go to UNSW home page
UNSW logo School of Chemistry Home Page
> Contacts   > Sitemap
About the School divider Research divider Graduate Study divider Undergraduate Study divider High School Resources

Intermolecular Forces

Review - 5

These are the normal boiling points of methane, dichloromethane, and tetrachloromethane (carbon tetrachloride):

CH4 -161.5 °C,   CH2Cl2 39.8 °C,   CCl4 76.7 °C

Interpret these values to decide what is the dominant type of intermolecular force in these substances.

Hints

First decide what types of intermolecular forces are present in these substances. One of these substances has a type of intermolecular force not found in the other two.

Answer

All of these substances have dispersion forces between their molecules and the strength of the dispersion forces increases as the size of the molecules increases. Chlorine atoms are larger than hydrogen atoms so the size of these molecules increases in this order:

CH4 < CH2Cl2 < CCl4

If dispersion forces were the dominant type of intermolecular force for these molecules you would expect the normal boiling points to increase in the same order. This is what is observed and so you can conclude that dispersion forces are the dominant intermolecular force for these molecules. CH4 and CCl4 are both non-polar molecules (they do not have a permanent dipole moment), however CH2Cl2 is a polar molecule (draw a picture or make a model to see this) and so CH2Cl2 molecules are attracted to each other by dipole-dipole forces as well as dispersion forces and this contributes to making the normal boiling point of CH2Cl2 higher than that of CH4. But if the dipole-dipole forces were much stronger than the dispersion forces you would expect the normal boiling point of CH2Cl2 to be higher than that of CCl4.

 

< Review - 4 Review - 6 >