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Chem 2 - Phase Transitions I

Chem 2 - Phase Transitions I

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Chem 2 - Phase Transitions I

  1. 1. Phase Transitions Pt. 1 By Dr. Shawn P. Shields This work is licensed by Shawn P. Shields-Maxwell, Ph. D., under a Creative Commons Attribution-NonCommercial- ShareAlike 4.0 International License.
  2. 2. Recall: Our Central Question The physical state of matter depends on the balance between the kinetic energies of particles and the strength of their interparticle attractions (intermolecular forces). Why is a particular substance a gas, liquid, or solid (at a given temperature)?
  3. 3. Phases of Matter and IM Forces Substances that are Gases at room temperature (RT) have relatively low intermolecular attractions Liquids at RT have intermediate strengths of intermolecular attractions Solids at RT have the strongest intermolecular attractions
  4. 4. Phase Transitions are Physical Changes Think about an ice cube at a constant 1 atm pressure and 5 C… Hydrogen bonds
  5. 5. Phase Transitions are Physical Changes Below 0C (at low enough temperature), the kinetic energy of the water molecules is not high enough to disrupt the hydrogen bonds.
  6. 6. Phase Transitions are Physical Changes Increase the temperature to 25C (const P)… Heat flows into the ice cube and the kinetic energy of the water molecules increases. The ice cube melts. (Ask yourself…Why?)
  7. 7. Phase Transitions are Physical Changes Keep on increasing the temperature to 100C… The kinetic energy of the water molecules increases and the water boils. Some of the water molecules have enough kinetic energy to overcome the attractive potential (IM forces) and escape the liquid.
  8. 8. Phase Transitions and External Pressure It is not as obvious, but the physical state of matter depends on the pressure, as well. The general idea… As external pressure on a given gas increases (at constant T), the phase changes from gas liquid solid
  9. 9. Phase Transitions and External Pressure Compare the 3 scenarios for gases in a piston that were discussed in context of the van der Waals eqn… As the pressure on the gas is increased (at constant T), the gas molecules are forced closer together.
  10. 10. Phase Transitions and External Pressure The gas molecules “feel” each other’s presence and begin to cluster (due to IM attractions).
  11. 11. Phase Transitions and External Pressure Continue increasing the external pressure (at const. T)… When the pressure is high enough, the gas sequentially condenses to a liquid, then transitions to a solid (exception- water).
  12. 12. Major Types of Phase Transitions Melting (fusion)- solid to liquid Evaporation (vaporization)- liquid to vapor Sublimation- solid to vapor Condensation- vapor to liquid Freezing (or solidification)- liquid to solid Deposition- vapor to solid
  13. 13. Next Up… Vapor Pressure (Pt 2)

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