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Intermolecular Forces December 5, 2016 Clarification • IntRAmolecular Forces: bonds that hold individual molecules together • These occur inside a molecule • Covalent or ionic bonds • IntERmolecular Forces of Attraction: these are forces, NOT BONDS, that attract many molecules to each other • These occur between molecules What are Intermolecular Forces? • Three types: • Dispersion Forces (London) • Dipole-Dipole Forces • Hydrogen Bonding • These are attractions BETWEEN molecules • They are weaker than either ionic or covalent bonds • The are responsible for determining whether a molecular compound is a gas, liquid, or a solid at a given temperature • Reminder: THEY ARE NOT BONDS, THEY ARE FORCES!!! Dipole-Dipole Forces • Occurs when POLAR molecules are attracted to one another • The electrical attraction occurs between the oppositely charged DIPOLES of polar molecules • The slightly negative region of a polar molecule is weakly attracted to the slightly positive region of another polar molecule • Similar to, but much weaker than, ionic bonds Dispersion Forces • Also known as London Dispersion Forces • These are the weakest of all intermolecular forces • They occur between all molecules and all substances • Caused by electrons moving more to one side of a molecule which is closest to a neighboring molecule. The neighboring molecule’s electrons are momentarily repelled away, causing a momentary attraction between the two molecules due to a temporary dipole • Dispersion forces are stronger as the molecule gets larger! • Predict which will have greater dispersion force: Methane (CH4) or Ethane (C2H6)? Hydrogen Bonding • Can be in the same molecule or a nearby molecule (eg. DNA vs water). • Always involves hydrogen and nitrogen, oxygen, and fluorine (or other halogens) (I have had e-NOF of this) • Water is a very good example • An attractive force between a hydrogen (covalently bonded to a very electronegative atom) and another electronegative element with an unshared pair of electrons Strength of Forces • The strength of intermolecular forces varies depending upon the type of force: • London Dispersion • Dipole-Dipole • Hydrogen Bonding Weakest Strongest Intermolecular Forces Animation Take a look at the animations at the following site: https://www.wisc-online.com/learn/naturalscience/chemistry/gch6804/intermolecular-forces What do you need to do? For each of the 20 molecules on the front of your long worksheet. Decide if the molecules have hydrogen bonding, dipole-dipole forces, and/or dispersion forces. Complete the pre-lab for intermolecular forces. This can be found under the files!