GASES Chapter 10

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

Like all matter, air molecules have mass.

Air pressure is equal in all directions. Pressure = force per unit area

Air molecules are everywhere.

Properties of Gases: Gas Pressure A. Gas Pressure = 1 m 2 1x10 5 N P (Pa) = Area (m 2 ) Force (N) Pascal = 1x10 5 N/m 2 Pressure = 1x10 3 kPa Air column 1m x 1m

High altitudes = lower pressure Low altitudes = higher pressure

Barometric Pressure Standard Atmospheric Pressure 1.00 atm 760 mm Hg, 760 torr kPa bar mbar 14.7 psi ins/Hg

The Aneroid 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 pressure in inches of mercury. However, meteorologists measure atmospheric pressure in 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 STP OK, 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

Manometers

B. Simple Gas Laws Boyle 1662 P  1 V Temp constant INVERSE RELATIONSHIP

 pressure decreases non-linearly  p  pressure will be less at P2 than at P1 due to pressure decreasing with height pressure differences responsible for  pressure differences responsible for movement of air movement of air  recording stations are reduced to sea level pressure equivalents

Boyle’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. P 1 V 1 = P 2 V 2 Robert Boyle ( ). Son of Early of Cork, Ireland.

Charles’s Law V  T Pressure constant To convert Celsius to Kelvin: T(K) = T(°C) DIRECT RELATIONSHIP

Charles’s Law If moles and pressure are constant …then V and T are directly proportional. V 1 V 2 = T 1 T 2 If one temperature goes up, the volume goes up!If one temperature goes up, the volume goes up! Jacques Charles ( ). Isolated boron and studied gases. Balloonist.

Gay-Lussac’s Law If moles and Volume are constant, then… P and T are directly proportional. P 1 P 2 = T 1 T 2 If one temperature goes up, the pressure goes up!If one temperature goes up, the pressure goes up! Joseph Louis Gay- Lussac ( )

twice as many molecules Avogadro’s Hypothesis Equal volumes of gases at constant T and P have the same number of molecules. V and moles are directly related V1 V2 = n1 n2

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

Combined Gas Law P1P1 V1V1 T1T1 P2P2 V2V2 T2T2 Boyle’s Law Charles’ Law Gay-Lussac’s Law =

Importance of Gases Airbags fill with N 2 gas in an accident.Airbags fill with N 2 gas in an accident. Gas is generated by the decomposition of sodium azide, NaN 3.Gas is generated by the decomposition of sodium azide, NaN 3. 2 NaN 3 ---> 2 Na + 3 N 22 NaN 3 ---> 2 Na + 3 N 2