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Intermolecular Forces

Intermolecular Forces. Dr. Nicholas Buker. Module Learning Goals. Review assessment of molecular polarity Use Coulomb’s law to understand attraction Learn to relate molecular structure to type of intermolecular force displayed Understand the relative strengths of covalent bonds and the IMF

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Intermolecular Forces

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  1. Intermolecular Forces Dr. Nicholas Buker

  2. Module Learning Goals • Review assessment of molecular polarity • Use Coulomb’s law to understand attraction • Learn to relate molecular structure to type of intermolecular force displayed • Understand the relative strengths of covalent bonds and the IMF • Connect intermolecular forces to important physical properties and biochemical phenomenon

  3. Review: Electronegativity and Bond Polarity • Electronegativity (EN)- Force of attraction for bonding electrons displayed by an element • Values range from 0.7 to 4.0 • ∆EN = 0.0 to 0.4 Nonpolar, ∆EN = 0.5 to 1.7 Polar, ∆EN > 1.7 Ionic Review assessment of molecular polarity

  4. Review: VSEPR Theory and Molecular Polarity • Valence Shell Electron Pair Repulsion Theory – Molecular shape can be predicted based upon electron repulsion • Overall molecular polarity is based upon symmetry • Polar molecules may contain nonpolar bonds and nonpolar molecules may contain polar bonds Review assessment of molecular polarity

  5. Coulomb’s Law Key points of Coulomb’s Law in chemistry: • Opposite charges attract each other • The greater the magnitude of these charges, the stronger the attraction • The smaller the distance between the charges, the stronger the attraction Coulomb’s law and attraction

  6. Intermolecular Forces: Dipole-Dipole Interactions • Polar molecules possess a positive region and a negative region • Attraction between these opposite charges is known as Dipole-Dipole Interactions Relate structure to type of intermolecular force

  7. Intermolecular Forces: Hydrogen Bonds • Not actually bonds! • Extra strong dipole-dipole interactions that exist between extremely polar molecules • Requisites: • H covalently bonded to F, O, or N • Lone pair on F, O, or N Relate structure to type of intermolecular force

  8. Intermolecular Forces: Dispersion Forces • Even nonpolar molecules may be temporarily polar due to electron motion • Interactions between these short-lived dipoles are known as Dispersion Forces Relate structure to type of intermolecular force

  9. Relative Strength of Intermolecular Forces Relative strengths of covalent bonds and the IMF’s • Intermolecular forces are much weaker than bonds • The more polar the molecule, the greater the partial charges and the stronger the interactions • Dispersion forces may be quite strong and in extremely large molecules

  10. Why are IMF’s Important? • Intermolecular forces control the majority of physical properties such as solubility, hardness, density, melting point, boiling point, vapor pressure and surface tension Boiling Points: CO2 -57°C H2O 100°C Connect IMF’s to physical properties

  11. IMF’s in Biology • Intermolecular forces are tremendously important in biology and biochemistry Connect IMF’s to biological phenomenon

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