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Vapor pressure is the pressure of gas above a liquid.

Intermolecular forces - those forces of attraction and repulsion that exist between adjoining molecules Technically, interparticle forces is the better term because that allow us to consider ion-dipole forces; however, most of the time we hear intermolecular forces (IMF).

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Vapor pressure is the pressure of gas above a liquid.

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  1. Intermolecular forces - those forces of attraction and repulsion that exist between adjoining molecules Technically, interparticle forces is the better term because that allow us to consider ion-dipole forces; however, most of the time we hear intermolecular forces (IMF).

  2. Dipole-dipole interactions (or attractions) are the forces between neighboring polar molecules. The negative dipole of one molecule is attracted to the positive dipole of another causing the molecules to stay together. • The strongest type of dipole-dipole interactions is called hydrogen “bond”ing. This occurs in molecules in which hydrogen is bonded to F, O, or N.

  3. The most familiar molecules with hydrogen bonding are H2O, HF, NH3, and alcohols like CH3CH2OH. • The force between ions and dipoles (full charges and partial charges such as NaCl in water) is the strongest type of interparticle force. • The weakest type of force which exists between ALL particles is called London dispersion forces (LDF). What causes these? Since electrons are always in motion, at any given time, the electrons in one molecule may repel the electrons in a neighboring molecule.

  4. So what are London dispersion forces? They are weak, temporary, unpredictable forces of attraction and repulsion that exist between all molecules. They are the ONLY forces that exist between nonpolar species. • The larger a molecule is, the more electrons it has. Accordingly, with more electrons, the molecule is more polarizable and will have greater LDF.

  5. Vapor pressure is the pressure of gas above a liquid. • Between which types of molecules - those with LDF, those with dipole-dipole interactions, or those with hydrogen bonding - will the vapor pressure be highest? Lowest? Why? • Vapor pressure is lowest over the substance with the greatest IMF because fewer particles are able to escape to the gaseous phase. Vapor pressure is highest over the substance with the weakest IMF because lots of molecules can escape to vapor.

  6. Clausius-Clapeyron equation • ln (VPT1) = ∆Hvap ( 1 - 1 ) • VPT2 R T2 T1 • Since heat of vaporization is an energy term, we use the energy R value which is 8.31 J/K-mol.

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