Experiment 1-5

• Melting point


  Temperature at which given solid material changes from a solid state to a liquid, or melts
• Melting point


  • A physical property
  • The amount of substance will not affect the melting point temperature
  • Used to identify the identity and purity of a compound
  • ALL compounds as long as they are solid, will have a melting point
  • Specific for a specific compound
• Intermolecular forces (IMF)


  The forces that hold molecules together
• Types of intermolecular forces

  • Hydrogen bonding
  • Dipole-dipole interaction
  • London dispersion forces
• Hydrogen bonding


  The strongest IMF, found in compounds with hydrogen bonded to fluorine, oxygen, or nitrogen
• Compounds with hydrogen bonding


  • Water, alcohols, carboxylic acids
• Dipole-dipole interaction


  IMF found in polar molecules with partial positive or negative charges
• London dispersion forces


  The weakest IMF, found in nonpolar compounds
• The type of IMF determines if a compound will have a high or low melting point
• Melting point range


  Melting point of pure compounds is expected to be narrow, while impure compounds will have a lower and broader melting point range
• Boiling point


  The temperature at which boiling occurs for a specific liquid
• Boiling point


  • Occurs when atoms/molecules of a liquid spread out enough to change from liquid to gas phase
  • Atmospheric pressure is equal to the vapor pressure of the liquid
• Vapor pressure


  The pressure or force exerted by molecules escaping the liquid state to the gaseous state
• Atmospheric pressure


  Greater atmospheric pressure requires more energy for liquids to boil, resulting in a higher boiling point
• Altitude/elevation


  Higher altitude/elevation results in lower atmospheric pressure and lower boiling point

  
• Vapor pressure is affected by the nature of the intermolecular forces present
• Intermolecular forces


  Stronger IMF result in higher vapor pressure and lower boiling point, weaker IMF result in lower vapor pressure and higher boiling point
• Boiling point elevation


  A colligative property where impurities increase the boiling point of a substance
• Recrystallization


  1. Dissolve impure compound in solvent at high temperature

  2. Cool solution to crystallize pure compound before impurities

  3. Separate pure crystals by filtration

  4. Can repeat process
• Spontaneous vs. induced crystallization

  • Spontaneous: pure crystals form naturally
  • Induced: crystals form with impurities occluded
• Decolorization


  Removing brightly colored organic impurities from a sample by adsorption onto materials like activated carbon
• Hot filtration is used to prevent premature crystallization and impurities
• Distillation


  Purification method for liquids that separates components based on differences in boiling points

  
• Distillation process


  1. Boil liquid, condense gas, collect liquid

  2. Simple distillation for widely different boiling points

  3. Fractional distillation for smaller boiling point differences

  4. Vacuum distillation for high boiling points

  5. Steam distillation for water-insoluble components

  
• Fractionating column


  Allows for many successive distillations to take place at once without dismantling the apparatus
• Sublimation


  Solid transforms directly into gas phase without first melting to liquid
• Sublimation


  • Occurs when a compound's vapor pressure equals its applied pressure
  • Substances with high vapor pressure at room temperature can sublime
  • Solid compounds that can sublime are very rare: weak intermolecular forces in the solid state
  • Purification technique for heat-sensitive organic solids
  • Solvents are not involved at all in the process
• polar molecules


  having different electronegativity
• Examples of London Dispersion forces 


  alkanes, alkenes, alkynes, hydrocarbons
• The factor affecting atmospheric pressure is altitude or we can call it as elevation
• Relationship of Atmospheric and Vapor Pressure


  As we go higher, boiling point decreases. As we go lower (at sea level) boiling point increases
• What are the 3 factors for the boiling point to be achieved?

  • Temperature
  • Atmospheric Pressure
  • Vapor Pressure
• Atmospheric Pressure


  Amount of force that the atmosphere exerts on an object
• Vapor Pressure


  The pressure or force that is being exerted by the molecules when they will escape the liquid state going to gaseous state
• High Vapor Pressure


  It will result to low boiling point because it will only take little energy for it to be equal to the atmospheric pressure
• Low Vapor Pressure


  It will result to a high boiling point because it will take more energy for it to be equal to the atmospheric pressure
• Recrystallization


  Purifying solid substances
• Distillation


  Purifying liquid substances
• 3 stages of crystallization

  1. removal of impurities
  2. produce nucleus
  3. crystal growth