Chem

• Intermolecular Force (IMF)


  The force that binds two or more atoms together to form a molecule or chemical bond
• Intermolecular Force (IMF)


  • A force of attraction; electrons are attracted to the positively charged nuclei in the other molecule
• Types of Intermolecular Forces

  • London Dispersion Forces
  • Dipole-Dipole Forces/ Van der Waals Forces
  • Hydrogen Bond
  • Ion-Dipole Forces
• London Dispersion Forces


  Named after a German Physicist, Fritz Wolfgang London (1930); Dispersion pattern of distribution; Only force present in non-polar molecules; Very weak and acts at very small distances; Formed due to attraction between the positive nucleus with negative electron cloud
• Examples of London Dispersion Forces


  • O2, N2, H2 (usually single element)
• Dipole-Dipole Forces/ Van der Waals Forces

  Named after Dutch Physicist, Johannes Diderik van der Waals (1873); Only in polar molecules, due to the partial positive pole and the partial negative pole of the molecule; More polar substance = stronger d-d forces; Less polar substance weaker d-d forces
• Examples of Dipole-Dipole Forces/ Van der Waals Forces

  • HCl, CO2, SO2
• Hydrogen Bond


  Very strong dipole-dipole interactions; Occurs in polar molecules with Hydrogen and any of the highly electronegative elements (ex. O, N, or F); Tends to be strongly positive due to the strong tendencies of F, O, and N to attract the electron
• Examples of Hydrogen Bond

  • HF, CHCl3, H2O (typically H is directly bonded with O, N, or F)
• Polar solvents


  Usually water
• Polar solvents


  • KBr in H2O
• Non-Polar Molecules


  • Electronegativity Difference 0-0.5
• Polar Molecules


  • Electronegativity Difference 0.6-up
• Liquid


  Definite volume, low density; Takes shape of the container, incompressible
• Surface Tension


  Ability to resist an external force, attributed to the strong attractive and cohesive forces between and among molecules
• Surface tension focuses on the hydrogen bond between molecules compared to buoyancy
• Viscosity


  Fluid's resistance to flow; decrease when temperature increases
• Strong IMF


  Higher viscosity
• Weak IMF and Higher temp

  Lower viscosity
• Capillary Action


  Ability of fluid to rise in narrow tubes or be drawn into small openings
• Forces involved in Capillary Action

  • Cohesion - like molecules
  • Adhesive - unlike molecules
• Boiling Point


  Temperature at which its vapor pressure is equal to the external or atmospheric pressure; influenced by the strength of its IMF
• Stronger IMF


  Higher boiling points
• Weaker IMF


  Lower boiling points
• Vaporization


  Includes evaporation and boiling: refers to the escape of molecules from the liquid state to the gas or vapor state
• Volatile substances


  Evaporate readily
• Vapor Pressure


  Point at which equilibrium pressure is reached between molecules from the liquid state to the gaseous phase
• Molar Heat of Vaporization


  Amount of heat required to vaporize 1 mole of a substance at its boiling point
• Heat disrupts the intermolecular forces

  As molar heat vaporization increases
• Properties of Water


  • Cohesion
  • Adhesion
• If a substance is dissolved in water


  The freezing point is lowered
• Energy is lost when water freezes. Thus, nights when ice freezes often feel warmer than nights when the ice melts
• Boiling point of water: 100 C, Freezing point of water: 0 C
• Specific Heat


  Amount of heat needed to change the temperature of 1g of water by 1 C
• Water has a very, very, very high specific heat, which is why it takes so long for a pot of cold water to boil
• The specific heat of water is 5x greater than that of sand
• Density


  Changes with temperature and salinity, only substance that contracts when cooled
• Density of ice is lesser than that of liquid water, that's why ice floats in water
• Strong Hydrogen bonding formed at 0°C locks water molecules away from each other. When ice melts, the structure collapses and molecules move closer together
• Water has the highest surface tension of all commonly occurring liquids, second to Mercury (H)