Motion and Properties of Gases: Kinetic Theory and Gas Pressure

1. GASES Unit 1 Chapter 1

2. Write this! Motion of particles: • In solids the particles: • are moving relatively slowly. • have low kinetic energy • In liquids the particles: • molecules move faster. • have higher kinetic energy. • In gases, the particles: • move fastest, • have high kinetic energy.

4. Kinetic Theory Model of States Gas Particles move freely through container. The wide spacing means molecular attraction is negligible. Liquid Particles vibrate, rotate, tumble and “flow”, but cohesion (molecular attraction) keeps them close together. Solid Particles vibrate but don’t “flow”. Strong molecular attractions keep them in place. 4

5. Write this! • Particles can have 3 types of motion: • Vibrational kinetic energy (vibrating) • Rotational kinetic energy (tumbling) • Translational kinetic energy (flying around) 5

6. Write this! Kinetic Theory of s, l & g. • When it is cold, molecules move slowly • In solids, they move so slowly that they are held in place (only vibrational energy) • In liquids they move a bit faster, they can tumble and flow, but they don’t escape from the intermolecular attraction with other molecules (mostly rotational energy, some vibration & translation) • In gases they move so quickly that can overcome the intermolecular attractions and leave the container (moretranslationalenergy, with a little bit of rotation & vibration). 6

7. Plasma, the “Fourth State” 7 When strongly heated, or exposed to high voltage or radiation, gas atoms may lose some of their electrons. As they capture new electrons, the atoms emit light—they glow. This glowing, gas-like substance is called “plasma”

8. Write this! Properties of gases: Gases: • can be atoms or molecules • IHave No Bright Or Clever Friends • I2 H2 N2 Br2 O2 Cl2 F2 are all diatomic gases, • have mass, • no definite volume or shape, • are compressible & can expand. • Therefore properties of gasses can only be compared under specific conditions. • Gas particles are very spaced out!

9. Find the properties of some common gases: Finish for homework! • Read pages 38-43 from your textbook. • For N2, O2, CO2, radon & methane gas: • List their: • Abundance • General use (Do we breath it? Do plants use it etc) • Technological applications • For O3 how is it a useful and harmful gas?

10. Fun Gases (of no real importance) • Nitrous Oxide (N2O) • AKA: Laughing gas, Happy gas, Nitro, NOS • Uses • anaesthetic in dentist offices, this sweet-smelling gas reduces pain sensitivity and causes euphoric sensations. • It is an excellent oxidizer, reigniting a glowing splint much like oxygen would. • It is used in racing where it is injected into the carburetor to temporarily increase an engine’s horsepower. • Sulfur Hexafluoride • One of the densest gases in common use. Fun with Sulfur hexafluoride 10

11. Write this! Matchthe gas with the problem it causes GasProblem • Carbon Dioxide Ozone layer depletion • CFCs Climate Change • Methane Toxic poisoning • Carbon monoxide Noxious smell • Sulfur dioxide Acid Rain

12. Last class: • We talked about the motion of molecules: • Vibrational • Rotational • Translational • Gasses have mostly translational motion. • As you increase temperature their motion also increases. • Increase in translational movement = increase in velocity

13. Write this! Pressure in gases: • A force applied over a unit of area. • For a gas, pressure results from gas molecules colliding with the wall of its container. • Measured in Pascals (Pa) or kiloPascals (kPa)

14. If container is not strong enough walls can rupture Adding a gas: • Adds more gas molecules • More collisions • Increased net pressure • Ex. Double # of molecules = double pressure

15. If container is not strong enough walls can collapse Removed a Gas: • Removes gas molecules • Less collisions • Pressure decreases

16. Change Size of Container: • Decrease container size • Decreases space for molecules to move • Increases collisions • Increases pressure

17. Change Size of Container: • Increase container size • Increases space for molecules to move • Decreases collisions • Decreases pressure

18. Heating a Gas: • Gas molecules absorb heat • Molecules move more rapidly • Increase collisions • Increase pressure

19. Cooling a Gas: • Gas molecules release heat • Molecules move more slowly • Decrease collisions • Decrease pressure

20. Write this! Gases exert a pressure as they collide with the walls of containers. The total pressure is dependent on magnitude & quantity of collisions. • Concentration: • Add more gas  Pressure • Remove gas  Pressure • Container size: • decrease  Pressure • increase  Pressure • Temperature: • Increase Temp  Pressure • Decrease Temp  Pressure

21. Write this! Kinetic Molecular Theory (K.M.T): • A gas is composed of particles • Gas particles move rapidly & are in constant random motion • All collisions are perfectly elastic • Kinetic energy is proportional to temperature

22. Write this! Kinetic energy & temperature: As temperature increases molecules move faster & have a greater KE. Not all molecules are moving at the same speed. The KE of moving objects is expressed by: This shows that the KE of molecules is dependent on both their mass & velocity.

