Intra vs Intermolecular Forces

Learned this in AP Chemistry

What is it? §

Intramolecular Forces - attractive forces that hold particles together in ionic, covalent and metallic bonds

• Intra - within

Intermolecular Force - attraction between adjacent molecules

• Much weaker than intramolecular forces
• Forces that hold solids and liquids together
  • When a substance undergoes a phase change, intermolecular

Graphs §

bond energy = 431 kJ/mol intermolecular attraction = 146 kJ/mol

Dipole-Dipole Forces §

• Forces that exist between neutral (no charge) polar molecules
• Attraction between ${\delta }^{+}$ and ${\delta }^{-}$ ends of different molecules

Hydrogen Bonding §

• Special case of dipole-dipole forces
• Requires H to be covalently bonded to one of the three most electronegative elements- F, O, or N (hydrogen bonding is FON)
• creates a very polar bond (greater partial ${\delta }^{+}$ and ${\delta }^{-}$ charges)
• Strength:
  • hydrogen bonds > dipole-dipole bonds

London Dispersion Forces (LDF) §

• weak forces that result from temporary shifts in the density of electrons in the electron cloud
• At given points the electrons are not even distributed
  • Results in an instantaneous asymmetric e${}^{-}$ distribution
    • Creates an instantaneous dipole moment
• The forces between instantaneous dipoles are called London Dispersion forces
• non-polar is always LDF
• Strengths (strongest to weakest):
  • Ionic
  • Hydrogen Bond
  • dipole-dipole
  • LDF
• Polarizability - the ease with which an electron cloud can be deformed
• More electrons = more polarizable -> more instantaneous dipoles -> more LDF’s
• larger molecules (the greater the # of e${}^{-}$) are more polarizable

Trends §

The stronger the forces between the particles:

• the higher the melting point.

• the higher the boiling point.

• the lower the vapor pressure.

• the greater the viscosity.

• the greater the surface tension.