Gen chem 2 (intermolecular forces)

• Intermolecular Forces


  The Forces that Holds Condensed States Together
• Intermolecular Forces


  • Generally attractive forces
  • Basis is electrostatic attraction (positive & negative attract)
• Chemical bond


  • Determine chemical behavior of substance
  • Strongly affected by physical changes
• Attractive forces


  Determine the state of matter
• Coulomb's Law


  • E = 1 q₁q₂ / 4πε₀ r
  • When q1 and q2 are opposite in sign, E is negative
• Bonding forces


  Result of large charges (protons and electrons) interacting at very close distances

  
• Nonbonding forces


  Result of smaller charges interacting at greater distances
• Types of Intermolecular Forces

  • Dispersion Forces
  • Dipole-dipole Forces
  • Hydrogen Bonds
  • Ion-dipole Forces

  
• Dispersion Force


  • Also called the London force or London-Dispersion Force
  • Instantaneous dipole or a temporary dipole
• As the molar masses and electron cloud volumes of the noble gases increase


  The greater dispersion forces result in increasing boiling points

  
• 1. pentane vs neopentane

  2. pentane has a higher boiling point than neopentane due to stronger dispersion forces from its straight shape allowing more interaction along the molecule

  
• Dipole-Dipole Force


  • Exists in all molecules that are polar
  • Acetone has a permanent dipole which can interact with other acetone molecules

  
• Polar molecules


  Have higher melting and boiling points than nonpolar molecules of similar molar mass
• Polarity of molecules composing liquids


  Important in determining miscibility
• Hydrogen Bond


  • A sort of super dipole–dipole force
  • Occurs in polar molecules containing hydrogen atoms bonded directly to small electronegative atoms – F,O,and N

  
• Hydrogen Bonding


  Increases the melting and boiling points of ethanol compared to dimethyl ether
• Ion-Dipole Force


  Occurs when an ionic compound is mixed with a polar compound, especially important in aqueous solutions of ionic compounds