[NOISE] Hello. [NOISE] Chapter 9. Gases. Exam 3, not Exam 2. [NOISE] Properties, mentioned last time. [NOISE] We'll talk about pressure first before we talk about how they interact. [NOISE] Section 9.1. [NOISE] Pressure is force divided by area. Metric unit of force, Newtons. Metric unit of area, square meters. Lots of other ways. [NOISE] You know pounds per square inch, psi. When you blow up your car tires or your bike tires, there's a gauge, psi. Pounds is a unit of force. Square inches is a unit of area. Weather report on TV at night. I know what's a TV. Barometric pressure. Inches of mercury. Page 463, barometer. Tub of mercury, tube sticking into it. How tall is the column of mercury? That's related to the atmospheric pressure. Typical day, 29.92 inches of mercury, or metric units, 760 millimeters of mercury. Some people don't like distance units being used to measure pressure. New unit, Torr, [NOISE] named after Evangelista Torricelli, who invented the barometer. [NOISE] Physics gets hold of it. New unit, call it one. How many atmospheres does the Earth have? Not a trick question. How many atmospheres does the Earth have? One. Atmosphere exerts pressure. [NOISE] Pressure unit, atmospheres, one. These all mean the same thing. Two most useful for us, Torr, atmospheres. Seven hundred and sixty Torr is one atmosphere. [NOISE] Need more time? Section 9.2. Pressure, volume, moles, and temperature. How are they related to each other? Seventeenth-century, Boyle's law. J-shaped tube, mercury. Did a lot with mercury then , low life expectancy. Closed end, opened end. Pour in more mercury. Air trapped inside. Volume goes down. Why? Pressure goes up. Equation. Pressure times volume equal to a constant or initial case, final case. Pressure times volume initially equals pressure times volume at the end. Make sense? [NOISE] Good artwork. [NOISE] Bad artwork. Same idea. Any of these Boyle's law. This one probably most useful for solving problems. [NOISE] Lecture notes, Page 82. [NOISE] [NOISE] Over time anyone? [NOISE] Page 82 the lecture notes, you have a balloon. Volume 2.54 litres, pressure one atmosphere. Look at the top of a mountain, where the pressure is 0.75 atmospheres, same temperature. What's the volume of the balloon? [NOISE] Bigger or smaller? [NOISE]. It's both. If pressure [NOISE] decreases, volume? Increases. Increases. Should be more than 2.54. [NOISE] Should be easy to solve. For variables, you know three of them. Plug in, [NOISE] rearrange, [NOISE] atmosphere is canceled [NOISE]. Answer in litres. In fact, larger than 2.54. [NOISE] Makes sense? Entire contribution of everyone in the 17th century to our understanding of gases. [NOISE] Open again. [NOISE] 18th century Charles, French, hot air balloon enthusiast decided to measure volume of balloon versus temperature. [NOISE] BAM direct proportionality. Increase temperature, increase volume. [NOISE] Several ways to write Charles law. [NOISE] Pressure, any units. [NOISE] Volume, any units. Temperature must be Kelvin. [NOISE] Everyone have this? [NOISE] Good now? [NOISE] Demo. [NOISE] [BACKGROUND] What's that? [OVERLAPPING] Liquid nitrogen. Liquid nitrogen. [BACKGROUND] Air. Two ninety eight Kelvin, 77 Kelvin. What's going to happen. It's going to shrink. [NOISE] Dry. [LAUGHTER] Pull it back out, 298. What happens? [NOISE] [LAUGHTER] Back up. [LAUGHTER] [BACKGROUND] [NOISE] Cold, small. Warm. [inaudible]. Gets crispy at low temperatures, [LAUGHTER] so do fingers. [LAUGHTER] Nobody touch. Make sense? Makes noise [inaudible]. [LAUGHTER] [APPLAUSE] Who needs audio? [BACKGROUND] [LAUGHTER] Lecture notes page 83. Balloon in a freezer, minus 10 Celsius, volume, 750 milliliters. Take out of the freezer, room temperature. Volume, bigger or smaller? Bigger. Bigger. You just saw it? Bigger. Take balloon out a very cold, becomes bigger. Answer should be more than 750 Kelvins. If you use Celsius, answer, positive or negative? Negative. Nonsense. No such thing as negative volume. Kelvins. [NOISE] Equation. Put in three variables, solve for the fourth. Kelvins cancel out. Eight hundred and fifty milliliters. More than 750 as predicted. [NOISE] Need more time? Charles' data plotted different gas samples. Volume, temperature. Straight lines converge here. Negative 273, absolute zero. Charles discovered it. Why named Kelvin? Should be Charles [NOISE]. But Charles starts with a C, Celsius starts with a C, confusion. Kelvin. [BACKGROUND] PV, constant. V over T, constant. Combine. PV over T, constant. P_i, V_i over T_i equals P_f, V_f over T_f. Combined gas law. Ridiculous problem, page 84 lecture notes. Balloon, 750 torr, 810 milliliters. Dry ice acetone bath, minus 77 Celsius. Take it out, let it warm up. At the same time, move it to a decompression chamber that's been pumped up to three atmospheres. Who does that? [LAUGHTER] Nobody. But here's why. Pressure, increasing or decreasing? Increasing. Less than one atmosphere, three atmospheres. [NOISE] Pressure goes up, volume should [NOISE] go down. Temperature, increasing or