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Edward Wen, PhD

Gases. Edward Wen, PhD. Properties of Gases. expand to completely fill their container take the shape of their container low density much less than solid or liquid state compressible when pressure is changed. mixtures of gases are always homogeneous (common air) fluid.

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Edward Wen, PhD

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  1. Gases Edward Wen, PhD

  2. Properties of Gases • expand to completely fill their container • take the shape of their container • low density • much less than solid or liquid state • compressible when pressure is changed. • mixtures of gases are always homogeneous (common air) • fluid

  3. Properties of Gas: Indefinite Shape & Volume Gas molecules: have enough kinetic energy and little attractions  keep moving around and spreading out  fill the container of whatever shape

  4. Comparison: Gas, Liquid, Solid

  5. Kinetic Molecular Theory

  6. Pressure: Gases Pushing What are Gas molecules doing? • constantly in motion • as they move and strike a surface, they push on that surface • push = force • Pressure of gas: total amount of force exerted by gas molecules hitting the entire surface at any one instant • pressure = force per unit area

  7. Measuring Air Pressure • use a barometer: column of mercury supported by air pressure • Force of the air on the surface of the mercury  Gravity on the column of mercury gravity

  8. Common Units of Pressure

  9. Practice: Convert Pressure between units • 735.0 mmHg = ? atm • 35. psi = ? torr Ans: 0.9671 atm Ans: 1.8 × 103 torr

  10. The Effect of Gas Pressure • whenever there is a Pressure difference, a gas will flow from area of High pressure  area of Low pressure • the bigger the difference in pressure, the stronger the flow of the gas • if there is something in the gas’ path, the gas will try to push it along as the gas flows

  11. Gas Pressure in Soda Straws Straw at idle: Pressure of the air inside the straw = Pressure of the air outside the straw  liquid levels is the same on both sides Suction of the straw: Pressure of the air inside the straw is < Pressure of the air outside the straw  liquid is pushed up the straw by the outside air

  12. Atmospheric Pressure & Altitude • Altitude↑ Atmospheric pressure↓ • At the surface, P = 14.7 psi, • At 10,000 ft altitude, P = 10.0 psi • Rapid changes in atmospheric pressure may cause your ears to “pop”  an imbalance in pressure on either side of your ear drum (driving or flying) Demo: Can you make a piece of paper uphold a bottle of water?

  13. Boyle’s Law For the gas contained at constant temperature: • Pressure of a gas is inversely proportional to its volume: P 1/V • Or P x V = constant • P1 x V1 = _______ https://www.youtube.com/watch?v=N5xft2fIqQU

  14. When you double the pressure on a gas, the volume reduces to one half, (as long as the temperature and amount of gas do not change)

  15. Information Given: P1 = 4.0 atm V1 = 6.0 L P2 = 1.0 atm Find: V2 = ? L Example:A cylinder equipped with a moveable piston has an applied pressure of 4.0 atm and a volume of 6.0 L. What is the volume if the applied pressure is decreased to 1.0 atm? Answer: 24 L

  16. We’re losing altitude. Quick Professor, give your lecture on Charles’ Law!

  17. Charles’ Law For the gas contained and at constant Pressure: • Volume is directly proportional to temperature V T • constant P and amount of gas • graph of V vs T is straight line • as T increases, V also increases • Kelvin K = °C + 273 • V = constant x T • if T measured in Kelvin

  18. Charles’ Law in Action https://www.youtube.com/watch?v=al5f9q845q0 https://www.youtube.com/watch?v=rQcLhH35RYo Egg sucked into bottle: https://www.youtube.com/watch?v=X-AldaPHQdE The density of common air depends on the temperature. Higher T, lower Density • Why the air vents for the air conditioning system are located at the ceiling?

  19. Information Given: V1 = 2.80 L V2 = 2.57 L T2 = 0°C Find: temp1 in K and °C Example:A gas has a volume of 2.80 L at an unknown temperature. When the sample is at 0°C, its volume decreases to 2.57 L. What was the initial temperature in kelvin and in celsius? T1 = 297 K = 24 °C

  20. Gay-Lussac’s Law For the gas contained at constant Volume: • Pressure is directly proportional to temperature P T • constant V and amount of gas • graph of P vs T is straight line • as T increases, P also increases • Kelvin K = °C + 273 • P = constant x T • if T measured in Kelvin

  21. Gay-Lussac’s Law in Action The pressure of gas in a sealed container depends on the temperature. Higher T, higher Pressure • Keep the propane container in a cool place, avoid from direct sunlight. • If the storage of propane container is on fire, the pressure of propane gas will increase!!! So what to do? • RUN! Call 911, Tell the FD about it!

