'A' Level Chemistry Problem Analysis: Melting Point of LiCl vs That of NaCl

QN: Why is the melting point of LiCl lower than that of NaCl??

Thought process:

Covalent bonds may be stronger or weaker than ionic bonds, (depending on factors such as effectiveness of orbital overlap and whether there is pi bonding involved for covalent bonds; for ionic bonds, the charge densities of the ions involved matter significantly).

But in explaining why LiCl has a lower melting point compared to NaCl, comparing the strengths of covalent versus ionic bonds per se is inadequate, not to mention wholly imprecise.

With the Li+ cation posessing a higher charge density compared to the Na+ cation, the electron densities of the Cl- anions are polarized to a greater extent by the cations in the LiCl ionic lattice than the NaCl ionic lattice.

Consequently, the LiCl lattice is slightly less ionic (relative to the NaCl lattice that is), and slightly more SIMPLE MOLECULAR (NOT more GIANT COVALENT, critical difference here), meaning that less energy is required to overcome the slightly weakened 'intermolecular' ionic bonds (and not the slightly strengthened 'intramolecular' ionic bonds) holding the Li-Cl units together in the solid lattice structure, hence a lower melting point for LiCl compared to NaCl.

Bear in mind melting LiCl (ie from solid to liquid) doesn't necessitate completely overcoming all ionic bonds (both 'intermolecular' and 'intramolecular') within the solid lattice structure, but rather just overcoming the weakened 'intermolecular' ionic bonds which results in a disruption of the orderly lattice of the solid state.

To help you understand things better, here's a different angle: due to the polarization of Li+ cations, there is now a more unequal distribution of electron densities and hence of electrostatic attractions or ionic bonds throughout the entire LiCl lattice structure, which in effect causes the Li+ cations and the Cl- anions to partially 'pair up' as if they were simply Li-Cl molecules (note though that LiCl as a whole is still actually more ionic than covalent).

The implication is that 'intramolecular' Li-Cl bonds (still mostly ionic, but now with slight covalent character) are subsequently strengthened, at the expense of the 'intermolecular' ionic bonds (ie. the electrostatic attractions holding neighbouring Li-Cl 'simple molecular' units together in the solid lattice structure) which are conversely weakened.

This results in lesser energy being required to overcome the weakened 'intermolecular' ionic bonds, and therefore a lower melting point for LiCl compared to NaCl.


The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums.


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