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Experiment 11 11—1 VAPORIZATION AND INTERMOLECULAR FORCES I. Learning Objectives… To understand how heats of vaporization vary with the strength of the intermolecular forces present in various solvents . To relate enthalpy of vaporization to intermolecular forces . II. Background Information Intermolecular Forces Intermolecular forces are involved when molecules associate with one another in the liquid or solid states. Intermolecular forces include ion-dipole interactions, dipole-dipole interactions, and induced-dipole interactions. Since the compounds used in this experiment are covalent, no ion-dipole interactions are involved. Dipole-dipole interactions are weaker than ionic interactions and arise from interactions between partial charges. Hydrogen bonding is a particular type of dipole-dipole interaction that involves a link between the highly electronegative N, O, and F atoms and an electropositive H atom (which is bonded to a N, O, or F). Normally, bond energies associated with hydrogen bonds are in the range of 20-30 kJ/mol. Many molecular interactions involve induced dipoles ( dispersion forces ). As two nonpolar molecules approach one another, repulsions and/or attractions between their electrons and nuclei lead to distortions in their electron clouds. These slight distortions result in momentarily induced dipoles that lead to intermolecular attraction. Generally, a larger molecule (with more electrons) produces greater fluctuation in charge distribution and thereby a larger induced dipole. Dispersion forces generally increase with molecular weight. Dispersion forces are also affected by the arrangement in 3-D space of the atoms in a molecule. Unbranched (or straight chain ) compounds tend to have stronger dispersion forces than their branched isomers . The atoms of neighboring branched molecules are unable to position themselves as close together as their unbranched counterparts. When compared to hydrogen bonding, typical bond energies for dispersion
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11—2 forces are considerably lower and typically near 2 kJ/mol. The strength of these forces, however, lies in the cumulative effect of the large number of interactions possible as molecular size and weight increases. Vaporization and Intermolecular Forces When moving from the world of the atom to the world around us, it becomes clear that the number of atoms and molecules in the universe is extremely large. Statistics are necessary when working with these huge numbers since there is no way to calculate the behavior and interactions of all of the molecules in even a very small collection. Ludwig Boltzmann and James Clerk Maxwell contributed to the science of statistical thermodynamics in which the properties of very large numbers of molecules is examined. The
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