Notes 11-3 Obj 11.5, 11.6

Rubbing alcohol feels cold when it evaporates from the skin due to the fact that rubbing alcohol has: a.an exothermic heat of vaporization. b.an endothermic heat of vaporization. c.an exothermic heat of formation. d.an endothermic heat of formation.

11.5 Vapor Pressure A.) Molecules can escape from the surface of a liquid into the gas phase (evaporation) 1.) upon placing a substance in a evacuated cylinder, some particles will evaporate to create a pressure. When they reach an equilibrium (when the rate of evaporation = rate of condensation), that pressure is called vapor pressure.

B.) At any temperature some molecules in a liquid have enough energy to escape. C.) As the temperature rises, the fraction of molecules that have enough energy to escape increases. © 2009, Prentice-Hall, Inc.

D.) As more molecules escape the liquid, the pressure they exert increases. © 2009, Prentice-Hall, Inc.

E.) The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. © 2009, Prentice-Hall, Inc.

F.) The boiling point of a liquid is the temperature at which it’s vapor pressure equals atmospheric pressure. G.) The normal boiling point is the temperature at which its vapor pressure is 760 torr. © 2009, Prentice-Hall, Inc.

H.) If a substance is allowed to vaporize in an open container, then equilibrium never occurs. If a substance has a very high vapor pressure (or evaporates quickly), it is considered to be volatile. 1.) gasoline 2.) alcohol 3.) acetone

Use the figure below to estimate the boiling point of diethyl ether under an external pressure of 0.80 atm. At what external pressure will ethanol have a boiling point of 60 °C?

The highest temperature at which a substance can exist as a liquid is referred to as its: a.boiling point. b.freezing point. c.triple point. d.critical point.

11.6 Phase Diagrams A.) Such a diagram allows us to predict the phase of a substance that is stable at any given temp or pressure.

B.) Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. © 2009, Prentice-Hall, Inc.

C.) The circled line is the liquid-vapor interface. D.) It starts at the triple point (T), the point at which all three states are in equilibrium.triple point (T), © 2009, Prentice-Hall, Inc.

E.) It ends at the critical point (C); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other (supercritical fluid). © 2009, Prentice-Hall, Inc.

F.) Each point along this line is the boiling point of the substance at that pressure. © 2009, Prentice-Hall, Inc.

G.) The circled line in the diagram below is the interface between liquid and solid. H.) The melting point at each pressure can be found along this line. © 2009, Prentice-Hall, Inc.

I.) Below the triple point the substance cannot exist in the liquid state. J.) Along the circled line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line. © 2009, Prentice-Hall, Inc.

K.) Phase Diagram of Water 1.) Note the high critical temperature and critical pressure. a.) These are due to the strong van der Waals forces ( hydrogen bonds) between water molecules. © 2009, Prentice-Hall, Inc.

Phase Diagram of Water 2.) The slope of the solid- liquid line is negative. 1. ) This means that as the pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid. © 2009, Prentice-Hall, Inc.

L.) Phase Diagram of Carbon Dioxide 1.) Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; CO 2 sublimes at normal pressures. © 2009, Prentice-Hall, Inc.

Referring to the figure below, describe any changes in the phases present when H 2 O is (a) kept at 0 °C while the pressure is increased from that at point 1 to that at point 5 (vertical line), (b) kept at 1.00 atm while the temperature is increased from that at point 6 to that at point 9 (horizontal line).

Using the figure below, describe what happens when the following changes are made in a CO 2 sample: (a)Pressure increases from 1 atm to 60 atm at a constant temperature of –60 °C. (b) Temperature increases from –60 °C to –20 °C at a constant pressure of 60 atm.