Vapor Pressure (and remaining PS12 topics) What is it? What affects it? Relation to boiling....

Vapor Pressure (and remaining PS12 topics)

• What is it?

• What affects it?

• Relation to boiling.

• Definition of boiling point.

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• Phase Diagrams

• Water’s unusual properties

Copyright © Houghton Mifflin Company. All rights reserved. 10–2

Fig. 11.25 Behavior of a Liquid in a Sealed Flask

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!)


Figure 10.40 a-b (Zumdahl) Measuring the Vapor Pressure of a Liquid

Chapter 09 Slide 5

Reminder: Kinetic Molecular Theory & Distribution Curves


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

10–7

Fig. 11.28 VP of Several Liquids as a function of T

VP of Water vs. T

Understanding “vapor pressure” is required to fully understand what “boiling point” is!

• Q1: What is the boiling point of water?


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!

Fig. 11.29 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”)



Figure 10.47 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

Pbubble = VP of liq!

So VP ≥ Pexternal for boiling


Table 11.8 Boiling Point of Water at Various Locations

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)


Fig. 11.37 The Phase Diagram for Water


Fig. 11.38 Navigation within a Phase Diagram


Fig. 11.35 The Heating Curve (Not Drawn to Scale) for a Given Quantity of Water Where Energy is Added at a Constant Rate

Problem 10.87 in Zumdahl

• How much energy does it take to convert 0.500 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

• Hvap= 40.7 kJ/mol

• Hfus= 6.02 kJ/mol



How about “Expt 5”?

Expt 5

What variable is changing, which variable is held constant, and what phase change occurs (and at what point)?

ANSWERS:1)P increases2)T is held constant3)Melting occurs (at crossing point)

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.

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 10.12 in Zumdahl (next slide)– Model passed around



Figure 10.12(c)

See also: http://www.edinformatics.com/interactive_molecules/ice.htm

http://www.lsbu.ac.uk/water/ice1h.html

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)



Fig. 11.39 The Phase Diagram for Other Substances


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!

Vapor Pressure (and remaining PS12 topics) What is it? What affects it? Relation to boiling....

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