^{-1} .\right)\)

Step-by-Step Solution

Answer

\(\mathrm{AlCl}_{3}\) becomes ionic in aqueous solution.

Step 1: Understand the question

The task is to determine if anhydrous \(\mathrm{AlCl}_{3}\), which is covalent in nature, will remain covalent or become ionic when dissolved in water. The decision will be based on provided ionization and hydration energy data.

Step 2: Identify energy components

Consider the ionization energy of \(\mathrm{Al}\) and hydration energies for \(\mathrm{Al}^{3+}\) and \(\mathrm{Cl}^{-}\). Ionization energy is \(5137 \mathrm{~kJ} \mathrm{~mol}^{-1}\), hydration energy for \(\mathrm{Al}^{3+}\) is \(-4665 \mathrm{~kJ} \mathrm{~mol}^{-1}\), and for \(\mathrm{Cl}^{-}\) is \(-381 \mathrm{~kJ} \mathrm{~mol}^{-1}\).

Step 3: Calculate total energy change if ionic in water

Calculate the total energy change if \(\mathrm{AlCl}_{3}\) becomes ionic:\(-\Delta H_{\text{hydration}}(\mathrm{Al}^{3+}) + 3(-\Delta H_{\text{hydration}}(\mathrm{Cl}^{-}))\). This will be \(-4665 + 3(-381)\).

Step 4: Compare total hydration energy with ionization energy

The total hydration energy from step 3 is \(-5808 \mathrm{~kJ} \mathrm{~mol}^{-1}\). Ionization energy is \(5137 \mathrm{~kJ} \mathrm{~mol}^{-1}\). Since the hydration energy exceeds the ionization energy, this means that sufficient energy is available to overcome the ionization requirement.

Step 5: Conclusion

The favorable total energy calculated in aqueous solution indicates that \(\mathrm{AlCl}_{3}\) likely becomes ionic in water because the hydration energy compensates for the high ionization energy.

Key Concepts

Anhydrous AlCl3

Anhydrous AlCl3 is a fascinating example of a compound that displays covalent characteristics in its dry form. In anhydrous state, which refers to the absence of water, the aluminum and chlorine atoms share electrons closely, creating covalent bonds. AlCl3 does not readily dissolve or dissociate in the absence of water. Since covalent bonds involve the sharing of electrons rather than the transfer of them, the compound maintains its covalency outside of an aqueous environment.

The intriguing property of AlCl3 observed when dissolved in water involves a transition in its bonding nature. Understanding this transition requires analyzing the relevant energies involved, which include ionization and hydration energies. When water is introduced, the surrounding water molecules play a significant role in altering AlCl3’s bonding nature.

Ionization Energy

Ionization energy is a critical concept when analyzing how compounds behave in different environments. It refers to the energy required to remove an electron from an atom or ion. For aluminum, this energy is quite substantial at 5137 kJ ext{mol}^{-1} , indicating that a lot of energy is necessary to ionize aluminum. The high ionization energy of aluminum suggests it prefers maintaining its electrons, contributing to AlCl3's covalent nature, at least in an anhydrous form.

In the context of abhyrous substances transitioning to aqueous solutions, the battle between ionization energy and hydration energy plays a pivotal role. High ionization energies often imply stability unless other forces provide compensation, such as those from hydration energies.

Hydration Energy

Hydration energy offers valuable insights into how polar solvents like water influence ionic compounds. It represents the energy change when ions become surrounded by water molecules. This process is energetically favorable as the ions interact with water, releasing energy. For Al^{3+} , the hydration energy is –4665 ext{ kJ mol}^{-1} , and for Cl^{-} , it's –381 ext{ kJ mol}^{-1} .

These figures illustrate how strongly water molecules stabilize these ions. The negative values indicate that energy is released when the ions become hydrated, making this process favorable in an aqueous solution. The larger the magnitude of the hydration energy, the more it compensates for ionization energy, adding to the potential for these ions to exist in water.

Aqueous Solutions

Aqueous solutions serve as a medium where many chemical reactions occur, and they have particular importance in altering the property of compounds like AlCl3 . When AlCl3 is introduced into water, the possibility of it becoming ionic increases because of the interaction with water molecules.

In an aqueous solution, the surrounding water molecules provide a polar environment that can stabilize ionic charges. By comparing energies, we can predict whether AlCl3 will remain covalent or become ionic. The hydration energy provided by the Al^{3+} and Cl^{-} ions can sometimes exceed the energy required for ionization, facilitating the change from a covalent to an ionic structure in water.

Energy Calculations

Energy calculations are the cornerstone for understanding the transformation of substances like AlCl3 when dissolved in water. By comparing ionization and hydration energies, we can predict changes in bonding nature.

The total hydration energy when AlCl3 becomes ionic is calculated as –5808 ext{ kJ mol}^{-1} , which includes the combined effects of Al^{3+} and Cl^{-} ions' hydration energies. When this is compared to the ionization energy of Al at 5137 ext{ kJ mol}^{-1} , we notice the stark difference indicating sufficient energy is available to overcome the ionization process.

• Negative hydration energies indicate energy release, showing stability in ions when solvated.
• The primary computation confirms that energy balance favors the formation of ionic AlCl3 in an aqueous solution.

By dissecting these calculations, students uncover the underlying beauty of energy interactions within chemical reactions.