Chap11 - Chem 200 Dr Saidane Lecture Notes Chapter 11...

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Chem. 200 Dr. Saidane Lecture Notes Chapter 11 Intermolecular Forces, Liquids, Solids INTERMOLECULAR FORCES Molecules are drawn together by intermolecular forces. These forces are sometimes called “Van Der Waals forces”. There are three main types of intermolecular forces. Their names and strengths are summarized in the following table: Types of Interaction Energy in kJ/mol Interacting species Dipole-Dipole 2 0.3 Stationary polar molecules Rotating polar molecules London Dispersion 2 All types of molecules Hydrogen Bonding 20 N, O, F; the link is a shared hydrogen atom London Forces London forces arise from the attraction between two instantaneous dipoles . The dipoles are due to fluctuations in electron locations in the molecules. As the electrons move about, they produce a fleeting instantaneous dipole moment. Electrons may pile up at one end of a molecule, or an atom leaving the nucleus at the other end exposed. One end of the molecule will have a fleeting partial positive charge and the other end will have a fleeting partial negative charge. Even a noble gas atom can have a small instantaneous dipole moment. The instantaneous partial charges on different molecules attract one another and the molecules stick together. London forces act between all types of molecules, polar as well as nonpolar. The strength of London forces increases with molar mass. Heavier molecules have more electrons, and there are bigger fluctuations in partial charges as the electrons flicker between different positions. The strength of London forces is also determined in part by the shapes of molecules. Rod-shaped molecules have stronger London forces than spherical 1
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molecules of the same mass. The instantaneous dipole moments in two neighboring rod shaped molecules tend to be close together and to interact strongly. Those on neighboring spherical molecules tend to be far apart and to interact weakly. The stronger the London forces, the higher the boiling and melting points of the molecular compound. Dipole-Dipole Interactions Polar molecules attract each other by the interaction between the partial charges of their electric dipoles and give rise to a dipole-dipole interaction . This interaction is in addition to the London force. The dipole-dipole interaction is weaker than the ion-ion interactions in ionic solids because only the partial charges of the dipoles are involved in dipole-dipole interactions. The strength of dipole-dipole interactions depends on the magnitudes of the bond dipoles and the shape of the molecules (similar to the London forces). Polar molecules form liquids and solids (condensed forms) partly as a result of dipole-dipole interactions. The stronger the interactions, the higher the boiling and melting points. Hydrogen Bonding
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Chap11 - Chem 200 Dr Saidane Lecture Notes Chapter 11...

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