Explain why a 1m solution of the compound AlCl3 in water has a lower freezing temperature than a 1m solution?

Question

of KCl in water

Answer 1

Because colligative properties are based on the number of species in solution. When AlCl3 dissolves, there are 4 species present in solution, one Al3+ ion and 3 Cl- ions. When KCl dissolves, only 2 species are present, K+ and Cl-. The effect isn't completely doubled, because the van't Hoff factor plays a role, but with more species present, there is more chance of intereference with the water molecules forming a orderly crystalline lattice, so the freezing point will be lower for an AlCl3 solution vs. a KCl solution at the same molality.

Answer 2

Aluminium chloride is a covalent conpound, despite being a compound between a metal and a non-metal. This is because of the high polarising effect of the Al3+ ion ( due to its high charge density)- Refer to Fajan`s rule. To solidify a liquid sample of it implies setting up weak Van Der Waals forces between the molecules of AlCl3. As these are weak, a small amount of energy is sufficient to bring the particles together in the solid state. KCl on the other hand is ionic. To solidify it, the ions need to be brought together with strong elctrostatic forces of attraction between them. A large amount of energy is needed to compensate for the energy released when the ions are brought together.

Note: Even if we were to compare 1m solution of AlCl3 with 0.00000000000000001m solution of KCl, or vice versa, the melting point of the former would still be lower as melting point is fixed, irrespective of the quantity.