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

Fig. 11.25 Behavior of a Liquid in a Sealed Flask Copyright © Houghton Mifflin Company. All rights reserved.

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!) Copyright © Houghton Mifflin Company. All rights reserved.

Figure 10.40 a-b (Zumdahl) Measuring the Vapor Pressure of a Liquid Copyright © Houghton Mifflin Company. All rights reserved.

Reminder: Kinetic Molecular Theory & Distribution Curves Chapter 09

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 Copyright © Houghton Mifflin Company. All rights reserved.

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

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? Copyright © Houghton Mifflin Company. All rights reserved.

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! Copyright © Houghton Mifflin Company. All rights reserved.

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”) Copyright © Houghton Mifflin Company. All rights reserved.

So VP ≥ Pexternal is the condition that must be met for boiling. Figure 10.47. 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 Copyright © Houghton Mifflin Company. All rights reserved.

Table 11.8 Boiling Point of Water at Various Locations Copyright © Houghton Mifflin Company. All rights reserved. 10–12

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

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 Copyright © Houghton Mifflin Company. All rights reserved.

Fig. 11.37 The Phase Diagram for Water Copyright © Houghton Mifflin Company. All rights reserved.

Fig. 11.38 Navigation within a Phase Diagram Copyright © Houghton Mifflin Company. All rights reserved.

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! Copyright © Houghton Mifflin Company. All rights reserved.

Fig. 11.35 The Heating Curve (Not Drawn to Scale) for a Given Quantity of Water Where Energy is Added at a Constant Rate Copyright © Houghton Mifflin Company. All rights reserved. 10–18

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 DHvap= 40.7 kJ/mol DHfus= 6.02 kJ/mol Copyright © Houghton Mifflin Company. All rights reserved.

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 Copyright © Houghton Mifflin Company. All rights reserved.

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. Copyright © Houghton Mifflin Company. All rights reserved.

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 Copyright © Houghton Mifflin Company. All rights reserved.

Figure 10.12(c) http://www.lsbu.ac.uk/water/ice1h.html See also: http://www.edinformatics.com/interactive_molecules/ice.htm Copyright © Houghton Mifflin Company. All rights reserved.

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.

Fig. 11.39 The Phase Diagram for Other Substances Copyright © Houghton Mifflin Company. All rights reserved.

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! Copyright © Houghton Mifflin Company. All rights reserved.