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Physical Characteristics of Gases

Physical Characteristics of Gases. Chapter 10. Kinetic-molecular theory. Particles of matter are always in motion. Ideal gas. An imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory. We can often treat real gases as ideal gases and still get good results.

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Physical Characteristics of Gases

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  1. Physical Characteristics of Gases Chapter 10

  2. Kinetic-molecular theory • Particles of matter are always in motion Chemistry Chapter 10

  3. Ideal gas • An imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory. • We can often treat real gases as ideal gases and still get good results. Chemistry Chapter 10

  4. Assumptions of KMT of gases • Large numbers of tiny particles that are far apart compared to their size • Low density • Easily compressed • Elastic collisions • No kinetic energy is lost when gas particles collide with each other or their container • It can be transferred between particles, but the total kinetic energy remains the same Chemistry Chapter 10

  5. Gas particles are in continuous, rapid, random motion • There are no attractive or repulsive forces between gas particles • When they hit, they don’t stick together • The average kinetic energy of gas particles depends on the temperature of the gas • Direct relationship Chemistry Chapter 10

  6. Expansion • Gases have indefinite shape and volume. • They completely fill any container they are in. • They also take the shape of that container. • Because: • they move rapidly in all directions and don’t stick together Chemistry Chapter 10

  7. Fluidity • Gas particles slide past each other. • They can flow • Fluid: something that can flow (can be gas or liquid) Chemistry Chapter 10

  8. Diffusion • Spontaneous mixing of the particles of two substances caused by their random motion. • Gas particles spread out to fill their new container Chemistry Chapter 10

  9. Effusion • When gas particles pass through a small opening • Particles leak out of the container Chemistry Chapter 10

  10. Real gases • Do not completely follow kinetic-molecular theory • Especially deviant at high pressures and low temperatures • Noble gases are closest to ideal • Very polar gases are farthest from ideal Chemistry Chapter 10

  11. Discuss • Describe the conditions under which a real gas is most likely to behave ideally. • Explain the following properties of gases using the kinetic-molecular theory: expansion, fluidity, low density, compressibility, and diffusion. Chemistry Chapter 10

  12. Describing gases • Needed: • Volume • Temperature • Number of molecules • Pressure • They are mathematically related. Chemistry Chapter 10

  13. Pressure • balloon • The force per unit area on a surface. Chemistry Chapter 10

  14. Force • A push or a pull • Measured in newtons (N). • At the Earth’s surface, 1 kg of mass exerts 9.8 N of force due to gravity. Chemistry Chapter 10

  15. Chemistry Chapter 10

  16. Pressure of gases • Gases exert pressure on any surface with which they collide. • Depends on volume, temperature, and number of molecules Chemistry Chapter 10

  17. Atmospheric pressure • Air around Earth exerts a pressure on it’s surface and everything on it. • Like the weight of all the molecules pressing down. Chemistry Chapter 10

  18. h Barometer • Used to measure atmospheric pressure. • Height of liquid (usually mercury) in tube can be used to express atmospheric pressure. • At sea level, the average is 760 mm Hg. Chemistry Chapter 10

  19. Manometer • Used to measure the pressure of gases. • The height difference between the two arms is the pressure. Chemistry Chapter 10

  20. Pressure Units Chemistry Chapter 10

  21. STP • Standard temperature and pressure. • 0 °C and 1 atm • Used to compare volumes of gases. Chemistry Chapter 10

  22. Example • A weather report gives a current atmospheric pressure of 745.8 mm Hg. Convert this to • Atmospheres • 0.9813 atm • Torr • 745.8 torr • Kilopascals • 99.43 kPa Chemistry Chapter 10

  23. Discuss • Define pressure • What is STP? • Convert 151.98 kPa to atmospheres • 1.4999 atm Chemistry Chapter 10

  24. Boyle’s Law • Fixed: mass and temperature • The volume varies inversely with pressure • Less volume, means the particles hit the walls more often. • This increases the pressure Chemistry Chapter 10

  25. Boyle’s Law • Mathematically: • Each sample of gas has its own k. Chemistry Chapter 10

  26. Example • A helium-filled balloon contains 125 mL of gas at a pressure of 0.974 atm. What volume will the gas occupy at standard pressure, assuming constant temperature? • 122 mL Chemistry Chapter 10

  27. You try • A weather balloon with a volume of 1.375 L is released from Earth’s surface at sea level. What volume will the balloon occupy at an altitude of 20.0 km, where the air pressure is 10.0 kPa, assuming constant temperature? • 13.9 L Chemistry Chapter 10

  28. Charles’s Law • Fixed: mass and pressure • Volume varies directly with temperature. • As temperature goes up, the particles have more energy, so they hit the walls more often and with more force • This pushes the walls outward. Chemistry Chapter 10

  29. Charles’s Law • Mathematically Chemistry Chapter 10

  30. Kelvin Scale • Charles’s law works more elegantly on the Kelvin Scale than the Celsius Scale. • If you double the temperature, the volume doubles. • Not true with Celsius • We must use Kelvin for Charles’s Law. Chemistry Chapter 10

  31. Kelvin Scale • Absolute zero: lowest possible temperature • All particle motion stops • 0 K, -273.15 °C • Often rounded to 273 Chemistry Chapter 10

  32. Example • A balloon filled with oxygen gas occupies a volume of 5.5 L at 25 °C. What volume will the gas occupy at 100. °C, assuming constant pressure? • 6.9 L Chemistry Chapter 10

  33. You try • A sample of nitrogen gas is contained in a piston with a freely moving cylinder. At 0.0 °C, the volume of the gas is 375 mL. To what temperature must the gas be heated to occupy a volume of 500. mL, assuming constant pressure? • 91 °C Chemistry Chapter 10

  34. Gay-Lussac’s Law • Fixed: mass and volume • Pressure varies directly with temperature (in Kelvin) • As temperature goes up, energy of particles goes up. • They go faster and hit the walls harder. • If the walls can’t move, the pressure goes up. Chemistry Chapter 10

  35. Gay-Lussac’s Law • Mathematically: Chemistry Chapter 10

  36. You try • The temperature within an automobile tire at the beginning of a long trip is 25 °C. At the conclusion of the trip, the tire has a pressure of 1.80 atm. What is the final Celsius temperature within the tire if its original pressure was 1.75 atm? Assume constant volume. • 34 °C Chemistry Chapter 10

  37. Combined gas law • Expresses the relationship between pressure, volume, and temperature of a fixed amount of gas. Chemistry Chapter 10

  38. You try • The volume of a gas at 27.0 °C and 0.200 atm is 80.0 mL. What volume will the same gas sample occupy at standard conditions? • 14.6 mL Chemistry Chapter 10

  39. Dalton’s Law • The total pressure in a container is the sum of the partial pressures of all the gases in the container. Chemistry Chapter 10

  40. Application • We can collect gases by displacing water. • When we do this, • Read Patm from the barometer. Look up Pwater in table A-8 in the appendix. Chemistry Chapter 10

  41. Example • A student has stored 100.0 mL of neon gas over water on a day when the temperature is 27.0 °C. If the barometer in the room reads 743.3 mm Hg, what is the pressure of the neon gas in its container? • 716.6 mm Hg Chemistry Chapter 10

  42. You try • A sample of nitrogen gas is collected over water at a temperature of 23.0 °C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mm Hg? • 764 mm Hg Chemistry Chapter 10

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