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Phase Changes - PowerPoint PPT Presentation


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Phase Changes. Kinetic Theory. Kinetic= motion All matter consists of small particles The molecules are in constant, random, rapid motion All collisions are elastic (no net loss of energy). As temperature increases, the molecules velocity increases, increasing the pressure on the container.

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kinetic theory
Kinetic Theory
  • Kinetic= motion
  • All matter consists of small particles
  • The molecules are in constant, random, rapid motion
  • All collisions are elastic (no net loss of energy)
slide4

As temperature increases, the molecules velocity increases, increasing the pressure on the container.

mean free path
Mean Free Path
  • The average distance traveled between collisions.
  • An oxygen molecules will collide with other molecules 4.5 billion times per second!
slide6

The temperature of a substance is a measure of the average kinetic energy of its particles.

  • Absolute zero – the temperature at which molecular motion stops (-273.15oC)
gas pressure
Gas Pressure
  • Caused by gas molecules colliding with the sides of a container
  • Force per unit area
  • Units:

1 atm = 101.3 kPa = 760 mmHg = 760 torr

measuring pressure
Measuring Pressure
  • Open Manometer – atmosphere exerts pressure on one side and gas sample exerts pressure on the other side
    • Add if gas pressure is greater
    • Subtract if air pressure is greater
  • Closed Manometer (barometer) – vacuum on one side, gas pressure on the other side
    • No addition or subtraction necessary
vapor pressure
Vapor Pressure
  • The pressure produced when vapor particles above a liquid collide with container walls; a dynamic equilibrium exists between the liquid and the vapor.
  • Vapor pressure increases with temperature.
  • A substance with weak intermolecular forces has a high vapor pressure and low boiling points. (these are volatile – alcohols, ethers)
  • A substance with strong intermolecular forces has a low vapor pressure and high boiling point (these are nonvolatile – water, molasses, glycerol)
phase diagrams
Phase Diagrams
  • Critical Point – above this temperature, no amount of pressure can liquefy it.
  • Triple Point – all three phases are in equilibrium