Intermolecular Forces

Intermolecular Attractions

The attraction between molecules. Named after Johannes VanDer Waals, these are called van der Waals. To determine which type of IMF a substance will exhibit you need to look at the types of atoms that are present in the sample and their polarity.

INTERMOLECULAR FORCES

Collectively called vander Waals forces. These are the attractive forces between molecules, between ions, or between ions and molecules

• London Dispersion Forces (LDF)

• Caused by the motion of the electrons

• Named for Fritz London, not the city in England

• Strength increases with increased number of electrons

• Strength increases with more mass Furthe
  Explains the behavior of noble gasses and halogens

• Everything has this!

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ONLY IN POLAR

DIPOLE-DIPOLE forces—

MOLECULES

• Dipole to dipole • LOF

• Hydrogen bonding,.

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• Dipole interactions occur when polar molecules are involved

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• Energy released when polar molecules interact with one another, and this is one of the reasons you must cool a gas consisting of polar molecules to convert it into a liquid, also reduces KE.

• Energy is released upon interaction; Energy is required to separate interacting dipoles [high MP, high BP, high AHvap.

• Hydrogen bonding-

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Dispension • attractive forces in which a hydrogen atom that is bonded to a highly electronegative atom has a very strong attraction to an unbonded pair of electrons.

really, it's not a bond, it's an attractive force that is NEVER to be confused with a bonded hydrogen!! It's a dipole-dipole interaction that involves an H that is bonded to a highly electronegative atom that THEN interacts with a pair of unbonded electrons on a highly electronegative atom on an adjacent molecule. It's a really big deal in Biology! That highly electronegative atom requirement keeps lots of hydrocarbons from exhibiting H-bonding

• N, O, or F-now these are some highly electronegative atoms

• Cl or S-weaker, yet still high in electronegative

• ION-DIPOLE forces-weaker than ion/ion interactions; occurs when a polar molecule is attracted to an ion

• Distance-the larger the interacting species, the weaker this force between them

• Chargefollow Coulomb's Law; the greater the charge the stronger the force of attraction

• Magnitudethe greater the magnitude of the dipole [larger dipole moment], the stronger the attraction

• hydrated-metals bond to several water molecules; porcelain also holds water

• heat or energy of hydration-when the water sets up an IMF with the metal, energy is released in the process

• energy of solvation-when "hydration" occurs with something other than water

• acids—H3O* is a hydrated proton

• Dipole-Induced Dipole forces—a polar molecule induces a temporary situation where a NON-polar molecule becomes slightly polar since the electron cloud is mushy •Induced Dipole-Induced Dipole forces-since the electron cloud is mushy and electrons move all the time, a molecule may build up a concentration of electrons on one side of the surface leaving a slight positive charge on the opposite side this sets off a chain reaction!

  Now very nonpolar molecules are behaving as if they were slightly polar-happens in iodine all the time! The larger the molecule, the more likely this will happen.

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  PROPERTIES OF SUBSTANCES DUE TO IMFS

  Property	increases
  Boiling/melting/freezing/etc. point	increases
  Enthalpy of fusion/vaporization etc.	decreases
  Vapor pressure Viscosity	increases increases
  Surface Tension	increases
  Lattice Energy	decreases
  Solubility (this depends a lot on the solvent and solute polarity)