Honors Chemistry

 

INTERMOLECULAR FORCES AND BOILING POINTS

 

Intermolecular bonds are responsible for a covalently bonded substance (the only substances that make molecules) to exist as a liquid or solid.  Boiling points for these substances are a good measure of the strength of the bonds holding molecules to each other.  (All the boiling points given below are in Celsius degrees.)

 

Elements: (Van der Waal's forces at work)

 

         Halogen Family                      Inert Gases

            F2    -188                              He    -269

           Cl2      -35                              Ne   -246

           Br2     +59                              Ar    -186

             I2   +184                              Kr    -152

                                                         Xe    -107

                                                         Rn      -62 

 

·        Why do the boiling points increase as we go down in these chemical families?

 

 

Hydrogen Compounds by Chemical Family:  (The effects of hydrogen bonding)

 

  Carbon Family      Nitrogen Family      Oxygen family        Halogen Family  

                                                                            

    CH4    -164            NH3    -33                 H2O   +100              HF    +20       

    SiH4   -112             PH3    -88                 H2S      -61              HCl    -85      

    GeH4    -89            AsH3   -55                 H2Se    -42              HBr   -67      

    SnH4    -52            SbH3   -17                 H2Te      -2               HI     -35      

 

·        Just as in the previous examples, boiling points increase as we go down in a family, but what’s happening with NH3, H2O and HF?

 

                        

Carbon Compounds: (Van der Waal's, and hydrogen bonding plus Van der Waal's)

 

                    Alkanes                           Alcohols

 

                    CH4     -164                   CH3OH    +65

                    C2H6     -89                   C2H5OH   +79

                    C3H8     -42                   C3H7OH   +97

                    C4H10     -0.5

 

·        C2H6  and CH3OH have the same number of electrons (18), and so should have the same strength of Van der Waals forces.  Why does CH3OH have a boiling point that is so much higher? (154o higher!)

 

·        C2H6O has 2 isomers.  One has a boiling point of 79oC and the other has a boiling point of –23oC.  Why so much difference?  Draw structural formulas for the two isomers.