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

Copyright © Houghton Mifflin Company. All rights reserved.10–2 Fig Behavior of a Liquid in a Sealed Flask

Fig Dynamic equilibrium results when R evap = R conden Copyright © Houghton Mifflin Company. All rights reserved.10–3 Constant because T is constant  KE avg 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!)

Copyright © Houghton Mifflin Company. All rights reserved.10–4 Figure a-b (Zumdahl) Measuring the Vapor Pressure of a Liquid

Chapter 09Slide 5 Reminder: Kinetic Molecular Theory & Distribution Curves

Copyright © Houghton Mifflin Company. All rights reserved.10–6 Fig Distribution of KE’s in a liquid at two different T’s E escape does NOT CHANGE with T; Avg KE does! (E escape )  IM forces! *Not speed here

10–7 Fig 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? Copyright © Houghton Mifflin Company. All rights reserved.10–8

What is the difference between “boiling” and “evaporating”? Operationally….it’s the bubbles! Copyright © Houghton Mifflin Company. All rights reserved.10–9 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 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”) Copyright © Houghton Mifflin Company. All rights reserved.10–10

Copyright © Houghton Mifflin Company. All rights reserved.10–11 Figure 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 P bubble ≥ P external P bubble = VP of liq! So VP ≥ P external for boiling

Copyright © Houghton Mifflin Company. All rights reserved.10–12 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 ≥ P external ) Definition of boiling point. (bp = T at which VP = P external )

Copyright © Houghton Mifflin Company. All rights reserved.10–14 Fig The Phase Diagram for Water

Copyright © Houghton Mifflin Company. All rights reserved.10–15 Fig Navigation within a Phase Diagram

Copyright © Houghton Mifflin Company. All rights reserved.10–16 Fig The Heating Curve (Not Drawn to Scale) for a Given Quantity of Water Where Energy is Added at a Constant Rate

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  H vap = 40.7 kJ/mol  H fus = 6.02 kJ/mol Copyright © Houghton Mifflin Company. All rights reserved.10–17

Copyright © Houghton Mifflin Company. All rights reserved.10–18 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 increases 2)T is held constant 3)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? Copyright © Houghton Mifflin Company. All rights reserved.10–19 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 H 2 O, 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 in Zumdahl (next slide) –Model passed around Copyright © Houghton Mifflin Company. All rights reserved.10–20

Copyright © Houghton Mifflin Company. All rights reserved.10–21 Figure 10.12(c) See also:

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) Copyright © Houghton Mifflin Company. All rights reserved.10–22

Copyright © Houghton Mifflin Company. All rights reserved.10–23 Fig The Phase Diagram for Other Substances

Copyright © Houghton Mifflin Company. All rights reserved.10–24 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!