1.6 Intermolecular Forces

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    Intermolecular Forces

    Intermolecular Forces
    Hydrogen Bonding -a super strong dipole-dipole force
    -must have an F-H, O-H, or N-H bond as a pure liquid
    -must only have F, O, N to hydrogen bond with water (or other H-bond donor)
    Dipole-Dipole Forces -interaction between molecules having permanent dipole moments
    -the larger the dipole moment, the larger the force
    London Dispersion Forces
    (van der Waals Forces)
    -weak interactions due to a transient (temporary) dipole
    -all molecules have these but is the only force present for nonpolar molecules
    -higher molecular weight, larger surface area, and greater polarizability result in higher London forces
    Ion-Dipole Forces -interaction between ions and polar molecules
    -present when ions are dissolved in polar liquids (such as NaCl in H2O)

    How to Rank Boiling Points

    Ranking Boiling Points
    1) Network Covalent (Cdiamond , SiO2)
    2) Ionic
    3) Hydrogen Bonding
    4) Dipole-Dipole
    (if all molecules are similar in size)
    5) London Forces
    Higher Intermolecular Forces Result In
    Higher Boiling Points
    Higher Melting Points
    Greater Viscocity
    Greater Surface Tension
    Lower Vapor Pressure

    Effects of Branching on Melting Points and Boiling Points

    Branching (usually) decreases the boiling pt, but increases the melting pt

    branching boiling point


    “Like dissolves like.” (in terms of polarity)