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Ch. 12 Behavior of Gases

Ch. 12 Behavior of Gases. Gases. Gases expand to fill its container, unlike solids or liquids Easily compressible: measure of how much the volume of matter decreases under pressure. Variables that describe a gas. Pressure (P) Measured in kilopascals, kPa

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Ch. 12 Behavior of Gases

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  1. Ch. 12 Behavior of Gases

  2. Gases • Gases expand to fill its container, unlike solids or liquids • Easily compressible: measure of how much the volume of matter decreases under pressure

  3. Variables that describe a gas • Pressure (P) • Measured in kilopascals, kPa • Pressure and number of molecules are directly related • increase molecules = increase pressure • Gases naturally move from areas of high pressure to low pressure, due to the available space to move into

  4. Variables that describe a gas • Volume (V) • Measured in Liters, L • Volume and pressure are inversely related • As volume decreases, the pressure increases • Smaller container = less room for movement, therefore molecules hit sides of container more often

  5. Variables that describe a gas • Temperature (T) • Measured in Kelvin, K • The temperature and pressure are directly related • Increase in temp = increase in pressure • Volume must be held constant • Molecules hit the walls harder (due to increase in K.E.) and more frequently. • Think about a tire in hot weather…

  6. Variables that describe a gas • Amount • Measured in moles, mol • Moles and pressure are directly related • Increase in # of moles = increase in pressure Ex: Inflating a balloon is adding more molecules. • Temperature must remain constant

  7. Gas Laws • Describe how gases behave • Change can be calculated • Know the math and the theory!!

  8. Boyle’s Law (1662) • Gas pressure is inversely related to volume (as volume increases, pressure decreases) • Temperature is constant P1V1= P2V2

  9. Ex: The pressure of a 2.5L of gas changes from 105 kPato 40.5 kPa. What will be the new volume?

  10. Charles’s Law (1787) • Volume is directly proportional to temp. (increase volume, increase temp) • Pressure is constant =

  11. Ex: A sample of Nitrogen occupies a volume of 250 mL at 25oC. What volume will the gas occupy at 95oC?

  12. Gay-Lussac’s Law (1802) • Pressure and temperature are directly related (Increase pressure= Increase temperature) • Volume is constant!

  13. Ex: A gas has a pressure of 710 kPa at 227oC. What will the pressure be at 27oC, if the volume does not change?

  14. Combined Gas Law • Combines 3 gas laws: Boyle’s, Charles’, and Gay-Lussac’s • Used when it is difficult to hold any one variable (P, V, or T) constant = • Can take away any variable that is constant • Take temp away = Boyle’s • Take Pressure away = Charle’s • Take Volume away = Gay-Lussac’s

  15. Ex: 3.0 L of Hydrogen gas has a pressure of 1.5 atm at 20oC. What would the volume be if the pressure increased to 2.5 atm at 30oC?

  16. Ideal Gas Law • Used for gases that behave “ideally” • Allows you to solve for # of moles of a contained gas when P, V, and T are known. • Use constant R=8.31 P(pressure)- must be in kPa V (volume)- must be in L n (# of moles)- muse be in moles of gas R- gas constant T (Temperature)- Must be in Kelvin (oC + 273= K)

  17. Ideal Gas Law • A gas behaves “ideally” if it conforms to the gas laws • Gases do not usually do this • Real gases only behave this way at: • High temps (molecules move fast) • Low pressure (molecules are far apart) • This is because gases will stay a gas under these conditions • Molecules are not next to each other very long so attractive forces can’t play a role b/c molecules are moving too fast • Ideal Gases do no exist because: • Molecules do take up space • There are attractive forces between molecules otherwise no liquid would form. (Molecules slow down to become liquids)

  18. Ex: What volume will 2.0 mol of N2 occupy at 720 torr and 20oC?

  19. Dalton’s Law of Partial Pressures • Used for mixture of gases in a container • If you know the P exerted by each gas in a mixture, you can calculate the total gas pressure • It is particularly useful in calculating pressure of gases collected over water. Ptotal = P1 + P2 + P3… *P1 represents the “partial pressure” or the contribution by the gas

  20. Ex: Helium, Nitrogen, and Oxygen exist in a container. Calculate the total pressure of the mixture for the following partial pressures:He = 200 kPa N= 500 kPa O= 400 kPa

  21. Graham’s Law of Effusion • Rate of effusion and diffusion are inversely proportional to the square root of the mm of molecules • Effusion: Gas escaping through tiny holes in a container • Diffusion: movement from area of high concentration to low concentration (ex: perfume spreading across a room) (Both depend of the mm of the molecule, which determines speed) = • Type of Molecule is important • Gases with lower mm effuse/diffuse faster • Ex: Helium diffuses/effuses faster than Nitrogen from a balloon b/c Helium moves faster due to lower mm. Big = Slow small = Fast

  22. Ex:

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