1. Changes of State
College Chemistry

2. Changes of State
Change of state or phase change – conversion of a substance from one of the three physical states of matter to another
Always involves a change of energy

3. Change of State
In order to move “up” from solids  liquids  gases, you must add more energy (endothermic)
In order to move “down”, energy is given off (exothermic)

4. Vaporization, Condensation, and Evaporation
Vaporization – change of state from a liquid to a gas
Occurs when heat and/or pressure is added to a system
Broad term – includes evaporation
Condensation – change of state from a gas to a liquid
Evaporation – process by which molecules escape from the SURFACE of a liquid to become a gas

5. Vaporization, Condensation, and Evaporation
Why do liquids evaporate?
Look to the kinetic-molecular theory – energy of a liquid depends on temperature, a rapidly moving molecule near the surface of the liquid occasionally possesses enough energy to overcome the intermolecular attractions and escape as a gas
Rate of evaporation depends on temperature (inc temp = inc rate)

6. Vapor Pressure
When a liquid evaporates, gaseous molecules exert a vapor pressure
As the concentration of molecules in the vapor state increases, some molecules condense and become trapped by intermolecular forces in the liquid
Equilibrium vapor pressure – equilibrium achieved between rate of condensation and rate of evaporation

7. Liquid-Vapor Equilibrium
Molecules that have gone through vaporization, can return to the liquid state by colliding with the liquid surface
At equilibrium, Rate of condensation = rate of evaporation
There IS still condensation and evaporation going on  the rates of both are equal

8. Liquid- Vapor Equilibrium
Equilibrium vapor pressure –the pressure of the molecules in the vapor state at equilibrium in a CLOSED container!

9. Boiling Point Extension of vapor pressure equilibrium…
As you wait for boiling to occur, small bubbles of vapor appear. This pressure of the vapor is the vapor pressure of water at that temperature.
As long as the vapor pressure is less than the atmospheric pressure, the bubbles collapse. Once they gain enough heat and the vapor pressure equals the atmospheric pressure, the bubbles of vapor escape

10. Heat of Vaporization
Heat of vaporization – amount of heat necessary to vaporize a given amount of liquid
We found the heat of vaporization for water
How close was your value to 80 cal/g??
Heat is needed to vaporize
For condensation, heat is released  heat of condensation
The energy does NOT change during a phase change
When you were melting your ice, was there a period where the temperature changed very little?

11. Molar Heat of Vaporization
Molar heat of vaporization, DHvap – energy (usually kJ) required to vaporize 1 mole of a liquid
usually directly related to strength of intermolecular forces holding liquid together

12. Enthalpy of Vaporization
This relationship between temperature and vapor pressure is expressed by the Clausius- Clapeyron equation
ln Pvap= (-∆Hvap/R) x (1/T) + A
Pvap= vapor pressure (torr) or mmHg
T=temperature (K)
∆Hvap= enthalpy of vaporization (J/mol)
R=8.31 J/mol*K
A=constant, each liquid has a specific value

13. Clausius- Clapeyron Equation
You can set this equation to solve for the vapor pressure of the liquid at different temperature
ln (P1/P2) = DHvap/R [(1/T2 – 1/T1)]

14. Question 1
At 100°C the vapor pressure of ethylene glycol is 14.9 torr and at 125°C it is 49.1 torr. What is the enthalpy of vaporization?
Use ∆Hvap=-[(ln Pvap2- ln Pvap1)/(1/T2)-(1/T1)] *8.314J/mol*K
∆Hvap=-[(ln 49.1 – ln 14.9)/(1/398)-(1/373)] *8.314J/mol*K
∆Hvap= kJ/mol
Pressure should be in mmHg or torr!

15. Freezing and Melting
Freezing point/melting point – point where a liquid freezes or ice melts
Heat of fusion – energy needed to melt a solid
Heat released when freezing a solid is EXACTLY the same as the heat of fusion  just a different sign

16. Molar Heat of fusion
Molar heat of fusion, DHfus – energy (usually in kJ) required to melt 1 mole of a solid
Molar heats of fusion are generally much smaller than molar heats of vaporization (liquid molecules are packed closer together and more energy need to rearrange from a solid to liquid)

17. Supercooling
Supercooling – a liquid is temporarily cooled below its freezing point
Occurs when heat is removed so rapidly that the molecules have no time to assume the order of a solid structure
Generally unstable

18. Superheating The same thing can occur when heating a solid or liquid
For example, if you heat water in the microwave, it heats up really quickly and can easily burn you when you take it out

19. Sublimation and Deposition
Some solids can actually go straight to a gaseous phase without becoming a liquid (sublimation)
The gases of these solids can convert back to a solid state without going through a liquid (deposition)
Ex: dry ice (CO2), snow on the ground

20. Molar heat of sublimation
Molar heat of sublimation, DHsub – energy (usually in kJ) required to sublime 1 mole of a solid
Equal to heats of fusion + vaporization
DHsub = DHvap + DHfus
Enthalpy is the same whether you go straight from a solid to gas or go through a liquid

21. Example 11.8
Calculate the energy needed to heat 346 g of liquid water from 0°C to 182°C. Assume that the specific heat of water is J/g*°C and the specific heat of steam is 1.99 J/g*°C
1. Heat water from 0°C to 100°C
2. Evaporating 346 g of water at 100C (phase change)
3. Heating steam from 100°C to 182°C

22. Example 11.8 1. q = mcDT 2. DHvap is 40.79 kJ/mol for water
q = (346 g)(4.184 J/g*C)(100 – 0)
q = 145 kJ
2. DHvap is kJ/mol for water
q = (346 g water) x (1 mol water/ g water) x (40.79 kJ/1 mol water)
q = 783 kJ
3. q = mcDT
(346 g)(1.99 J/g*C)(182 – 100)
q = 56.5 kJ
Add them up: = 985 kJ

23. Heating Curves
Heating curves – explain how temperature is changing throughout phase changes for a certain compound or element
Ex of water:

24. Critical Temperature and Pressure
Critical temperature – temperature above which its gas phase CANNOT be made to liquefy, no matter how much pressure is applied
Critical pressure – minimum pressure needed to bring about liquefaction at the critical temperature

25. Phase Diagrams
Phase diagrams – relates the states of a substance to their temperature and pressure
Water
Carbon Dioxide