Question

Why does aluminium chloride, $A{{l}_{2}}C{{l}_{6}}$, sublime at the relatively low temperature of $180{}^\circ C$?
(1) The intermolecular forces between the $A{{l}_{2}}C{{l}_{6}}$molecules are weak.
(2) The coordinate bonds between aluminium and chlorine are weak.
(3) The covalent bonds between aluminium and chlorine are weak.
(A)- 1,2 and 3 are correct.
(B)- 1 and 2 are correct.
(C)-2 and 3 are correct.
(D)- 1 only is correct.

Answer 1

Hint: The transformation of a substance from the solid to the gaseous state, without transforming through the liquid state is known as Sublimation. Aluminium chloride and phosphorus chlorides are two tricky chlorides of period 3. Aluminium chloride and phosphorus chlorides modify their structure from ionic to covalent when the solid compound turns to a liquid or vapour state of the compound.

Complete answer:
-Sublimation process is an endothermic process, as it requires heat energy to occur. This phenomenon occurs at temperatures and pressures lower than a substance’s triple point in its phase diagram, which corresponds to the lowest possible pressure at which the substance can exist in a liquid state.
-In thermodynamics, the temperature and pressure at which the all the three phases (solid, liquid, and gaseous) of the substance coexist in thermodynamic equilibrium are called as the triple point of a substance.
-Sublimation occurs when the molecule absorbs enough heat energy that it can overcome the attractive forces of their neighbours and escape into the vapour state.
-Electronegativity of elements increases as we go across the period and aluminium being the third element in period 3 is not more electronegative. The electronegativity of aluminium is 1.61 and the electronegativity of chlorine is 3.16, which implies that there isn’t enough electronegativity difference between aluminium and chlorine for there to be a simple ionic bond.
-At room temperature, the aluminium atom in Aluminium chloride is six-coordinated, which means each aluminium atom is surrounded by six chloride atoms and the structure is an ionic lattice with a lot of covalent character.
-At ordinary atmospheric pressure, aluminium chloride sublimes at temperature $180{}^\circ C$. If the pressure is raised to just over 2 atmospheres, it melts at a temperature of $192{}^\circ C$. At this temperature, the aluminium modifies and thus becomes 4-coordinated. Each aluminium is now surrounded by 4 chlorines rather than 6.
-In simple words, aluminium chloride on raising the temperature loses its ionic character due to which aluminium chloride sublimes.
Therefore, only sentence 1 is correct.

Hence, the correct answer is option D.

Note:
Let us now see how the reactivity of aluminium chloride changes at different states and temperatures. Aluminium chloride at room temperature is six-coordinated and present in dimer state. Due to the absence of free ions, aluminium chloride in this state does not conduct electricity. Aluminium chloride at higher temperature and pressure is again non-conductive because it loses its ionic character. Moreover, aluminium chloride reacts drastically with water. Aluminium chloride in presence of water rather than simply dissolving reacts with water producing hydrogen chloride gas.