Ask A Scientist Chemistry Archive

name         Ilia
status       student
grade        9-12
location     NY

Question -   Hello!
Why are the melting points of sodium halides (NaCl, NaBr, and NaI)
decrease as the molar masses increase, whereas the melting points of
hydrogen halides (HCl, HBr, and HI) increase as the molar masses
increase?
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Hi Ilia,

The three hydrogen halides you mentioned
are molecular compounds, and the molecules
are attracted to one another within
the solid mostly by van der Waals forces
since the individual molecules (HCl, HBR, HI)
have no overall charge.

The van der Waals forces are primarily
dipole-dipole and dispersion forces
in this case. The dispersion force
tends to increase rapidly with the
molar mass (all other things being equal) because
the molecule becomes more "fluffy" and polarizable
as the number of electrons increases,
so the intermolecular forces are stronger in HI.
This makes the melting point of HI higher than that of HCl.

The three sodium halides mentioned are ionic
compounds made out of charged atoms (ions).
Although dispersion forces are important
here as well, they are much weaker than the Coulombic
interaction between the ions and so we need to only
focus on the Coulomb interactions to understand the trend.

Each Na+ ion is attracted to all of the halide
ions within the crystal lattice, just as each
halide is attracted to all of the Na+ cations
in the lattice, according to Coulomb's law:

V = - M * q1 * q2 / r

V = energy of coulombic attraction

q1 = charge on the cation = +1
q2 = charge on the anion  = -1
r = distance between the nucleus of the cation and
the nucleus of the anion
M = a constant (depends on the kind of lattice and units)

The more negative V is, the stronger the attraction
between the ions.

This law says that the attraction get weaker as the
halide ion gets bigger, because the center of positive
charge on the Na+ is farther away from the
center of negative charge on the halide.
Since the ionic radius increases as you go
down the column, the Coulomb attraction gets
weaker as you go from NaCl to NaBr and the melting
point becomes lower as a result - although the
dispersion force increases, it does not increase
by nearly as much as the coulomb attraction decreases.

I hope this helps,

Dr. Topper
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