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Announcements

Announcements. Final: Monday, April 6, 7:30 am (ugh!) 300 pts comprehensive, multiple choice, Scantron, 10% No-calc bonus Very strict exit policy! Buy Scantron Now and fill out your name and other info before coming!. Here’s what people said they learned.

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Announcements

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  1. Announcements Final: Monday, April 6, 7:30 am (ugh!) 300 pts comprehensive, multiple choice, Scantron, 10% No-calc bonus Very strict exit policy! Buy Scantron Now and fill out your name and other info before coming!

  2. Here’s what people said they learned.

  3. Here’s what people said they wanted to know better.

  4. Tequila (don’t YOU drink it) b-damascenone gives tequila some of its woody/fruity flavor

  5. Chapter 11Gases: what if there were no intermolecular forces?

  6. At a molecular level, the states of matter differ primarily by the level of cohesion. • Solids (examples: a penny, ice, glass???) • Molecules have attracted each other • Atoms that are bound together by sea of electrons • Sometimes in a specific array (crystals) • Sometimes not (glasses) • Hold their own shape indefinitely • Liquids (example: honey, water, glass???) • Molecules have attracted each other • – permanent or temporary dipoles. • Not in any pattern (exception: liquid crystals) • Have a surface: adopt shape of bottom of container • Gases (example: air, helium in balloon) • Molecules not attracted to each other (or weakly attracted) • No pattern • No surface: adopt shape of all of the container.

  7. How things are: Everything wiggles, only some things translate. Solids: molecules or atoms can wiggle about fixed, central location. They don’t translate far. Liquids: molecules or atoms move along (translate) until they bounce into something, and this happens frequently because the molecules or atoms have cohesion. Gases: like liquids, only collisions not as often. No cohesion.

  8. How fast? Depends on molecule. isopentyl acetate, CH3COOCH2CH2CH(CH3)2 About 500 miles per hour—pretty fast! Let’s measure it.

  9. How do we KNOW that structure?

  10. Why it took so long to get to back of room: collisions! The isopentyl acetate molecule only goes about 200 nm before hitting something, which then sends it in a different direction. It takes forever for it to spread around the room…..unless there is a draft.

  11. Diffusion: 10000 steps forward and 10001 steps backward The “hydrodynamic” world of diffusion—dominated by collisions—is very different from our normal “ballistic” world. An object set into motion comes almost immediately to a stop, rather than coasting as we are used to seeing.

  12. Gases—pressure matters! Gas particle hits a piston, imparts a force. With a small number of gas atoms in the cylinder, this force would be quite random. With a large number, the force would be quite steady. We usually see almost perfectly steady pressure because there are billions of billions of particles in any normal sample.

  13. You could measure pressure this way, but it’s pretty darn inconvenient. We still show it because it’s conceptually easy. P = F/A = mg/p r2 Mass of bricks 9.8 m/s2 Radius of piston

  14. Slightly more practical…using water height to measure pressure • How high will the water rise? • depends on the weather • depends on whether you're in Louisiana • or Colorado • is always a little more than 30 feet • Why? Because that is how far • atmospheric pressure can push it. • The vacuum pump doesn't suck; • the air pushes. Vacuumreviewer.com

  15. Density of the water Yeah, but how far will the water rise? Patm = rgh = 101,325 nt/m2 9.8 m/s2

  16. Rather than use the Super Hoover, we could draw a hose full of water, sealed at one end, out of the swimming pool. • Fill a hose with water, taking care to introduce no bubbles. • Put the hose in a large tub of water. • Seal one end of the hose. • Drag the sealed end out with a pulley. • When the hose tip exceeds about 34 feet, you will see the water goes no higher! You get an air gap at the top of the hose. • You now have a barometer. As weather changes, the height of the column of water will fluctuate.

  17. BOB = Big Ol’ Barometer

  18. Remember: P = rghso… h = P rg Denser liquids mean shorter barometers! Mercury density: 13.6 g/mL Why mercury is a liquid http://antoine.frostburg.edu/chem/senese/101/periodic/faq/why-is-mercury-liquid.shtml http://www.madsci.org/posts/archives/may97/862179191.Ch.r.html

  19. No matter what the gas, always extrapolates to -273.15 oC when P = 0 Rigid box Experiments with barometers: #1 Pressure vs. temperature at fixed volume: put barometer into fixed, rigid box and raise temperature. Make pressure readings.

  20. Kelvin concepts T (K) = T(oC) + 273.15 Room temperature: 25oC = 298K Ice water: 0.00 oC = 273.15 K Boiling water: 100.00 oC = 373.15 K All evidence suggests that NOTHING can be colder than 0K = -273.15 C If T is in Kelvins, P is directly proportional to T. You can make a barometer into a thermometer!

  21. P V Experiments with barometers: #2 Pressure vs. volume at fixed temperature: put barometer into horror chamber with descending ceiling. Keep at same Temperature while ceiling lowered. Make pressure readings. Movable ceiling! Lets you change room volume.

  22. P increases with inverse of volume

  23. Experiments with barometers: #3 Pressure vs. mass of gas at fixed T and V. More gas particles (e.g., weigh in more gas) leads to more pressure, other things (like T and V) being equal. Where N is the number of gas particles

  24. Combined gas law: usually called the ideal gas law Combine

  25. We can rearrange that: The constant is called the gas constant. This relation only works for “ideal” gases (but most gases are ideal).

  26. The ideal gas law is good for handling transitions in closed systems. Before: “State 1” After: “State 2” (example: low T) (example: high T)

  27. Suppose a gas is at 1.5 atm of pressure and occupies 250 mL. If the pressure is reduced to 0.5 atm, what volume does the gas now occupy?

  28. 765 mL of a gas is held in a container at a pressure of 0.95 atm. With temperature held constant, the pressure is decreased to 0.82 atm. What is the new volume?

  29. Ideal gas law is PV=nRT. But number of moles, n, is grams, g, divided by Mwt….. So……

  30. 765 mL of a gas is held in a container at a pressure of 0.95 atm. With temperature held constant, the pressure is decreased to 0.82 atm. What is the new volume?

  31. Standard Temperature & Pressure (STP) is arbitrarily set at…… T = 0oC = 32oF = 273.15K P = 1 atm = 101,325 Pa (1 Pa = 1 nt/m2) 1 mol of a “simple” gas occupies 22.4 liters at STP 16 g CH4 = 22.4 liters 44 g CO2 = 22.4 liters 32 g O2 = 22.4 liters 28 g N2 = 22.4 liters http://generationsofaithfulness.wordpress.com/2011/08/03/dear-milk-man/

  32. Mixing Gases, Partial Pressures Gases can mix. When they do, their pressures add (almost): Patmospheric = PO2 + PN2 + PH2O + PAr+ PCO2 + minor pressures from trace gases This is why Baton Rouge is so uncomfortable: PH2O is way too high! All those water molecules crashing into you replenish the heat your body was trying to get rid of by evaporating water molecules as you sweat. Air is approximately 80% N2, 20% O2

  33. Wonderful things, everywhere you look! Water crystals exclude the “grape stuff” (color and sugar and flavor” to make beautiful and tasty popsicle-like treat. PE Container: H-[CH2]n-H

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