Question

Which of the following will be formed, if we heat an aqueous solution of \(\mathrm{AlCl}_{3}\) to dryness? (a) Solid \(\mathrm{AlCl}_{3}\) (b) Dimer \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) \(\mathrm{Al}_{2} \mathrm{O}_{3}\)

Short answer

On heating \r\(\mathrm{AlCl}_{3}\) aqueous solution to dryness, dimer \r\(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) will be formed.

Step-by-step solution

Understand Heating Aqueous Solutions

Consider what happens to an aqueous solution upon heating. When we heat an aqueous solution, the water starts to evaporate, and the solute eventually remains. The nature of the solute determines what will be left behind after all the water has evaporated.

Properties of Aluminum Chloride

Aluminum chloride (\r\(\mathrm{AlCl}_{3}\)) is known to exist as a covalent compound in the gaseous and liquid state. In a solid state, it forms a lattice structure found in a dimeric form as \r\(\mathrm{Al}_{2}\mathrm{Cl}_{6}\).

Heating Aluminum Chloride Solution

On heating \r\(\mathrm{AlCl}_{3}\) solution to dryness, \r\(\mathrm{AlCl}_{3}\) dissociates to form the dimer \r\(\mathrm{Al}_{2}\mathrm{Cl}_{6}\), which is the solid form of aluminum chloride.

Key concepts

Aluminum Chloride Properties

Aluminum chloride (\r\( \text{AlCl}_{3} \r\)) is a chemical compound with several unique properties that make it valuable for a variety of industrial processes and chemical reactions. One of the key characteristics of aluminum chloride is its state of aggregation where it can exist in different forms based on temperature and the presence of other substances.

\r\( \text{AlCl}_{3} \r\)’s physical form changes from solid to liquid and gas at varying temperatures, signifying its volatility. In the gaseous phase or when dissolved in solvents like benzene, it forms a covalent bond and exists as a dimer, \r\( \text{Al}_{2}\text{Cl}_{6} \r\). This dimerization is an essential aspect for students to understand as it reflects aluminum chloride's tendency to form compounds where two \r\( \text{AlCl}_{3} \r\) molecules come together. In its solid-state, this compound features a layered lattice structure, which imparts the ability to conduct electricity and also contributes to its high melting and boiling points.

These properties make aluminum chloride a useful catalyst in many chemical reactions, most notably the Friedel-Crafts reaction for organic compound synthesis. The physical properties of \r\( \text{AlCl}_{3} \r\) must be thoroughly comprehended to predict its behavior under various conditions, particularly when involved in complex chemical processes.

Chemical Behavior of Aqueous Solutions

The chemical behavior of aqueous solutions is central to a multitude of chemical reactions and processes. An aqueous solution is created when a substance (the solute) is dissolved in water (the solvent). The nature of the solute and its interactions with water molecules dictate the resulting behavior.

Upon heating an aqueous solution, the water molecules gain energy and eventually evaporate. As the water evaporates, the solute's concentration increases until the solution becomes saturated. If heating continues, the water will eventually evaporate completely, leaving behind the solute. Depending on the solute's properties, it may remain intact or undergo a chemical transformation due to the elevated temperature.

Aluminum chloride in an aqueous solution dissociates into its respective ions, \r\( \text{Al}^{3+} \r\) and \r\( \text{Cl}^{-} \r\). However, upon drying, these ions may recombine or convert into different species depending on numerous factors, including temperature, pressure, and the presence of other chemical compounds. Understanding these interactions and behaviors is fundamental for predicting the outcomes of evaporation and the subsequent state of the solute.

Dimerization in Chemistry

Dimerization in chemistry refers to the process whereby two monomer molecules join to form a dimer, a compound composed of two similar units connected by bonds. Dimerization is not limited to simple pairings but can also involve complex interactions between different atoms within the monomers leading to the formation of a stable structure.

In the context of aluminum chloride, dimerization occurs when two \r\( \text{AlCl}_{3} \r\) molecules combine to form \r\( \text{Al}_{2}\text{Cl}_{6} \r\). This process is reversible and highly dependent on the conditions such as temperature and the medium in which it is present. Dimerization can affect the chemical reactivity, solubility, and volatility of the compound involved. For students to grasp the concept fully, it’s important to highlight that dimerization is a dynamic process which plays a critical role in the behavior of many chemical compounds, especially those used as catalysts or those that are prone to polymerization.