INTERMOLECULAR FORCES, INSTANTANEOUS (TEMPORARY) DIPOLE INDUCED DIPOLE FORCE

 

INTERMOLECULAR FORCES:

             (Van der waals, forces): Weak attractive forces which exist between the molecules are called intermolecular forces or often called van der waals forces. These forces are much weaker than ionic covalent or metallic bonding forces. Thus are three types of intermolecular forces:

            (1)              Instantaneous (temporary) dipole-induced dipole forces.

(2)              Dipole- dipole forces (permanent dipole forces)

(3)              Hydrogen-bonding

                       The increasing strength order of van der waals forces describes that hydrogen bonding is the strongest of all. Temporary dipole forces < Permanent dipole forces <           Hydrogen bonding.

 (1)      INSTANTANEOUS (TEMPORARY) DIPOLE INDUCED DIPOLE FORCE:

             Instantaneous dipole induced dipole forces are weakest of all the van der walls forces.  Such forces are generated due to temporary dipole created in non-polar molecules. These forces are also termed as Landon  forces or dispersion forces.

             These intermolecular forces are important in extremely short distance gas molecules. These forces also exist in all types of molecules in condensed phase, in mono atomic molecule gases and among symmetrical non polar molecules. These forces are more effective at low temperature and high pressure. Let us consider the example of mono atomic molecules of noble gas. Normally the electron cloud is symmetrically distributed around the nucleus. Actually electrons are moving rapidly around the nucleus, they may get unsymmetrical at any instant producing instantaneous (temporary) dipole. This instantaneous dipole induces a temporary dipole in the adjacent molecule. As a result weak attractive forces are generated between the molecules.

             The dispersion forces increase with the increasing number of electrons in a molecule. Electrons in large molecules could easily be distorted because electrons are situated away from the nucleus and are less firmly held by the nucleus. Thus in large molecules distribution of electrons may easily get unsymmetrical and produces instantaneous dipole. It can easily influence the neighboring large size molecule and induce dipole in the neighboring molecule. Hence, due to short distance this chain process induces dipole in all the molecules and they are held by weak Vander Walls forces.

 

            Energy is required to convert liquids into gas hence enthalpy change of vaporization or boiling points increases from small to large molecules. e.g; Among halogens fluorine has the least boiling point and iodine has the highest boiling point.


Alkines also show similar treated in their enthalpy energies i.e; increases with the increasing (number of atoms per molecule). For different isotopes straight chain compound have high boiling point and branched chain have low boiling point.

 

            Shape                                     Molecule                                           Boiling point

 

                                                            CH3-CH2-CH2-CH2-CH3                       360C

 

                                                            CH3-CH-CH2-CH3                                280C

 


                                                                    CH3

                                                                   

        CH3         

 


CH3-C-CH3                                          100C

  

       CH3

 

                                                                                                CH3

 


                  CH3CH2CH2CH2CH3 > CH3-CH-CH2-CH3 >   CH3-C-CH3

                                                           

 CH3                           CH3

                        360C                            280C                         100C

 

            Increased branching gives the molecules an almost spherical shape and unbranched gives have must more chance of dispersion than other molecules consequently dispersion forces decreases with increased branching and the boiling points decreases. PTFE (Teflon) is a non stick plastic dispersion attractive forces are weak in plastic and oil or grease whereas these forces are strong in oil or grease itself. This difference gives the polymer a non-stick properties.

 

            Another example is low density and high density poly (ethane) low density polymer have high degree of branching  whereas high density plastic have  less degree of branching. It is observed that branching decreases intermolecular forces hence, low density plastic due  to high degree of branching is less strong and have low boiling point whereas high density plastic due to less degree of branching is more opaque to light and have greater strength.

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