1. Equilibrium Of phases!

2. Equilibrium Definition: situation where two changes are occurring at equal rates in a closed system Closed system: energy can move, but matter can't Phase: part of a system that is separate from other parts due to its properties and composition When phase changes occur in a closed system, equilibrium means that one change occurs at the same rate as the opposite change

3. Phase Changes Melting: change from solid to liquid Sublimation: change from solid to gas Freezing: change from liquid to solid Vaporization: change from liquid to gas Condensation: change from gas to liquid Deposition: change from gas to solid

4. Phase Equilibrium In a closed system, opposing phase changes may occur at the same rate Example: freezing – melting H 2 O(l) H 2 O(s) The double arrow indicates reversible reactions

5. LeChâtelier Henri Louis LeChâtelier: 1850-1936 Studied chemical reactions and rates LeChâtelier's Principle: in a system in equilibrium, a stress will cause a shift that relieves the stress and equilibrium will be re- established with different conditions

6. LeChâtelier For example, in a closed system where H 2 O(l) H 2 O(s) Forward reaction is exothermic If heat is added to the system, this reaction slows, and reverse reaction (endothermic) occurs more rapidly until the amount of solid and liquid re-establish equilibrium

7. LeChâtelier If pressure or volume is changed in a system with gas, equilibrium is changed Equilibrium vapor pressure is the pressure exerted by a gas over its liquid phase Vapor pressure is affected by surface area of liquid, temperature, and type of molecule

8. Vapor Pressure

9. Volatile vs Nonvolatile Volatile liquid: relatively weak attractive forces, so loss of particles to gas is easy Volatile liquid would have high vapor pressure Volatile liquid has low boiling point

10. Read! Pages 372-382!