AP Chemistry Unit 3.1 Intermolecular Properties

Intramolecular vs Intermolecular

• Intramolecular bonding: what holds atoms together within the molecule
  • Covalent and ionic
• Intermolecular forces: forces between (rather than within) molecules

 Intermolecular Forces

•  Weaker interactions that occur between molecules (3 types)

1. 1. Hydrogen bonding (strongest)
   2. Dipole-dipole forces (intermediate)
   3. London-dispersion forces (weakest)

• Intramolecular forces are a lot stronger than intermolecular forces
• The weaker the forces, the more likely the substance is to exist as a gas → bcuz particles are able to move far apart since they are not held together very strongly
• Molecules orient themselves to maximize the ⊕,⊝ interactions and to minimize ⊕,⊕ and ⊝,⊝ interactions

 Dipole-Dipole Forces

• Dipole-Dipole Attraction: Molecules with dipole moments (polar molecules) can attract each other electrostatically by lining up so that the positive and negative ends are close to each other
  • Can be attractive or repulsive
• The strength of the interactions is directly related to the magnitude of the dipole
  • The more polar the molecule is (bigger electronegativity difference) → stronger the D-D attraction
    • Force becomes weaker as the distance between the dipoles increases (larger atoms)
  • Molecules with D-D have atoms held more tightly together → higher MP and BP → more likely to be a liquid at room temp

 Hydrogen “Bonding”

• Hydrogen bonds: A very strong type of dipole-dipole attraction that occurs when hydrogen is covalently bonded (intramolecular) to N, O, or F
• 

• • → not HB bcuz H is covalently bonded to Carbon

• It is very strong due to the large difference in electronegativity and hydrogen’s small size that allows dipoles to be close → need more energy to overcome bonds → higher MP and BP

 London Dispersion Forces

• ALL covalent compounds experience LDF (aka van der waal forces)
  • Do not use the word “has”
• Is the only force present in noble gas atoms and nonpolar molecules
  • Forces also exist between polar molecules but not as strong as D-D
• In non-polar molecules the electron charge is usually evenly distributed but it is possible that at a particular moment in time, the electrons might not be evenly distributed (e- are always moving in their orbitals) → distorts electron cloud → separation of charge creates instantaneous dipoles in molecule → induces another instantaneous dipole in another molecule → short/weak attraction between molecules
• Polarizability: how easy the electron cloud of an atom can be induced into a dipole
  • Elements with more electrons (greater molar mass) = more polarizable molecule = electron cloud can be more easily induced into a dipole = stronger LDF = higher MP/BP
• Force is usually weak but can be significant in large atoms/molecules (Ex: F2, Cl2, Br2)

Ion-Dipole Forces

• Attractive force between an ion and the oppositely charged end of a polar molecule → strongest force

• • Ex: NaCl and water 

Forces Between Polar and Nonpolar Molecules

• Dipole-induced dipole interaction: a permanent dipole on a polar molecule can induce a dipole on a neighboring nonpolar molecule
  • Thus polar molecules and nonpolar molecules exhibit an attraction for one another
    • Explains why oxygen and CO2 can be dissolved (slightly) in water
• Factors that affect strength → nature of both molecules
  • The larger the magnitude of the dipole in a polar molecule → the better able it is to induce a dipole in a neighboring molecule.
  • Nonpolar molecules with a greater number of electrons have an increased polarizability (LDF) → dipole more easily induced

Factors Affected by IMF’s

Properties that increase with stronger InterMF’s → bcuz molecules experience stronger attractions for each other and it takes more energy to overcome that attraction

• Melting/Freezing and boiling points
• Surface tension
• Heat of vaporization
• Viscosity

Properties that decrease with stronger InterMF’s

• Vapor pressure
• Volatility: how easily a liquid evaporates

Comparing IMF’s

• If the size of molecules is similar → H-bonding > D-D > LDF
  • If sizes are comparable → LDFs are similar bcuz have electron clouds of similar size and polarizability
• If the molecules are of different size → more complex → very imp. for AP exam
  • LDF’s can become more significant than D-D or H-B when the size of molecules is bigger
    • Occurs when the difference in magnitude of LDFs in A exceed the magnitude of all the other interactions in B, giving A more IMFs total
• More linear compounds (less branched) = greater surface area for contact between molecules = stronger InterMFs
  • Ex: for questions involving isomers