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Unformatted text preview: GENERAL CHEMISTRY CHAPTER 5 GASES PAGE 1 OF 23 Gases Gas = a large collection of particles moving at random through a volume that is primarily empty space Expand to completely fill their container Take the shape of their container Low density Much less than solid or liquid state Compressible Mixtures of gases are always homogeneous Fluid Have movement Molecular Collisions and Pressure Gas molecules are constantly in motion As they move and strike a surface, they push on that surface Push = force If we could measure the total amount of force exerted by gas molecules hitting the entire surface at any one instant, we would know the pressure the gas is exerting Pressure = force per unit area Pressure = Force Unit Area = F A Each collision is only a small force, but when these forces are summed over the many molecules in the air they can add up to a substantial force The pressure of a gas depends on several factors: Number of gas particles in a given volume Volume of the container Average speed of the gas particles When more molecules are added, more molecules hit the container at any one instant, resulting in higher pressure Also higher density Atmospheric Pressure Whenever there is a pressure difference, a gas will flow from area of high pressure to low pressure The bigger the difference in pressure, the stronger the flow of the gas If there is something in the gas path, the gas will try to push it along as the gas flows GENERAL CHEMISTRY CHAPTER 5 GASES PAGE 2 OF 23 Example : Soda Straws and Gas Pressure Atmospheric pressure = pressure above any area in the Earths atmosphere caused by the weight of the air As elevation increases, fewer air molecules are above Atmospheric pressure decreases with increasing altitude The higher up in the atmosphere you climb, the lower the atmospheric pressure is around you At the surface the atmospheric pressure is 14.7 psi (pounds per square inch) , but at 10,000 ft it is only 10.0 psi Measuring Atmospheric Pressure Barometer = device used to measure atmospheric pressure Invented by Italian scientist Evangelista Toricelli A long thin tube that is sealed at one end, filled with mercury, and then is inverted into a dish of mercury Some mercury runs from the tube into the dish until the downward pressure of the mercury inside the column is exactly balanced by the outside atmospheric pressure This pressure pushes on the mercury in the dish and pushes it up the column Standard atmospheric pressure at sea level = 760 mm Hg The height of the mercury column varies slightly from day to day depending on the altitude and weather conditions In Breckenridge, Colorado (elevation 9600 ft) atmospheric pressure = 520 mm Hg Air is thinner = less air molecules pushing down on the Earths surface The pressure of the air inside the straw is the same as the pressure of the air...
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This note was uploaded on 01/21/2011 for the course PHYS 4A 60865 taught by Professor L. oldewurtel during the Fall '09 term at Irvine Valley College.

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