1. Chapter 8: Physical Equilibria In this chapter, we are going to discuss the equilibrium between phases of matter and the thermodynamics behind the process What can we use this for? –Environmental Sciences: Toxin cleanup, wastewater purification –Biochemistry/Biology: Gas exchange, blood flow –Materials Science: Colloids, Biomaterials

2. Equilibrium Remember: i.At equilibrium, the forward rate of change is equal to the reverse raate  G=0 iii.A phase change is the change of matter from one state to another Solid -> liquid -> gas -> liquid -> solid

3. Vapor Pressure If we place some water vapor in the headspace above the Hg in a barometer, it will exert pressure on the Hg This pressure is proportional to the amount of water we put into the space UNTIL we add so much that liquid water starts appear on the surface of the Hg. No more H2O vapor can form There is an equilibrium between the 2 states

4. Vapor Pressure The pressure exerted on the surface of the Hg increased until we started to get liquid appearing on the Hg At this point, the rate of vaporization equals the rate of condensation and the pressure measure is called the Vapor Pressure

5. At a fixed temperature, as long as some liquid is present, the vapor exerts a characteristic pressure regardless of the amount of liquid present The vapor pressure of a given phase of a substance is the pressure exerted by its vapor when the vapor is in dynamic equilibrium with the condensed phase Vapor Pressure

6. The vapor pressure of a liquid @ a given temperature is expected to be low and its enthalpy of vaporization high if the intermolecular forces are strong

7. 8.3: Variation of Vapor Pressure with Temperature What determines the vapor pressure of a liquid? The Intermolecular Forces or How easily the molecules can escape and enter the gas phase

8. Variation of Vapor Pressure with Temperature

9. We can model this behaviour with the Clausius-Clapeyron Equation: The Vapor Pressure Increases with increasing temperature Or The more molecules move, the more they go into the gas phase The higher the  H vap, the more energy required to vaporize the molecule 

10. 8.4: Boiling What happens when we boil a liquid (let’s say H 2 O) at atmospheric pressure? At 1 atm and 100°C, the water boils throughout the volume –You see bubbles coming from all over/throughout the pot, not just vapor coming off the surface This is the normal boiling point of water –The Temperature at which the vapor pressure equals 1 atm When the atmospheric/outside pressure is greater than 1 atm, the boiling point is ____? –Conversely, at lower pressures, the boiling point is ____

11. Boiling Boiling Occurs when the vapor pressure of a liquid is equal to the atmospheric pressure Strong Intermolecular forces usually lead to higher normal boiling points

12. 8.5: Freezing and Melting Remember: In a liquid, the molecules can slip past each other and in a solid, they can’t At the freezing temperature, the liquid and solid phases are in equilibrium The melting temperature is the same as the freezing temperature, it just depends on which way you look at it (are you trying to melt a solid or freeze a liquid?) The higher the pressure, the higher the melting (freezing) point. Why is this? –Less pressure, less inhibition of the molecules to eject into the gas phase Water is unique –Higher pressures equals a lower melting point

13. 8.6: Phase Diagrams A phase diagram plots the pressure versus temperature for a given sample We use them to determine the state of a sample at given conditions Phase boundaries separate the different phases and represent where the phases are in equilibrium with each other Triple point: Where 3 phase lines meet. All 3 phases are in equilibrium

14. Representative Phase Diagrams Sulfur has two solid forms, rhombic and monoclinic They differ in the way they are packed. There are 3 triple points

15. Representative Phase Diagrams There are at least 10 kinds of ice.

16. 8.7: Critical Properties What happens when we move to point C on the phase curve. As the temperature and pressure increase, the liquid and vapor phases remain in equilibrium, but the density of the gas phase increases At 218 atm and 374°C, the interface between liquid and vapor phases disappears

17. 8.7: Critical Properties Critical Temp (T c ) or Critical Pressure (P c ) reached

18. 8.7: Critical Properties A gas can be liquified at any pressure or temperature BELOW T c or P c Above the critical point, the material becomes a Supercritical Fluid Many commercial uses of these