Introduction

• Intramolecular forces - ‘primary bonds’, electrostatic attraction between atoms in a compound.

  • “intra” - within

  • “the forces that hold atoms together in a molecule”

• Intermolecular forces - ‘secondary bonds’, “the forces that exist between molecules”

  • Also known as van der Waals forces.

  • Strength of intermolecular forces determine the physical properties (melting/boiling point) of a molecular (not network aka continuous covalent, since the entire structure is considered to be covalent) molecule.

Properties of Secondary Bonds

• Strength of secondary bonds decreases with distance

• secondary bonds between non-polar molecules diminish incredibly quickly with distance.

Dipole-dipole interactions

• Dipole - molecule where the ends have opposing charges.

• Dipole-dipole interactions - Attractive force between polar molecules.

  • (Polar molecules have permanent dipoles, because they are polar opposites)

  • Attraction between partially positive pole in one molecule, and partially negative pole in another molecule.

• Strength of dipole-dipole interactions depend on:

  • Distance between molecules; the closer the distance the greater the interaction

  • Magnitude of electric charge:

    • The greater the difference of electronegativity → the greater the magnitude → the greater dipole-dipole interaction

Hydrogen Bonds

• Hydrogen bonds - A type of dipole-dipole interaction that occurs between: a partially positive (𝛿+) hydrogen and a partially negative (𝛿-) oxygen/nitrogen/fluorine.

• Oxygen/nitrogen/fluorine = most electronegative elements, therefore making hydrogen bonds a quite strong form of dipole-dipole interactions.

  

Dispersion forces

• Dispersion forces - Weak forces of attraction between both polar and nonpolar molecules.

  • Because of random electron movement, even nonpolar molecules can have temporary dipoles

• Random movement of electrons causes temporary dipoles - asymmetrical distribution

  

• Temporary dipoles form temporary attractions.

Induced Dipoles

• Induced dipoles - formed from when one molecule is temporarily polarised, as with dispersion forces (process is called induction)

  • This results in another molecule having its symmetrically-distributed electrons repelled by the extensive partial negative charge of the first molecule.

  • Electrons of second molecule are forced to one side, causing a partial negative on the opposite side of the first molecule’s partial positive.

  

• Induction is more successful with larger atoms and molecules - the negatively charged electrons are further away from the positively charged nucleus and thus are easier to shift.

  • Increased size - greater number of electrons; increases size of potential temporary polarisation of molecule; leads to greater induced dipole-dipole interactions.

Ion-Dipole Interactions

• Ion-Dipole interactions - Occurs between ion and a polar molecule.

• Used to determine solubility of ionic compounds.

• Strongest type of secondary force.

Strength of Secondary Bonds

Melting/Boiling Points

• Nonpolar molecules have greater boiling points as their size increases.

• Polar MP/BP vary widely.

• Depending on structure of molecule, both hydrogen and dipole-dipole bonds can be involved.

  • Hydrogen bonds → higher boiling point because hydrogen bonds are the second strongest type of intermolecular force.