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dustbus 2013-10-23 11:48 PM

melting point

why boiling point can be determined by intermolecular force

but melting point cannot?(just like the shape,whether they are symmetry or not):smile_41::smile_41:

chungkin81 2013-10-25 02:52 PM

For melting point, we need to consider how well the particles are packed in the crystal structure.

If the pacticles are packed well in the crystal structure (that means the particles are symmetrical or look like a sphere), a large amount of energy is required to break the crystal lattice. As a result, the melting point is higher.

On the other hand, if the pacticles cannot pack effectively in crystal lattice, the melting point would be lowered.



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jmlo 2013-10-26 11:47 AM

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First of all, remember that intermolecular force is crucial in the determination of both melting point and boiling point of a substance.

Here we limit the discussion to molecular solids for the sake of simplicity.

The process of melting (or boiling) is dictated by the free energy change which is given by

dG = dH - TdS

The interaction between molecules in a solid is included in the enthalpy term dH, while symmetry aspect is described by the entropy term dS. The effect of symmetry is manifested in solid state because molecules are packed and cannot move freely. Therefore, the higher the symmetry, the better the molecules are packed. This will lead to a smaller change of entropy of fusion (i.e., melting) and thus a higher melting point of the molecular solid.

The effect of symmetry (and entropy) becomes less significant for liquid phase because the lattice structure is destroyed and molecules move more freely.

Let's look at a real example. Consider the following pairs of molecules: meta- and para-xylene
Since para-xylene is more symmetrical than meta-xylene, its entropy of fusion is higher. Using the experimentally determined enthalpy of fusion for these compounds:

Compound          meta-xylene         para-xylene
dH(fusion)          11.569kJ/mol        17.113kJ/mol
dS(fusion)           51.34J/K/mol        59.74kJ/mol

we can calculate the temperature at which the Gibbs free energy of fusion is zero (i.e., the melting point):

meta-xylene:    0 = 11.569 - T*(51.34/1000)   ---->  T = 225.34K
para-xylene:     0 = 17.113 - T*(59.74/1000)   ---->  T = 286.46K

So, you can see that both intermolecular force and symmetry are important in determining the trend of melting point.

On the other hand, the entropy changes of vaporization (i.e., boiling) of these compounds are:

meta-xylene: 88.25 J/K/mol
para-xylene: 87.65 J/K/mol

As you can see, the entropy changes are essentially the same for both compounds; it implies that the difference in boiling point is mainly due to the difference in the intermolecular force (or in other words, enthalpy) between these molecules in liquid phase.

[[i] 本帖最後由 jmlo 於 2013-10-26 11:49 AM 編輯 [/i]]

dustbus 2013-10-26 09:41 PM

多謝詳盡的解釋
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