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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