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Notes 11-3 Obj 11.5, 11.6
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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.
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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.
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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.
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D.) As more molecules escape the liquid, the pressure they exert increases. © 2009, Prentice-Hall, Inc.
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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.
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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.
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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
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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?
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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.
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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.
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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.
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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.
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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.
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F.) Each point along this line is the boiling point of the substance at that pressure. © 2009, Prentice-Hall, Inc.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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