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An Introduction to Gases

An Introduction to Gases. Chapter 13. Kinetic Molecular Theory. Postulate #1 Gases consist of tiny particles (atoms or molecules) Postulate #2

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An Introduction to Gases

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  1. An Introduction to Gases Chapter 13

  2. Kinetic Molecular Theory • Postulate #1 • Gases consist of tiny particles (atoms or molecules) • Postulate #2 • These particles are so small, compared with the distances between them, that the volume (size) of the individual particles can be assumed to be negligible (zero). • Gases are COMPRESSIBLE

  3. Kinetic Molecular Theory • Postulate #3 • The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas.

  4. Kinetic Molecular Theory • Postulate #4 • The particles are assumed not to attract or to repel each other. • Postulate #5 • The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas.

  5. twice as many molecules Avogadro’s Hypothesis • At the same temp & pressure, equal volumes of gas hold same number of molecules. • V and n are directly related.

  6. Pressure Pressure of air is measured with a BAROMETER (developed by Torricelli in 1643)

  7. Pressure • What is Pressure? • What tool do we use to measure it?

  8. Pressure • mmHg (or Torr) • Atmospheres (atm) • Pascals (used in physics: 1 pascal = 1 newton per square meter) 4. psi Equivalences: 1 atm = 760 mmHg 1 atm = 101,325 Pa = 101.325 kPa 1 atm = 14.7 psi

  9. Pressure Calculation What is 475 mm Hg expressed in atm? 475 mm Hg 1 atm = 0.625 atm 760 mm Hg

  10. Pressure Calculation The pressure of a tire is measured as 294,000 pascals. What is this pressure in atm? 294, 000 Pa 1 atm 2.90 atm = 101, 325 Pa

  11. Dalton’s Law “The Law of Partial Pressure” • The total pressure of a mixture of gases is the sum of the partial pressures of the gases in the mixture. Ptotal = PA + PB + PC

  12. Gas Laws Calculations Get out a calculator!!!

  13. The Gas Law PV=nRT P = pressure ( atm or kPa ) V= volume ( L ) n= number of moles (mol) T= temperature (K)

  14. R – The Proportionality Constant Value depends on units Or

  15. The Gas Law – Problem If 7.0 moles of an ideal gas has a volume of 12.0 L with a temperature of 300. K, what is the pressure in kPa? PV = nRT P = 1454.95 kPa P = 1500 kPa

  16. Combined Gas Law Let’s say we have some O2 gas AND we change some conditions. Would there be anything similar between the two gases?

  17. Combined Gas Law – Problem You have 3 moles of a solution at 300. K and 15 atm in a 2 L container. If the container is heated to 350. K and the volume decreased to 1 L, what will the new pressure be?

  18. Combined Gas Law – Problemc If we know that R1 = R2 and the mass is constant then Replace with numbers

  19. Combined Gas Law – Problem P2 = 35 atm

  20. Pressure & Volume • At constant Temperature • Pressure and Volume vary inversely. • Why? • More collisions  More pressure P1V1 = P2V2

  21. P & V – Example Problem If you start with 0.500 L of a gas at 7.0 atm and you move the gas to a container with 3.5 L available, how much pressure will the gas exert? P1 (V1) = P2 (V2) 7.0 atm (0.500 L) = P2 (3.5 L) 1.0 atm = P2

  22. Demo Now flip the can over into cold water. Predict what do you predict will happen? Put a few drops of water in a can. Heat the can until the water boils. What is happening to the gas inside?

  23. On a Larger Scale

  24. On a Larger Scale

  25. Temperature & Volume At constant Pressure Volume & Temperature vary directly. • Why? • More collisions  More Volume

  26. T & V – Example Problem If a gas is in a balloon with a volume of 12.0 L and at a temperature of 300. K, what will the volume be if you place the balloon in a freezer at 250. K?

  27. S.T.P. • Standard Temperature and Pressure These are conditions that are universal Standard Temperature: 0ºC or 273K Standard Pressure: 1atm or 101.325kPa

  28. S.T.P. – Example Problem What is the volume of 1 mole of a gas at STP? PV = nRT (1atm)V = (1 mole)(0.0821 [Latm/Kmole])(273K) V= 22.4 L

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