Chapter 11 - Condensed Phases
- Preview of thermochemistry
- Transfer of HEAT
- HEAT is not the same as TEMPERATURE
- Heat measured in Joules
- 4.184 J - 1g water T 1C
- often use kJ
- Chemical/physical process usually accompanied by energy
- KM Theory for Condensed phases
- liquids packed more closely than gases
- enough energy to move, but not escape
- definite volume/variable shape
- solids
- energy less - molecules 'locked' into position
- definite volume and shape
- Crystalline/amorphous
- phase change - increase/decrease energy of system
- cause heat flow by T with surroundings
- can force some gases to liquid by pressure (more later)
- Intermolecular Forces
- Much weaker than intramolecular forces (bonds)
- BP/MP often indication of strength of IM forces
- 2 types of ionic forces
- Ion-Ion
- Strongest IM force
- long rang - over several ions
- Ion-Dipole
- very important for the solubility of ionic compounds in water!
- 3 types of van der Walls forces
- Dipole-Dipole
- molecules of approx same size/mass, BP increases with polarity!
- London dispersion forces
- Instantaneous dipole/induced dipole
- Size and shape dependent
- larger - more easily polarized
- more spherical - less easily polarized
- Occur in both polar and non-polar molecules
- HBr BP higher than HCl BP
- Hydrogen bonding
- Special case of dipole-dipole
- occurs because of high polarity and small size
- H almost bare + charge - no shielding from inner electrons
- H bonded to F, O, N.
- BP water about 200C higher than predicted!
- Energy - 4kJ to 20 kJ
- larger than other IM forces but not as strong as bonds.
- Open structure of ice
- Viscosity and Surface tension
- viscosity
- resistance to flow
- related to attractive forces and shape
- Motor oil
- Surface tension
- force on molecule at surface different than bulk
- tend to minimize surface area
- form spherical drops
- manufacture of ball bearings/spheres in space
- Surface tension
- allows bugs to 'walk on water'
- molecules more closely packed.
- Cohesive and Adhesive forces
- cohesive - between same type of molecules
- adhesive - different molecules
- 'Adhesive' tape
- meniscus - strength of cohesive vs adhesive
- capillary action
- Changes of State
- Vaporization
- Sublimation
- Condensation
- Melting (fusion)
- Freezing
- All require energy
- Explain why ice water remains 0C while surroundings at RT
- Heating curve
- Critical temp/pressure
- Temp - highest temp you can liquefy a gas by pressure
- Pressure - pressure required to liquefy at crit. temp
- Vapor Pressure
- Pressure due to evaporation for saturated atmosphere
- Molecular level explanation
- Volatility - ease of evaporation
- more volatile - weaker IM forces
- Reaches dynamic equilibrium
- Link between vapor pressure and temperature
- Link between vapor pressure and boiling point
- Phase diagrams
- plot of pressure vs temp
- lines show equilibrium conditions for phase change (melting, subliming, boiling)
- Define triple point and critical point
- Show 'unique' phase change for icewater
Learning Goals
1. Employ the kinetic-molecular model to explain the differences in motion of particles in
gases, liquids, and solids and how these relate to their states.
2. Describe the various types of intermolecular attractive forces, and state the kinds of
intermolecular forces expected for a substance given it's molecular structure.
3. Explain the meaning of the terms viscosity, surface tension, critical temperature, and critical
pressure, and account for the variation of these properties in terms of intermolecular forces
and temperature.
4. Explain the way in which the vapor pressure of a substance changes with intermolecular
forces and temperature.
5. Describe the relationship between the pressure on the surface of a liquid and the boiling
point of the liquid.
6. Draw a phase diagram of a substance given appropriate data, and use the phase diagram to
predict which phase(s) are present at any given temperature and pressure.
7. Distinguish between crystalline and amorphous solids.
Comments? E-mail me.
Return to my home page.