decreasing? [OVERLAPPING] Increasing. Increasing. Temperature goes up, volume should go up. Which one wins? That's why you have the equation. Who needs more time with this? [BACKGROUND] [NOISE] Okay now? [NOISE] Solution to ridiculous problem. First, [NOISE] temperature in kelvins. Add 273, here you are. Pressures, atmosphere is okay. Torr, okay. But they must both be the same. Seven hundred and sixty torr in one atmosphere. Convert. Rearrange equation to get the final by itself and your numbers. [NOISE] It turns out the balloon shrinks. Change in pressure mattered more than change in temperature. Not obvious until you do the arithmetic, so here. Will this always be the case for atmospheric pressure [inaudible] Units have to be the same. It doesn't matter what they are. Temperature, yes, kelvins. [NOISE] Everyone okay? Moles. Who else? Avogadro's law, and the number of moles? Blow more air into the balloon, the balloon gets bigger. I don't think we need a problem for that one. But we've seen this at a high temperature, water is a gas too. Balanced equation works for molecules, work for moles. But if all of them were gases, also works for volumes. [NOISE] If pressure and temperature remain constant, two liters of hydrogen reacts with one liter of oxygen to form two liters of water gas. Each balloon is a liter. It's called the law of combining volumes. Small whole numbers of the volumes, whole numbers of molecules, whole numbers of moles. Logical. [NOISE] We're good? Gas laws, easy as ABC, [NOISE] Avogadro, Boyle, Charles. Combine all three, you'll get this, called the constant R. Here's the value of R. You will be given this on an exam. Lots of people like to chop it off, [NOISE] 0.082, makes people think only two sig place, no. Funny units. You'll see why soon. Universal gas constant fits into this equation. PV equals nRT. How many have seen this before? [NOISE] Ideal gas law. Ideally, all gases obey it. Reality none do. Good first approximation. Page 479 in your textbook. Main thing from chapter 9, PV equals nRT. Second main thing from chapter 9, STP; standard, temperature, and pressure. Zero Celsius, one atmosphere. Remember, standard thermodynamic pressure, temperature, one atmosphere, 25 Celsius. I don't know why they did temperature differently, but they did. STP means this wherever you see it. Good place to start practicing after exam number 2. Everyone done? Who knows the answer to this question? 22.4. 22.4 liters. Where does it come from? PV equals nRT. Rearranged to get volume? n, 1 mole, R, given T, standard temperature in Kelvins, P static pressure. All units cancel except liters. 22.4, is a famous number. If you know it, great, if not calculate it. Sometimes hopeful. Page 87, lecture notes. How to find molecular weight using PV equals nRT. Gas, 50.00 milliliters, 27 degrees Celsius, 754 mass, 88 milligrams. Find molecular weight. What must we do? [inaudible] moles convert milligrams of the gas to moles? Well, we don't know the molecular weight. Find the number of moles. How do we do that? [inaudible]. Okay. Units, anyone? Check units for R. [inaudible]. Atmospheres. Volume units, liters, temperature, whole room. [BACKGROUND] Kelvins. Convert pressure to atmospheres by dividing by 760. Volume to liters by dividing by 1000. Temperature to Kelvins. Then use PV equals nRT. When you do, n comes out 2 times 10 to the negative third moles. Find the molecular weight, milligrams divided by millimoles, or convert milligrams to grams and then divide by moles. Same answer either way. 44. Name a gas with a molecular weight of 44. Yes. CO_2? CO_2. There are others, but if you're trying to figure out what gas it is, find molecular weight, it narrows down the possibilities. [BACKGROUND] [NOISE] Questions? Need more time? [BACKGROUND] Page 483. Uh-oh. [BACKGROUND] Chalk, calcium carbonate. Heat it. Lime, calcium oxide, CO_2. Three hundred and four grams of chalk, heat it up. How many liters of CO_2 at STP can we get? Stoichiometry. What's step 1? Convert to moles. Convert to moles. How if we have grams? Divide by [inaudible] Divide by the [inaudible]. Divide by the molecular weight of calcium carbonate, which is 100. So easy. 3.04 moles. What's step 2? [inaudible]. Not yet. Yes? Go through the mole ratio. Mole ratio. Mole ratio here. One. 3.04 moles of CO_2. Could use PV equals nRT. But it's an STP. What's the shortcut? Yeah? 22.4 liters per mole. If you know the magic number, 22.4 liters equals one mole at STP, you can use that as a conversion part. Or if you don't know that, you can plug this into PV equals nRT and you should get the same answer. Questions? Do you like monosyllables? I can do this when I don't have a sore throat. Seven days from now, exam number 2. Today, done. Nice weekend. [BACKGROUND] Well, I think you can see this? Yes. [BACKGROUND] [inaudible] [NOISE] [BACKGROUND] [NOISE] [inaudible] this one sometimes. Yes. [BACKGROUND] [NOISE]
chem103-090-20181018-140001.mp4
From Dana Chatellier October 25, 2018
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