LeSirPotatoes asked "Why is the melting point of LiCl lower than NaCl?"

I replied :

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

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 and thus an imprecise explanation, with no marks awarded on its own.

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

Consequently, the LiCl lattice is slightly less ionic (compared to the NaCl lattice), and slightly more SIMPLE MOLECULAR (and 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 formula units together in the lattice structure of the solid state, and hence a lower melting point for LiCl compared to NaCl.

Bear in mind to melt LiCl from (s) to (l) 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 of the lattice structure results in a disruption of the orderly lattice of the solid state, melting the solid into liquid, which is why the melting point of LiCl(s) is lower than that of NaCl(s).

To help you understand this better, think of it as : instead of having a more equal distribution of electron density and hence of electrostatic attractions or ionic bonds throughout the entire lattice structure as is the case with NaCl ; in LiCl due to the polarization of Li+ cations, there is now a more unequal distribution of electron density and hence of electrostatic attractions or ionic bonds throughout the entire lattice structure, which in effect causes the Li+ cations and the Cl- anions to partially 'pair up' as if they were partially simple Li-Cl molecules (but note that LiCl is still actually more ionic than covalent).

This means the 'intramolecular' Li-Cl bonds (still mostly ionic, but now with slight covalent character) are 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 thus weakened.

This results in a less energy required to overcome the weakened 'intermolecular' ionic bonds, and thus a lower melting point, for LiCl compared to NaCl. As stated earlier, melting LiCl(s) to form LiCl(l) only requires overcoming the weakened 'intermolecular' ionic bonds of the lattice, and not the strengthened 'intramolecular' ionic bonds of the lattice.

So the confusion most students have here, is that when they consider the LiCl ionic bonds gaining partial covalent character, they think the ionic lattice thus 'becomes' more like a strong giant covalent lattice (which would indeed increase not decrease the melting point), when in fact the opposite occurs : the ionic lattice 'becomes' (when comparing LiCl to NaCl) more like a weakened lattice of simple molecules (which would therefore decrease the melting point, as seen in the lower melting point of LiCl compared to NaCl), due to the strongly polarizing power of the high charge density Li+ cation causing the unequal distribution of electron density and thus unequal strengths of ionic bonds in the lattice structure (stronger 'intramolecular' ionic bonds, weaker 'intermolecular' ionic bonds).

Now, the above long-winded explanation is really just to help JC students UNDERSTAND the concepts involved (and to dispel the common misconception JC students have that from ionic lattice it becomes more giant covalent lattice, when in fact from ionic lattice it becomes more simple molecular lattice instead, hence a lower melting point).

Rest assured Cambridge will NOT require such a long-winded explanation. Summarily stating "the lower melting point of LiCl compared to NaCl is the result of the ionic LiCl having partial covalent character due to the high polarizing power of the Li+ cation due to its high charge density" OWTTE (or words to that effect) will gain you full marks for the question. That's all Cambridge requires as an explanation at A levels, no worries.
Quote 0 0