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Chapter 13

Chapter 13. Gases. The Kinetic-Molecular Theory. Ideal Gas : an imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory Ideal Gas does not exist—most gases behave close to this at lower pressure and higher temps

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Chapter 13

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  1. Chapter 13 Gases

  2. The Kinetic-Molecular Theory Ideal Gas: an imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory Ideal Gas does not exist—most gases behave close to this at lower pressure and higher temps Gases that are unreactive (He, Ne) or non-polar (N2, H2) are closer to ideal • 5 assumptions of the theory

  3. 1. Gases consist of large numbers of particles that are far apart relative to their size. Gas particles are small— Gases have a low density Gases are easily compresses Most of a gas is empty space

  4. 2. Collisions between gas particles and between particles & container are elastic. Elastic collision—no net loss of kinetic energy Total energy of the particles is the same as long as temp is constant

  5. 3. Gas particles are in constant, rapid, random motion. Move in all directions Move in straight lines

  6. 4. There are no forces of attraction or repulsion between gas particles. Gas particles are like tiny billiard balls When they collide, the immediately bounce apart.

  7. 5. The average kinetic energy of gas particles depends on the temperature of the gas. KE = ½ mv2 KE depends only on speed As temperature increases, both kinetic energy & speed increase All gases at the same temp will have the same KE…lighter gases have higher speeds (less mass)

  8. Properties of Gases Expansion Particles move rapidly in all directions Fill container given Fluidity Particles glide past each other and are able to flow (liquids & gases) Low Density Particles are very far apart

  9. Properties of Gases (cont.) Compressibility Volume can be greatly decreased—100x as many particles in a pressurized container Diffusion Spontaneous mixing of the particles caused by random motion Rate depends on speed, diameter, and attractive forces Effusion Process where gas particles under pressure pass through a tiny opening

  10. Diffusion Diffusion: describes the mixing of gases.The rate of diffusion isthe rate of gas mixing.

  11. Effusion Effusion: describes the passage of gas into an evacuated chamber.

  12. Pressure • Is caused by the collisions of molecules with the walls of a container • is equal to force/unit area • SI units = Newton/meter2 = 1 Pascal (Pa) • 1 standard atmosphere = 101,325 Pa • 1 standard atmosphere = 1 atm = 760 mm Hg = 760 torr

  13. Force Area Pressure = Pressure = the force per unit area on a surface SI unit for force = Newton = 1kg m/s2 Smaller area will increase the pressure

  14. Gas molecules exert pressure due to collisions with surface Air pressure pressure decreases with altitude! Caused by the weight of the gases that compose the atmosphere http://www.physicalgeography.net/fundamentals/7d.html At sea level: pressure is 10.1 N/cm2 = 1.03kg /cm2 = 14.7 lb / in2

  15. That's alot of pressure pushing down on us!! Why don't we feel it? The pressure inside our body pushes outward to balance the force.

  16. "The most common type barometer used in homes is the aneroid barometer . Inside this instrument is a small, flexible metal capsule called an aneroid cell. In the construction of the device, a vacuum is created inside the capsule so that small changes in outside air pressure cause the capsule to expand or contract. The size of the aneroid cell is then calibrated and any change in its volume is transmitted by springs and levers to an indicating arm that points to the corresponding atmospheric pressure." http://www.physicalgeography.net/fundamentals/7d.html http://www.physicalgeography.net/fundamentals/7d.html How is air pressure measured? Barometer: device used to measure atmospheric pressure invented by Evangelista Torricelli in 1643 Height of column is measured--reported as "mm Hg"

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