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Nature of Gases

Nature of Gases. Properties of Gases. Gases have mass (seems obvious) Easy to compress gases Completely fill container they are in. Properties of Gases. Different gases can move through each other rapidly (diffusion) Gases exert pressure

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Nature of Gases

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  1. Nature of Gases

  2. Properties of Gases • Gases have mass (seems obvious) • Easy to compress gases • Completely fill container they are in

  3. Properties of Gases • Different gases can move through each other rapidly (diffusion) • Gases exert pressure • Pressure of a gas depends on its temperature, amount and volume

  4. Kinetic Molecular Theory • Sometimes abbreviated KMT • Assumes a gas is made of points without volume that have mass • Most gases are molecules • Noble gases = single atom gases

  5. KMT • Particles in a gas must be separated by relatively large distances (can be compressed) • Particles must be in rapid, constant motion (explains diffusion)

  6. KMT • Gases exert pressure on containers because their particles collide frequently • Collisions must be perfectly elastic (no energy lost because collisions keep occurring)

  7. KMT • Gas pressure increases when temperature increases (and volume is held constant) because temperature is a measure of average kinetic energy of particles • KE = mv2/2

  8. KMT • Gas particles exert no force on one another (attractive forces between molecules or atoms are so weak the KMT model assumes the force to be zero)

  9. KMT • What are the assumptions of the kinetic molecular theory? • (Hint: the points highlighted, underlined, italicized, and in PURPLE in the previous slides)

  10. Measuring Gases • We will deal with 4 variables when it comes to measuring gases • Amount • Volume • Temperature • Pressure

  11. Amount of a gas (n) • Measured in moles • 1 mole of ANY gas at STP occupies 22.4 L (molar volume) • Therefore, 1 mole of ANY gas at STP has _________ particles (1 mole of ANYTHING has how many of those things?)

  12. Volume • Gas completely fills any container it is in • The volume of the container IS the volume of the gas • Measured in liters, cm3, or mL

  13. Temperature (T) • Measured in degrees Celsius, on a practical basis • Celsius is always changed to degrees Kelvin for calculation (WHY?) • 0 °C = ? K • T (K) = T (°C) + 273

  14. Pressure (P) • Collisions of gases cause pressure • Gas particles in atmosphere exert pressure • Atmospheric pressure is due to the fact that gas has mass which is attracted by Earth’s gravity

  15. Pressure (P) • Force per unit area = pressure • Force in SI = Newton, area = m2 • Pressure in SI = Pascal (Pa) • 1,000 Pascal = 1 kPa

  16. Pressure • Another way to measure atmospheric pressure = 1 atm • Pressure exerted by atmosphere at sea level • Torricelli invented way of measuring weight of atmosphere

  17. Pressure • Torricelli inverted a glass tube of mercury in a bowl of mercury • Atmospheric pressure pushed the column of Hg inside the tube to a certain height • Unit of pressure = torr

  18. Torricelli’s Barometer

  19. Pressure • 1 atm = 760 torr (mm of Hg) = 101, 325 Pa = 1013.25 hPa = 1013.25 mbar • Aloha!

  20. Now for the laws… • Boyle’s law • Boyle trapped air in a U-shaped tube and measured what happened to the volume when pressure was applied

  21. Boyle’s Results • Trial Pressure Volume • 1 1.30 atm 3.10 cm3 • 2 1.60 2.51 • 3 1.90 2.11 • 4 2.20 1.84

  22. Boyle’s Results • 1 1.30 atm x 3.10 cm3 = 4.03 • 2 1.60 x 2.51 = 4.016 • 3 1.90 x 2.11 = 4.009 • 4 2.20 x 1.84 = 4.048 • Boyle's law demo

  23. Results? • Pressure is proportional to the inverse of the volume • Greater the pressure, _____________ the volume • Practical applications? • Diving • Scuba Bill

  24. Mathematical statement • PV = k (at constant temp) • P1V1 = P2V2 • If you know three of the variables, you can calculate the remaining one

  25. Charles’s Law • Jacques Charles determined the relationship between temperature and volume • At constant pressure, temperature (in K) is directly proportional to volume • By extrapolation, the idea of “absolute zero” is hinted at

  26. Charles’s Law

  27. Charles’s Law • If temperature increases and pressure remains constant, volume will ____________. • Practical applications? • Checking tire inflation? When should you do this?

  28. Charles’s Law • Mathematical statement • V=kT, so V/T = k • V1T2 = V2T1

  29. Gay-Lussac’s Law • Direct relationship between temperature and pressure • P = kT • Aero can pressure

  30. Gay-Lussac’s Law • At constant volume, P1 = P2 T1 T2

  31. Avogadro’s Law • Equal volumes of gases at same temperature and pressure contain an equal number of particles • All gases exhibit same physical behavior • Larger volume must have greater number of particles

  32. Avogadro’s Law • V = kn • From the previous laws, you can note volume changes with temperature and pressure • But for 1 mole at STP, all gases will occupy 22.41 L

  33. Dalton’s Law of Partial Pressures • In mixture of gases, each gas exerts a pressure as if it were alone in the container • Reason why air (a mixture of gases) in some ways acts like a single gas

  34. Dalton continued • Sum of the partial pressures of all gases in a mixture is equal to the total pressure • PT = pa + pb + pc + …

  35. Putting It All Together • Ideal Gas Law combines the work of Boyle, Charles, Gay-Lussac and Avogadro • PV = nRT • Animated gas lab

  36. Ideal Gas Law • P = pressure in atm • V = volume in liters • n= number of moles • R = gas constant • (0.0821 atm-L/mol-K) • T = temperature in Kelvins

  37. Deviant Behavior • Ideal gas law very useful, but doesn’t apply under certain conditions • Very high pressure (experimental values do not agree with predicted values)

  38. Deviant Behavior • Why? • High pressure = gas particles close together, volume of gas molecules (or atoms) becomes significant (remember, we assumed gases are single points of mass with no volume)

  39. Deviant Behavior • Low temperature failure of KMT • Gas particles slow down and attractive forces between particles become significant

  40. Just for grins…

  41. Uh-oh….

  42. MLIA

  43. Deviant Behavior • DONE • DONE • DONE • DONE • Whatever floats your boat!

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