Explain the structure and bonding of Al2Cl6 and MgCl2 and the reaction of CaCO3 with HCl.

• i) The formula of aluminium chloride is given as $\text{Al}_2\text{Cl}_6$ because it exists as a dimer in the vapor phase and in non-polar solvents. Aluminium, being electron-deficient in $\text{AlCl}_3$, forms coordinate bonds with chlorine atoms from another $\text{AlCl}_3$ molecule to achieve a stable octet, forming the $\text{Al}_2\text{Cl}_6$ structure.

  • Reason: These compounds have simple molecular structures with weak intermolecular forces (van der Waals forces). Little energy is required to overcome these forces, resulting in low melting and boiling points, making them liquid at room temperature.

• iii) $\text{Al}_2\text{Cl}_6$ has a lower melting point than $\text{MgCl}_2$ because $\text{MgCl}_2$ is an ionic compound with a giant ionic lattice structure held together by strong electrostatic forces, requiring a lot of energy to melt. In contrast, $\text{Al}_2\text{Cl}_6$ is a covalent compound with a simple molecular structure where only weak intermolecular forces need to be overcome to melt it.

The balanced equation for the reaction is:

• Effervescence (fizzing) due to the production of carbon dioxide gas.
• The marble chips would decrease in size or dissolve.