GENCHEM2

• Importance of Intermolecular Forces

  • They determine the physical properties of molecules: boiling point, melting point, density, enthalpies of fusion and vaporization, viscosity
• Liquids have a definite volume independent of the size and shape of their container
• Liquids have varying surface tension properties depending on the nature of their intermolecular forces
• Ion-Dipole Force


  • Attractive force between an ion and a neutral molecule with a dipole, becomes stronger with increased ion charge or dipole magnitude
• Cohesion


  Force of attraction between the same molecules
• London Dispersion Force


  • Weakest intermolecular force, depends on the number of electrons in a molecule, explains why nonpolar covalent compounds freeze at low temperatures
• Intermolecular Forces (IMFs)


  Forces between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles, e.g., atoms or ions
• Surface Tension is the resistance of a liquid to spread out and increase its surface area
• Cohesive Force


  Force that holds the body of a liquid together with a minimum surface area, with strong cohesive force, liquid will maintain its shape
• Liquids are more dense and less compressible than gases
• Types of Intermolecular Forces (IMFs)

  • Dipole-Dipole Interaction
  • Hydrogen Bonding
  • London Dispersion Force
  • Ion-Dipole Force
• Intramolecular force


  Force of attraction between atoms within a molecule by the sharing of electrons
• Attractive forces in liquids are not strong enough to keep neighboring molecules fixed in position, allowing molecules to move past or slide over one another
• Intermolecular attractive forces are strong enough to hold molecules close together
• Adhesive Force

  Force that t
• Concepts Related to Surface Tension: '"AS INTERMOLECULAR FORCES INCREASE, SURFACE TENSION INCREASES"'
• Adhesive Force


  Force that tries to make a body of a liquid spread out
• Surface Tension also allows insects to walk on water
• As intermolecular attraction increases

  Viscosity increases
• Capillary Action


  Rise of liquid through a very small diameter tube
• A paper clip can float on water because of surface tension
• As intermolecular attraction increases

  Boiling point increases
• Vapor Pressure


  Measurable quantity that exists when a liquid and its vapor are in equilibrium
• Activity 1: Enumeration of forces of attraction, differentiation of forces, and types of IMF
• Surface tension


  With strong cohesive force, liquid will maintain its shape
• Molar heat of vaporization is an important part of energy calculations
• Viscosity


  Internal property of a fluid that offers resistance to flow
• Problem Solving: Calculation of heat energy required to vaporize 45.0 g of H2O given the formula q = (∆ Hvap ) x (mass) and Hv of H2O = 40.7 kJ
• Adhesion


  Force of attraction between different molecules
• Meniscus


  Concave or convex surface of a liquid caused by the attraction between the molecules of the container (glass) and liquid
• Molar Heat of Vaporization


  Amount of heat necessary to boil (or condense) 1.00 mole of a substance at its boiling point
• As intermolecular attraction increases


  Vapor pressure decreases and boiling point increases
• Boiling Point


  Temperature at which vapor pressure becomes equal to the atmospheric pressure or the pressure exerted on the surface of the liquid
• Units for molar heat of vaporization are kilojoules per mole (kJ/mol) or sometimes J/g
• High Specific Heat


  • Specific Heat – the amount of energy absorbed or lost by 1 gram of a substance to change the temperature by 1 degree Celsius
  • Water can moderate temperature because of its high specific heat and high heat of vaporization
  • A lot of energy is needed to break down the hydrogen bonds formed by water molecules
  • When bonds break, water molecules move freely and create more friction. More friction will produce a higher temperature
• Temperature and movement of molecules in water

  • When heat is first added, there is no rise in temperature because the energy is breaking the bonds
  • Only once the bonds are broken and the energy starts increasing the movement of water molecules (friction) does the temperature rise (to boiling)
  • Water has a unique ability to absorb heat without a corresponding rise in temperature
• Lower density of ice - hydrogen bonds of water molecules form ice crystals - ice is less dense than water because of the hydrogen bonds being apart
• High Heat of Vaporization

  • Heat of Vaporization - the amount of heat energy needed to change a gram of liquid into gas
  • The property of water that is responsible for moderating temperature
  • Water needs a lot of energy to break down the hydrogen bonds
  • The evaporation of water off a surface causes a cooling effect
• During daytime, sand gets heated more quickly compared to water. The air layer associated with sand becomes hot and rises up. This causes the sea breeze to move towards the land
• Molar Heat of Vaporization of Water (Hv) = 40.66 kJ/mol or 40,660 J/mol