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Exp11_S08 - Experiment 11 VAPORIZATION AND INTERMOLECULAR...

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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
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