PHASE CHANGES

• Intermolecular Forces


  Forces between molecules that hold them together in liquids and solids
• Phase Changes


  Transformations of matter from one physical state to another
• Phase changes occur when energy (usually in the form of heat) is added or removed from a substance
• Molecular order in different phases

  • Molecules in solid phase have the greatest order
  • Molecules in gas phase have the greatest randomness or disorder
• Types of phase changes

  • Melting
  • Vaporization
  • Sublimation
  • Freezing
  • Condensation
  • Deposition

  
• Endothermic processes


  Change in state where heat is absorbed (heat gained)
• Exothermic processes


  Change in state where heat is given off (heat lost)
• Heating curve


  Shows how temperature changes as heat is added to a substance

  
• Cooling curve


  Shows how temperature changes as heat is removed from a substance

  
• During phase change, two physical states of the substance exist at the same time

  
• When addition or removal of heat is stopped at the phase change temperature, the two physical states will interconvert from one state to the other, and will be at equilibrium

  
• Melting


  Transformation of solid to liquid
• Freezing


  Transformation of liquid to solid
• Normal melting (or freezing) point


  Melting (or freezing) point of a substance when the external pressure is 1 atm
• Heat of fusion


  Amount of heat needed to convert the solid to the liquid state at the melting point
• Molar heat of fusion (ΔHfus)


  Energy required to melt 1 mole of a solid
• Boiling


  Transformation of liquid to gas
• Condensation


  Transformation of gas to liquid
• Normal boiling point


  Boiling point of a liquid at 1 atm pressure
• Heat of vaporization


  Amount of heat absorbed by the sample as the liquid transforms into gas

  
• Molar heat of vaporization (ΔHvap)


  Energy required to vaporize 1 mole of a liquid at the boiling point

  
• Sublimation


  Transition of a substance directly from the solid to the gas phase, without passing through the liquid phase
• Deposition


  Transition of a substance directly from the gas to the solid phase, without passing through the liquid phase
• Molar heat of sublimation (ΔHsub)


  Amount of energy that must be added to a mole of solid at constant pressure to turn it directly into a gas, without passing through the liquid phase
• The sum of the heat of fusion and the heat of vaporization can give a good estimate of the heat of sublimation of a substance

  
• TYPES OF PHASE CHANGES

  IDENTIFY

  A) ENDOTHERMIC PROCESS

  B) EXOTHERMIC PROCESS

  C) SUBLIMATION

  D) DEPOSITION

  E) MELTING

  F) EVAPORATION

  G) CONDENSATION

  H) FREEZING