Vapor Pressure

Evaporation H 2 O(g) molecules (water vapor) H 2 O(l) molecules

Evaporation H 2 O(g) molecules (water vapor) H 2 O(l) molecules

Evaporation H 2 O(g) molecules (water vapor) H 2 O(l) molecules

How Vapor Pressure is Measured 1 atm = 760 mm Hg 760 mm mm = 880 mm Hg Animation by Raymond Chang All rights reserved

Manometer Atmospheric Pressure Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 401

Manometer A BIG = small + height ________ = small + __________ 760 mm Hg h = 120 mm 760 mm 120 mm Small = 640 mm Hg ? Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 401

Manometer B BIG = small + height BIG = ________ + _________ 760 mm 120 mm BIG = 880 mm Hg 760 mm Hg h = 120 mm ? Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 401

The Manometer and Vapor Pressure

Barometer & Manometer atmospheric pressure = kPa atmospheric pressure = kPa atmospheric pressure = kPa confined gas confined gas confined gas 600 mm 200 mm 325 mm 150 mm 100 mm 500 mm 750 mm (a) (b) (c) (d)

Pressure and Temperature STP (Standard Temperature and Pressure) standard temperaturestandard pressure 1 atm kPa 760 mm Hg 273 K 0oC0oC Equations / Conversion Factors: K = o C o C = K – atm = kPa = 760 mm Hg

Convert 25 o C to Kelvin. K = o C How many mm Hg is kPa? How many kPa is 1.37 atm? 25 o C K = X kPa = 1.37 atm kPa 1 atm = kPa X mm Hg = kPa 760 mm Hg kPa = 1737 mm Hg

PaPa CONFINED GAS AIR PRESSURE Hg HEIGHT DIFFERENCE manometer: manometer: measures the pressure of a confined gas higher pressure

CONFINED GAS AIR PRESSURE Hg HEIGHT DIFFERENCE manometer: manometer: measures the pressure of a confined gas

101.3 kPa Atmospheric pressure is 96.5 kPa; mercury height difference is 233 mm. Find confined gas pressure, in atm. SMALL + HEIGHT = BIG atm atm = X atm X = 1.26 atm 96.5 kPa 1 atm mm Hg 760 mm Hg 1 atm = X atm 96.5 kPa mm Hg = X atm 233 mm Hg 96.5 kPa X atm BIG small 1.26 atm

Vapor Pressure more “sticky” less likely to vaporize In general: LOW v.p. not very “sticky” more likely to vaporize In general: HIGH v.p.  measure of the tendency for liquid particles to enter gas phase at a given temp.  a measure of “stickiness” of liquid particles to each other NOT all liquids have same v.p. at same temp.

TEMPERATURE ( o C) PRESSURE (kPa) CHLOROFORM ETHANOL WATER Volatile substances evaporate easily (have high v.p.’s). BOILING  when vapor pressure = confining pressure (usually from atmosphere) b.p. = 78 o C b.p. = 100 o C atmospheric pressure is kPa

Vapor Pressure o C78.4 o C100 o C chloroform ethyl alcohol water Pressure (KPa) Temperature ( o C) 101.3

ETHANOL WATER AIR PRESSURE (~100 kPa) VAPOR PRESSURE (~5 kPa) VAPOR PRESSURE (~10 kPa) BOILING  when vapor pressure = confining pressure (usually from atmosphere) At sea level and 20 o C…

ETHANOL WATER NET PRESSURE (~95 kPa) NET PRESSURE (~90 kPa)

Water Molecules in Liquid and Steam

Microscopic view of a liquid near its surface The high energy molecules escape the surface.

Behavior of a liquid in a closed container

Water rapidly boiling on a stove

Copyright © 2006 Pearson Benjamin Cummings. All rights reserved. Pressure Cooker

120 o C

Formation of a bubble is opposed by the pressure of the atmosphere Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 452

Vapor Pressure o C78.4 o C100 o C chloroform ethyl alcohol water Pressure (KPa) Temperature ( o C) 101.3

Boiling Point and Pressure

Vapor Pressure and Boiling Keys Vapor Pressure and Boiling

Heating / Cooling Curve of Water Temperature ( o C) Heat added at a constant rate liquid water water and steam steam ice and water ice Heating Cooling

Gas Collected Over Water

Measuring the Vapor Pressure of a Liquid Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 376

Gas Mixtures and Dalton’s Law

Gases Dissolved in Liquids