1. Weak Intermolecular Forces:
* AlCl₃ exists as a dimer, Al₂Cl₆, in the solid state.
* The dimeric structure is held together by relatively weak van der Waals forces and dipole-dipole interactions.
* These weak forces are easily overcome by thermal energy, allowing the molecules to transition directly from the solid to the gas phase.
2. Covalent Bonding:
* The Al-Cl bonds within the Al₂Cl₆ dimer are strong covalent bonds.
* This strong intramolecular bonding contributes to the stability of the gaseous phase, making sublimation favorable.
3. Polarity:
* AlCl₃ is a polar molecule due to the electronegativity difference between aluminum and chlorine.
* The polar nature of the molecule further contributes to its tendency to exist in the gas phase.
4. Low Lattice Energy:
* The lattice energy of AlCl₃ is relatively low due to the weak intermolecular forces.
* This low lattice energy means that less energy is required to break apart the solid structure and transition into the gaseous state.
5. Sublimation is an Endothermic Process:
* Sublimation is an endothermic process, meaning it requires heat energy to occur.
* At 180°C, the thermal energy provided is sufficient to overcome the intermolecular forces and allow the AlCl₃ molecules to escape into the gaseous phase.
In summary, the combination of weak intermolecular forces, strong covalent bonding, polarity, low lattice energy, and the endothermic nature of sublimation allows aluminium chloride to sublime at 180°C.
