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HIST498 (2)

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Bryan Hogeveen

1 2 London dispersion forces (LDF) See fig 12.15 All molecules and atoms have London dispersion forces. London dispersion forces are generally weak. However their importance increases in molecules without a permanent dipole  Disturbance of the electron cloud, caused by applying an electric/magnetic field or by asymmetric movement of molecules or by a polar solvent  Induced (or instantaneous) dipole  Induced dipole in adjacent molecule 3 4 Two types: Look at the halogens.  Dipole–induceddipole Homogeneous diatomics No dipole–dipole forces Ex.CO 2dissolving in H2O   Inducedipole–inducddipole Only London dispersion forces Ex.I 2 At room temperature F weLkD  gas Weak forces 2 Dipole–induced dipole typically 2–10 kJ/mol Cl2 still weak LD  gas Induced dipole–induced dipole 0.05–40 kJ/mol Br2 mediumLD  liquid But LDF increase in strength with increase in atomic I2stronerD  solid size. More e –  greater opportunity to disturb e cloud Increase size  strongerLD   stronger induced dipole hardtrreak  higher mp and bp 5 6 Summary of bonding and nonbonding forces Liquids Gases condense when intermolecular forces become significant. Gases Liquids Freeotion Partimalotion Fill entire space Partially filled space No structural order Short range structural order ~ few molecules Remember IMF (solid) > IMF (liquid) > IMF (gas) 7 8 Qualitative solubility Demo 5 "Like dissolves like"
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