It is usually the case that, among chemically similar compounds, the one with the higher molecular weight?

Question

will have the higher melting point and boiling points. However, this is obviously not the case in SnCl2 and SnCl4.

I'm aware that SnCl4 has a lower melting point due to the covalant character, but could someone explain this to me?

Cheers

Answer 1

Here's a newsflash:  The melting point and boiling points of substances have NOTHING to do with the molar masses.  Melting and boiling points have everything to do with the intermolecular forces which attract one molecule to another.   These intermolecular forces are called van der Waals forces and consist of dipole-dipole interactions, London dispersion forces and hydrogen bonding.

Dipole-dipole interactions require polar molecules with positive and negative ends that attract other polar molecules.  London dispersion forces are based on the polarizability of a molecule which, in turn, depends on the number of electrons, and hydrogen bonding requires that hydrogen be bonded to to F, O, or N so that the H of one molecule be attracted to the F, O or N of an adjacent molecule.

High melting and boiling point are also attributable to the networks formed by the extremely polar covalent bonds that we sometimes call "ionic bonds".

We can judge the degree to which a covalent bond displays more or less ionic character by calculating the electronegativity difference.    The published electronegativity for chlorine is 3.16, and for tin, it is 1.96.  This produces a DEN of 1.2.  We can compute* the percent ionic character for this bond.  It is 30.2 percent ionic (~70 percent covalent).

Tin (IV) chloride is a liquid at room temperature.  What accounts for the higher melting point of SnCl2 (246C) when compared to SnCl4 (-33C)?    The difference is that SnCl2 forms a network consisting of long chains of SnCl2 units, where SnCl4 does not.  

The end result is that Sn(IV) has a much higher effective electronegativity than what is published for Sn.  There is a general trend among polyvalent elements where the effective electronegativity goes up as the oxidation number goes up.  With a higher electronegativity, the electronegativity difference for Sn(IV) and chlorine is much less, indicating a much more covalent bond.