Melting/Boiling points

[jm01]

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Hi, why do melting and boiling points of a compound depend on its molecular mass?

Thanks

[Pwnage101]

if its a non-polar substance, because the greater the molecular mass, the stronger the dispersion forces. Stronger intermolecular forces mean it is harder to break up 2 molecules, and thus it has a higher melting nd boiling point.

[jm01]

What if it's polar?

Thanks

[shady145]

Originally Posted by Pwnage101

if its a non-polar substance, because the greater the molecular mass, the stronger the dispersion forces. Stronger intermolecular forces mean it is harder to break up 2 molecules, and thus it has a higher melting nd boiling point.

o i thought that all molecules, polar and non polar had dispersion forces according to their size and mass, thats y we can get all elements as a solid because there are dispersion forces present in all molecular bonds.
the non-polar substances have the dipole-dipole bonds requiring more energy to break these bonds. (lol im hesitant about questioning you because i saw what ur HSC results were =s)

so to op this i what i think.
as mass and size of the molecules increase they create stonger dispersion forces, requiring larger amounts of energy to break these bonds.

[jetblack2007]

The way I understand it, the larger the molecule, the more electrons that are displaced to create the temporary dipole and hence the greater the force between molecules.

In order for a substance to melt and then boil, intermolecular forces must be overcome. Thus, the stronger the intermolecular forces, the higher the melting/boiling points.

Consequently, the larger the molecule, the higher the melting and boiling points of that substance.

[kaz1]

Originally Posted by shady145

o i thought that all molecules, polar and non polar had dispersion forces according to their size and mass, thats y we can get all elements as a solid because there are dispersion forces present in all molecular bonds.
the non-polar substances have the dipole-dipole bonds requiring more energy to break these bonds. (lol im hesitant about questioning you because i saw what ur HSC results were =s)

so to op this i what i think.
as mass and size of the molecules increase they create stonger dispersion forces, requiring larger amounts of energy to break these bonds.

I think what Pwnage101 was trying to say was that since non-polar molecules only have dispersion forces and nothing else as their intermolecular bonds. That is why the higher the molecular mass the higher the molecular mass the higher the melting/boiling point. Polar substances also have dispersion forces but it is more difficult to find the melting/boiling point as you have to take into account the differences in electronegativity.