107 Ch11-REV01.pptx - Chapter 11 Liquids Solids and...

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Chapter 11: Liquids, Solids, and Intermolecular Forces
Properties of the Three Phases of Matter There is a competition between: Thermal energy, where molecules move Intermolecular attraction, that holds them together
Intermolecular Forces A molecule’s structure determines the strength of its intramolecular forces . Intermolecular forces are the attractive forces that exist between all molecules and atoms. Intermolecular forces are the forces that hold many liquids and solids together; The states/phases of matter depend on the magnitude of these intermolecular forces, relative to the amount of thermal energy.
Coulomb’s Law Force Charge 1 Charge 2 Potential Energy Electrical Permittivity of a vacuum (8.854 x 10 -12 C 2 •J -1 •m -1 or F/m) 1 2 2 q q F k r
Why Are Molecules Attracted to Each Other? Intermolecular attractions are due to attractive forces between opposite charges. Larger charge = stronger attraction Longer distance = weaker attraction Intermolecular attractive forces are small relative to the bonding forces between atoms (intramolecular). Generally smaller charges Generally over much larger distances
London Dispersion Forces They are also known as London forces or van der Waals forces. Instantaneous dipole that occurs accidentally in a given atom induces a similar dipole in a neighboring atom. Significant in large atoms/molecules. Occurs in all molecules , including nonpolar ones .
Effect of Molecular Size on Dispersion Force As the number of electrons increases, the strength of the dispersion forces increases. The stronger the attractive forces between the molecules, the higher the boiling point will be.
Effect of Shape on Dispersion Force The more molecules can interact, the stronger their Dispersion forces.
Trends in the Strength of Intermolecular Attraction Order the following in order of INCREASING intermolecular forces: SF 6 , He, CH 4 , CF 4 He-268.9 o C CH 4 -161.5 o C CF 4 -127.8 o C SF 6 -64 o C The stronger the intermolecular attractive forces, the higher the boiling point of the liquid.
Bond Polarity: Bond Dipole Moments Dipole moment , μ, is a measure of bond polarity. A dipole is a material with a + and − end. It is directly proportional to the size of the partial charges ( q ) and directly proportional to the distance ( r ) between them. μ (dipole moment) = ( q )( r ) Measured in Debyes, D Generally, the more electrons two atoms share and the larger the atoms are, the larger the dipole moment.

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