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Unformatted text preview: 1 Intermolecular Forces Chapter 12 Kinetic Molecular Theory Average kinetic energy is related to the particles average speed, is proportional to the absolute temperature. We can use this theory to study gases liquids and this theory to study gases, liquids and solids. Gas: low intermolecular force, little attraction between molecules. Particles are far apart. More kinetic energy causes an increase in volume or pressure as the particles move faster. Liquids, solids Liquids: intermolecular forces are stronger, the particles are touching, and mobile. Increased kinetic energy causes increased motion of particles (vs their neighbors) and increased vibration. Notice that there is a slight increase in volume (orthats why a mercury or alcohol thermometer works). Solids: Intermolecular forces are dominant. Most particles dont move. They remain in place and can vibrate, but not move. Increased kinetic motion causes vibrations and some increase in volume. Definition Intermolecular forces are the forces between atoms, molecules, and ions that hold solids and liquids together. Intermolecular forces also play a part in the solubilities of substances. Ion-dipole forces Attraction of an ion to one side of a polar molecule. This interaction allows water to easily dissolve many ions. Dipole-Dipole forces Attraction of the oppositely charged sides of polar molecules molecules. May occur between molecules of the same or different compounds 2 Dipole induced dipole Occurs when a permanent dipole causes a non-polar species to polarize, forming a temporary dipole. This is a weak interaction. This phenomenon allows non-polar molecular to dissolve slightly in polar solvents (and vice-versa). Polarizability: The ease with which a particles electron cloud can be distorted. Smaller atoms are less polarized than larger atoms Polarizability increases as you go down a column and decreases left to right (big radius=more polarizable) Cations are less than atoms, anions are more than atoms. Induced dipole induced dipole: London dispersion forces Dispersion forces occur between non-polar molecules. These forces occur when a temporary dipole forms by chance in one molecule and induces a dipole in a second. This is a weak interaction. Note: greater MW=more Deformation=more London Bonding=higher boiling point: F 2 bp= -188.14 deg C Cl 2 bp= -34.6 deg C Br 2 bp= 58.78 deg C I 2 bp= 184.35 deg C Special Case: Hydrogen bonding A hydrogen bond is an especially strong dipole-dipole interaction. This is not a bond like a covalent bond. Occurs when hydrogen is covalently Occurs when hydrogen is covalently bound to N, O, or F (all small, very electronegative atoms) and has a dipole-dipole interaction with a highly electronegative atom that has a lone pair. (usually another N, O, or F atom in a compound) ....
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