Prentice Hall ©2004 Gases-Intro A state of matter, not a “kind” of matter or substance Operational Definition: no fixed volume or shape (assumes those

Prentice Hall ©2004

Gases-IntroGases-Intro

• A state of matter, not a “kind” of matter or substance

• Operational Definition: no fixed volume or shape (assumes those of the container in which it is in)

• Conceptual Definition: nanoscopic “units” (“particles”) are completely separated from one another


Gases-IntroGases-Intro

• A state of matter, not a “kind” of matter or substance

e.g., Many say “CO2” is a gas; “He” is a gas….BUT─Under certain conditions, nearly all substances can exist

as a gas!─Flipside of that is this: All gases can be turned into

a liquid or solid if the conditions are adjusted!• What conditions favor the gaseous state?

Consider H2O! It tends to be a gas at high _________ Consider a refrigeration cycle in an air-conditioning system?

Refrigerant turns from liquid to gas after “expansion”; at low ___________

T (temperature)

P (pressure)


Compression and Expansion (“Decompression”)

of refrigerant occurs in a refrigerator!Compression and Expansion (“Decompression”)

of refrigerant occurs in a refrigerator!

Chapter 09 Slide 3


Gases are favored at “High T” and “Low P”Gases are favored at “High T” and “Low P”

• BUT specific T’s and P’s depend on the substance Water:o turns to a gas at 100C at 1 atm pressure; o if the pressure is about 2 atm (as in a pressure cooker),

won’t turn to a gas until about 120C O2 and N2 and CO2:oAre gases at room temperature and 1 atm pressureoNeed to be cooled to much lower T’s to turn to a liquid at 1

atm. (or compressed greatly to turn to a liquid at moderate T’s)

Chapter 09 Slide 4


Gases’ properties (with respect to T, P, V, and n) are independent of the type of particle (substance)

Gases’ properties (with respect to T, P, V, and n) are independent of the type of particle (substance)

• As long as a substance is under “favorable conditions” to be in the gaseous state (appropriate T and P for that substance) ,

it turns out that it will behave the same way (with respect to

certain variables) as any other substance “acting like a gas under its favorable conditions”!

Chapter 09 Slide 5

--i.e., it doesn’t matter if the “gas” is water, methane, carbon dioxide. The size or mass of molecules doesn’t change the way it behaves! (Pretty amazing!)

--under such ”ideal” conditions, a substance is said to behave “ideally” as a gas. Called an “ideal gas”

--the behaviors of “ideal gases are described by “gas laws”

Prentice Hall ©2004 Chapter 09 Slide 6

Gas Pressure 02Gas Pressure 02

• Units of pressure: atmosphere (atm)

Pa (N/m2, 101,325 Pa = 1 atm) Torr (760 Torr = 1 atm)

bar (1.01325 bar = 1 atm)

mm Hg (760 mm Hg = 1 atm)

lb/in2 (14.696 lb/in2 = 1 atm)

in Hg (29.921 in Hg = 1 atm)


How Can Pressure be Measured?How Can Pressure be Measured?

• Importance of mechanical equilibrium ideao If something is not moving, and it stays not moving,

there is no net force on ito “equal but opposite” forces

• Consider a syringe with gas in it.o If the only external pressure (force/area) on it is the

air in the room (Pbar), then Pgas = Pbar

o If Pgas were to immediately increase (say, because you raised T), it would not stay that way because the barrel would move!

• Vgas would increase, decreasing P until it was equal to Pbar again


Using a Manometer to Determine PgasUsing a Manometer to Determine Pgas

• Uses “mechanical equilibrium” idea

liquid not moving and stays not moving

• levels equal => Pgas = Pbar

• left level higher => Pgas + P”extra” = Pbar

• right level higher => Pgas = Pbar + P”extra”

o If liquid in manometer is Hg(l), then the height difference, in mm, is the P”extra” in units of mmHg (or torr)


• Pressure–Volume Law (Boyle’s Law):

Boyle’s Law 01Boyle’s Law 01


http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html

Chapter 09 Slide 10


Boyle’s Law 02Boyle’s Law 02

• Pressure–Volume Law (Boyle’s Law):

• The volume of a fixed amount of gas maintained at

constant temperature is inversely proportional to

the gas pressure.

(n, T constant)1

Volume Pressure

1 1 1 (n, T const) PV k11

1

k

VP


http://www.grc.nasa.gov/WWW/K-12/airplane/aglussac.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/aglussac.html

Chapter 09 Slide 12


Charles’ Law 01Charles’ Law 01

• Temperature–Volume Law (Charles’ Law):


Charles’ Law 01Charles’ Law 01

• Temperature–Volume Law (Charles’ Law):

• The volume of a fixed amount of gas at constant

pressure is directly proportional to the Kelvin

temperature of the gas.

V T

V1

T1=k1V1 = k1T1 (?, ? const)

(?, ? const)


Avogadro’s Law 01Avogadro’s Law 01

• The Volume–Amount Law (Avogadro’s Law):

What variable is being held constant here? ___ T


Avogadro’s Law 01Avogadro’s Law 01

• The Volume–Amount Law (Avogadro’s Law):

• At constant pressure and temperature, the volume of a gas

is directly proportional to the number of moles of the gas

present.

• Avogadro’s Hypothesis: Equal volumes of different gases at

the same T and P contain equal numbers of gas particles.

nV

11

1 knV V1 = k1n1 (?, ? const)

(?, ? const)


Gases Worksheet-I PracticeGases Worksheet-I Practice

• Proportional relationships / calculations• If A is proportional to B then:

If B doubles, then A doubles!If B triples, then A triples!If B becomes 3.21 times what it was, then A becomes

____ times what it was.If B becomes x times what it was, then A becomes

___ times what it was.x

3.21


Gases Worksheet-I PracticeGases Worksheet-I Practice

• If A is inversely proportional to B then:If B doubles, then A becomes ½ of what it was!If B triples, then A becomes 1/3 of what it was!If B becomes 3.21 times what it was, then A becomes

____ times what it was.If B becomes x times what it was, then A becomes

___ times what it was.1/x

1/3.21

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Prentice Hall ©2004 Gases-Intro A state of matter, not a “kind” of matter or substance Operational Definition: no fixed volume or shape (assumes those