GASESGASESChapter 10Chapter 10

Gravity pulls the air molecules Gravity pulls the air molecules toward the earth, giving them toward the earth, giving them weight. The weight of the air weight. The weight of the air molecules all around us is molecules all around us is called the air pressure.called the air pressure.

Like all matter, air molecules have mass.

kgkg

Air pressure is Air pressure is equal in all equal in all directions.directions.

Pressure = force per unit areaPressure = force per unit area

Air molecules are Air molecules are everywhere.everywhere.

Properties of Gases: Gas Pressure

A. Gas Pressure

amF =

1 m2

1x105N

)/8.9()000,10( 2smkg

25101

s

mkgx

Nx 5101

P (Pa) =

Area (m2)

Force (N)

Pascal = 1x105 N/m2

Pressure = 1x103 kPa

Air column 1m x 1m

High altitudes = lower pressureHigh altitudes = lower pressure

Low altitudes = higher pressureLow altitudes = higher pressure

Barometric PressureStandard Atmospheric Pressure

1.00 atm

760 mm Hg, 760 torr

101.325 kPa

1.01325 bar

1013.25 mbar

14.7 psi

29.92 ins/Hg

The The AneroidAneroid Barometer!! Barometer!!

•No fragile tubes!No fragile tubes!

•No toxic chemicals!No toxic chemicals!

•No batteries!No batteries!

•Never needs winding!Never needs winding!

Get yours today!!

• relies on evacuated, collapsible chamber

• atmospheric weight compresses chamber

• initially calibrated to local altitude, temperature and latitude

Aneroid Barometer

An aneroid barometer uses a cell which has had most of the air removed.

As the air pressure around the cell increases, it presses on the cell, which causes the needle to move.

Television weather forecasters usually give barometric Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists pressure in inches of mercury. However, meteorologists measure atmospheric pressure in measure atmospheric pressure in millibars.millibars.

MILLIBARS

Measurement of Pressure

• barometer – ‘weight’ meter– Mercury barometer –

developed by Torricelli– Aneroid barometer – uses

no liquid

• 3 corrections required:– Elevation– Temperature– Gravity changes with latitude

The standard atmosphere is defined as the pressure

exerted by a mercury column of exactly 760 mm in height

when the density equals 13.6 g/mL.

And now, we pause for this commercial message from

STPOK, so it’s really not THIS kind

of STP…

STP in chemistry stands for Standard Temperature and

Pressure

Standard Pressure = 1 atm or equivalent

Standard Temperature =

0 C or 273 K

STP allows us to compare amounts of

gases between different pressures and temperatures

STP allows us to compare amounts of

gases between different pressures and temperatures

Manometers

B. Simple Gas Laws

• Boyle 1662 P 1V

Temp constant

INVERSE RELATIONSHIP

pressure decreases non-linearlypressure decreases non-linearly

ppressure will be less at P2 than at P1

due to pressure decreasing with

height

pressure differences responsible for pressure differences responsible for

movement of airmovement of air

recording stations are reduced to sea

level pressure equivalents

Boyle’s LawBoyle’s Law

Pressure and Volume are INVERSELY PROPORTIONAL if moles and temperature are constant (do not change).

For example, P goes up as V goes down.

P1V1 = P2 V2

Robert Boyle (1627-1691). Son of Early of Cork, Ireland.

Charles’s Law

V T

Pressure constant

To convert Celsius to Kelvin:

T(K) = T(°C) + 273.15

DIRECT RELATIONSHIP

Charles’s LawCharles’s LawIf moles and pressure are If moles and pressure are

constant …then constant …then

V and T are directly V and T are directly proportional.proportional.

VV11 V V22

==

TT11 T T22

• If one temperature goes If one temperature goes

up, the volume goes up!up, the volume goes up!

Jacques Charles (1746-1823). Isolated boron and studied gases. Balloonist.

Gay-Lussac’s LawGay-Lussac’s LawIf moles and Volume are If moles and Volume are

constant, then…constant, then…

P and T are directly P and T are directly proportional.proportional.

PP11 P P22

==

TT11 T T22

• If one temperature goes If one temperature goes

up, the pressure goes up, the pressure goes up!up!

Joseph Louis Gay-Joseph Louis Gay-Lussac (1778-1850)Lussac (1778-1850)

twice as twice as many many moleculesmolecules

Avogadro’s HypothesisAvogadro’s Hypothesis

Equal volumes of gases at constant T and P Equal volumes of gases at constant T and P have the same number of molecules.have the same number of molecules.

V and moles are directly relatedV and moles are directly related

V1 V2V1 V2 ==n1 n2n1 n2

• Molecular movement in a sealed container: pressure increased by increasing density (b) or temperature (c)

Combined Gas Law

P1 V1

T1

P2 V2

T2

Boyle’s Law

Charles’ Law

Gay-Lussac’s Law

=

Importance Importance of Gasesof Gases

• Airbags fill with NAirbags fill with N22 gas in an accident. gas in an accident.

• Gas is generated by the decomposition of Gas is generated by the decomposition of sodium azide, NaNsodium azide, NaN33..

• 2 NaN2 NaN33 ---> 2 Na + 3 N ---> 2 Na + 3 N22