1. Vapor Pressure (and remaining PS12 topics)
What is it?
What affects it?
Relation to boiling.
Definition of boiling point.
Phase Diagrams
Water’s unusual properties

2. Fig. 11.25 Behavior of a Liquid in a Sealed Flask
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3. Fig. 11.26 Dynamic equilibrium results when Revap = Rconden
Constant because T is constant
 KEavg is constant (Only those with enough KE can escape [there’s a “barrier” to escape])
Increases because [gas] increases (NO barrier to be “captured” by liquid!)
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4. Figure 10.40 a-b (Zumdahl) Measuring the Vapor Pressure of a Liquid
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5. Reminder: Kinetic Molecular Theory & Distribution Curves
Chapter 09

6. Fig. 11.24 Distribution of KE’s in a liquid at two different T’s
Eescape does
NOT CHANGE with T; Avg KE does!
(Eescape)
 IM forces!
*Not speed here
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7. Fig. 11.28 VP of Several Liquids as a function of T
VP of Water vs. T

8. Q1: What is the boiling point of water?
Understanding “vapor pressure” is required to fully understand what “boiling point” is!
Q1: What is the boiling point of water?
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9. What is the difference between “boiling” and “evaporating”?
Operationally….it’s the bubbles!
Evaporation happens only at the surface of a liquid; boiling [bubble formation] can occur from within the liquid
See board / next slides: forming a bubble means “creating a pocket of gas inside the liquid”
This means “pushing the liquid out of the way” (to create space)
How hard must you push? At least as hard as the external pressure pushes “down” on you!
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10. Fig Boiling Occurs when the VP of the liquid is great enough to form a bubble in the interior of the liquid (“push away the atmosphere or external pressure”)
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11. So VP ≥ Pexternal is the condition that must be met for boiling.
Figure Zumdahl Water in a Closed System with a Pressure of 1 atm Exerted on the Piston
To form a bubble here, the piston would have to move upwards! This won’t happen unless
Pbubble ≥ Pexternal
But Pbubble = VP of liq!
So VP ≥ Pexternal is the condition that must be met
for boiling.
But VP depends on T, so…
Bp = T at which VP = Pexternal
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12. Table 11.8 Boiling Point of Water at Various Locations
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10–12

13. Vapor Pressure (Quick Review)
What is it? (P exerted by a gas “over” its liquid)
What affects it? (IM forces and T)
Relation to boiling.
(boiling occurs only when VP ≥ Pexternal)
Definition of boiling point.
(bp = T at which VP = Pexternal)
 bp varies with external pressure

14. Phase Diagram for a Substance
Set of all T, P combinations at which solid, liquid, or gas (phase) is present.
Lines separate the regions (solid, liquid, gas)
See next slide
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15. Fig. 11.37 The Phase Diagram for Water
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16. Fig. 11.38 Navigation within a Phase Diagram
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17. Heating Curve for a substance
Bit of a misnomer—no “curves” (only “lines”)
Plot of T vs. “heat added”
Review!
Add heat, raise T: q = C x m x DT
Always?
T does NOT raise during phase transitions
E.g., melting, boiling
Heat goes into “doing” the phase change (increasing PE) not raising T (inc. KE)
Relation to phase diagram!
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18. Fig The Heating Curve (Not Drawn to Scale) for a Given Quantity of Water Where Energy is Added at a Constant Rate
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10–18

19. Problem in Zumdahl
How much energy does it take to convert kg ice at -20.C to steam at 250.C?
Specific heat capacities:
Ice, 2.03 J/g C
Liquid, 4.2 J/g C
Steam, 2.0 J/g C
DHvap= 40.7 kJ/mol
DHfus= 6.02 kJ/mol
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20. How about “Expt 5”?
What variable is changing, which variable is held constant, and what phase change occurs (and at what point)?
ANSWERS:
P increases
T is held constant
Melting occurs (at crossing point)
Expt 5
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21. Why does this occur? (It should seem odd to you!)
What state (s,l,g) is usually the most dense?
Not true for water! Ice is less dense than liquid water.
Water expands on freezing
Don’t leave full water or soda containers in your car during the winter!
Matter generally compresses when subjected to high pressure. For H2O, this means going to the liquid rather than solid state.
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22. Why is water unusual?
At low T, structure that maximizes H-bonding between molecules is favored
This leads to hexagonal symmetry
Snowflakes!
This leads to open “channels” in ice structure
increases net distance between molecules.
See Figure in Zumdahl (next slide)
Model passed around
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23. Figure 10.12(c)
See also:
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24. Some “Real World” Implications
Ice floats on water
Glaciers float! Sea level would be much higher; no Los Angeles? Hawaii? Etc.
Lakes only freeze at surface
Life in lake can survive winter
Water expands when frozen
Weathering (helps erode mountains)
Street maintenance (cracks, potholes! )
Pipes / hoses crack (use antifreeze; hoses inside)
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25. Fig. 11.39 The Phase Diagram for Other Substances
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26. Dry Ice. How can we get this in the lab?
(Clearly it’s not “storable” at typical conditions)
Stored at high pressure as a liquid in a tank. Open the valve on tank, with a “collector” device on end. What happens?
1) P drops, and liquid turns to gas.
2) Endothermic phase change; T drops! Eventually turns to solid!
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