  22. Avogadro’s Law • Volume directly proportional to the number of gas molecules • V = constant x n • constant P and T • more gas molecules = larger volume • count number of gas molecules by moles • Equal Volumes of gases contain Equal numbers of molecules • the gas doesn’t matter

  23. Avogadro’s Law

  24. Combined Gas Law • Boyle’s Law : Pressure and Volume • at constant temperature • Charles’ Law : Volume and absolute Temperature • at constant pressure  Volume of a sample of gas when both the Pressure and Temperature change

  25. Information Given: V1 = 158 mL, P1 = 755 mmHg, t1 = 34°C V2 = 108 mL, t2 = 85°C Find: P2, mmHg Example:A sample of gas has a volume of 158 mL at a pressure of 755 mmHg and a temperature of 34°C. The gas is compressed to a volume of 108 mL and heated to 85°C, what is the final pressure in mmHg? P2 = 1.29  103 mmHg

  26. Ideal Gas Law • Combined Gas Law + Avgadro’s Law  Ideal Gas Law • R is called the Gas Constant • the value of R depends on the units of P and V • R = 0.0821 atm/K · mol • convert P to atm and V to L • Application of Ideal Gas law: when T, P, V of a gas all changes

  27. Information Given: V = 3.2 L, P = 24.2 psi, t = 25°C Find: n, mol Example:Calculate the number of moles of gas in a basketball inflated to a total pressure of 24.2 psi with a volume of 3.2 L at 25°C

  28. Air: Mixtures of Gases • Air is a mixture (N2 , O2) • Each gas in the mixture behaves independently of the other gases • though all gases in the mixture have the same volume and temperature • all gases completely occupy the container, so all gases in the mixture have the volume of the container

  29. Pgas = PH2O + PH2 Collecting gas over water Zn metal reacts with HCl(aq) to produce H2(g). The gas flows through the tube and bubbles into the jar, where it displaces the water in the jar. Because water evaporates, some water vapor gets mixed in with the H2.

  30. Standard Conditions (STP) • Common reference points for comparing Standard Temperature & Pressure • Standard Pressure = 1.00 atm • Standard Temperature = 0°C = 273 K

  31. Molar Volume of a Gas at STP Definition: The volume of 1 (exact) mole gas at STP • Use the Ideal Gas Law: PV = nRT • 1 mole of any gas at STP will occupy 22.4 L ==> Molar volume • can be used as a conversion factor • as long as you work at STP 1 mol  22.4 L

  32. Molar Volume So much empty space between molecules in the gas state,  the volume of the gas is not effected by the size of the molecules, (under ideal conditions).

  33. Density of Gas at STP • Since every exactly one mole of any gas has a volume of 22.4 L, whereas the mass of such gas would be as the molar mass in grams • Density of Gas = Molar mass / Molar Volume Example: Find the Density of Oxygen gas at STP. 1.43 g/L

  34. At STP, the density of common gases (in g/L) as: H2 0.0900 He 0.179 CH4 0.716 N2 1.25 Air 1.29 O2 1.43 CO2 1.96 Cl2 3.17 Which one, hydrogen gas or helium gas, is better in blimps in providing lift? Why carbon dioxide is used in putting out fire? What if its density is less than the air? Density of Common Gases

  35. Real Gases • Ideal gas laws assume • No Attractions between gas molecules • No Volume: gas molecules do not take up space • based on the Kinetic-Molecular Theory • Real gases: often do not behave like Ideal gases at High pressure (“Squeezed”) or Low temperature (“Frozen”)

  36. Ideal vs. Real

  37. Information Given: T1 = 18°C, P1 = 30. psi T2 = 35°C Find: P2 in psi. The tire on a bicycle stored in a cool garage at 18C had a pressure of 30. psi. What is the pressure inside the tire after riding the bike at 35C? Assume the volume of the tire remains constant. T1 = 291 K, T2 = 308 K, P2 = 32 psi

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