November 14: Intermolecular Forces

In  class, we learned about 4.4- Intermolecular Forces 

The Physical Properties of molecular substances result from different types of forces between their molecules

Our key understandings for this topic were:

        Intermolecular forces include London (dispersion) forces, dipole-dipole forces and hydrogen bonding
    The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds << covalent bond

London (dispersion) forces: 

  These are the weakest form of intermolecular force        
       Arise due to electrons constantly moving around → electrons are unevenly spread
    This produces temporary instantaneous dipoles
      An instantaneous dipole can induce another dipole in a neighboring particle 
    This results in a weak attraction between the two particles
      Their strength increases with increasing molecular mass

Dipole-Dipole Forces:

                   Still relatively weak, but the attraction is stronger than London (dispersion) forces

                     Occurs when polar molecules are attracted to each other by electrostatic forces                         
                     Permanent dipole: when one end of the molecule is electron deficient with a partial positive charge (δ+), while the other end is electron rich with a partial negative charge (δ–) 
                     Dipole-dipole forces occur when the opposite charges on neighboring molecules    attract each other

Hydrogen Bonding:

                  
 Hydrogen bonds are the strongest form of intermolecular attraction 
 Occurs when hydrogen is (covalently) bonded directly to a highly electronegative element, such as fluorine, oxygen or nitrogen

 

This causes boiling points of substances that contain them to be higher than what would be expected from their molar mass

Physical Properties → related to bonding type:

Melting/Boiling Points 
-       The melting & boiling points (of simple covalent molecules) depend on the type of forces of attraction between the molecules:

Hydrogen bonding > Dipole-dipole > London (dispersion) forces

  Solubility
-       ‘Like dissolves like’
-       Polar substances tend to dissolve in polar solvents
-       Non-polar substances tend to dissolve in non-polar solvents

Ex: Water H₂O (which shows hydrogen bonding) will be good for dissolving Ethanol C₂H₅OH (which also shows hydrogen bonding), but it won’t dissolve Hexane (only London forces) → oil & water won’t mix

  Volatility
-       The weaker the intermolecular forces, the more volatile the substance

  Electrical conductivity
-       For conductivity to occur the substance must possess electrons or ions that are free to move