23. The range of kinetic energies can be represented as a “bell curve.” Maxwell’s Velocity Distribution Curve. The mean & mode can help establish “average” molecules Most molecules “Average” molecules “Slow” molecules Increasing # molecules “Fast” Molecules mean mode Increasing kinetic energy Average kinetic energy

24. Distribution of Particles Around Average Kinetic Energies. Conclusion: As temperature increases. Curve broadens. Average KE increases. Average kinetic energy of molecules Average kinetic energy of warmer molecules Average kinetic energy of colder molecules Number of molecules Slower than average molecules Faster than average molecules Kinetic Energy of molecules (proportional to velocity of molecules)

25. Write this! Two different gases at the same temperature will: • Have the same AVERAGE EK= ½ mv2 • Lighter molecules will move faster. • Heavier molecules will move slower. Fun Fact The average speed of oxygen molecules at 20°C is 1656km/h. • At that speed an oxygen molecule could travel from Montreal to Vancouver in three hours…If it travelled in a straight line.

26. Observing gases As scientists observed gases, they saw mathematical relationships that very closely, but not perfectly, described the behaviour of many gases. They have developed theories & mathematical laws that describe a hypothetical gas, called “ideal gas.” It is an approximation that helps us model and predict the behavior of real gases.

27. Write this! Kinetic Theory for ideal gases. • The particles of a gas are infinitely small. Explains effusion & compressibility. • The particles of a gas are in constant motion, and move in straight lines. Until they run into another particle or wall. Explains diffusion. • The particles do not attract or repel each other. Explains why gases expand to fill a space. • The average kinetic energy of particles is proportional to the absolute temperature. Explains observed changes in pressure. Fun fact  Each air molecule has about ten billion collisions per second 27

28. Textbook questions Do Page 62 # 3,4,5,8,9,10,11 To be done for next class

29. And in my spare time I invented dialysis, which has saved the lives of thousands of kidney patients Write this! Thomas Graham (1805-1869) • Graham studies the speed of diffusion & effusion. • Diffusion is when gas molecules spread throughout a container until they are evenly distributed • Effusion is when gas molecules pass through tiny opening in container. • He derived his law on Ek = ½ mv2 m = mass (kg) v = velocity (m/s)

30. Write this! • Graham’s Law • Rate of diffusion of a gas is inversely related to the square root of its molar mass. • The equation shows the ratio of Gas 1’s speed to Gas 2’s speed. v = velocity M = molar mass (Leave a space for one more variation!) Same as

31. Write this! Ex. 1 Determine the relative rate of diffusion for krypton and bromine. Relative rate means find the ratio “v1/v2”! Ans: Krdiffuses 1.381 times faster than Br2.

32. Write this! Ex. 2 A molecule of oxygen gas has an average speed of 12.3 m/s at a given temp and pressure. What is the average speed of hydrogen molecules at the same conditions? Put the gas with the unknown speed as “Gas 1”. C. Johannesson

33. Write this! The ratio “v1/v2” is 4.0. Ex. 3 An unknown gas diffuses 4.0 times faster than O2. Find its molar mass. Square both sides to get rid of the square root sign. C. Johannesson

34. Graham’s Law • Rate of diffusion of a gas is inversely related to the square root of its molar mass. • The equation shows the ratio of Gas 1’s speed to Gas 2’s speed. v = velocity M = molar mass Same as

35. Graham’s Law Version #2, Effusion Time Add the equation! Careful the relationship is not inverted! Sometimes it’s easier to measure the time it takes for a gas to effuse completely, rather than the speed. Graham’s law can be changed for this, but the relationship between time and molar mass is direct as the square root:

36. Kinetic Theory Trivia • The average speed of oxygen molecules at 20°C is 1656km/h. • At that speed an oxygen molecule could travel from Montreal to Vancouver in three hours…If it travelled in a straight line. • Each air molecule has about 1010 (ten billion) collisions per second • 10 billion collisions every second means they bounce around a lot! • The number of oxygen molecules in a classroom is about: • 722 400 000 000 000 000 000 000 000 • that’s more than there are stars in the universe! • The average distance air molecules travel between collisions is about 60nm. • 0.00000006m is about the width of a virus. 36

37. Videos • Kinetic Molecular Basketball • http://www.youtube.com/watch?v=t-Iz414g-ro&NR=1 • Average Kinetic Energies • http://www.youtube.com/watch?v=UNn_trajMFo&NR=1 • Thermo-chemistry lecture on kinetics 37