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Why Balloons Float (and why they don’t)

Why Balloons Float (and why they don’t). Unit 3: Phases of Matter Lesson 3: Gases and Pressure. How does a gas behave?. Kinetic Molecular Theory (KMT)- Describes an “ideal” gas. We imagine how it would behave. It would have five properties: Be made of particles with negligible volume

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Why Balloons Float (and why they don’t)

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  1. Why Balloons Float(and why they don’t) Unit 3: Phases of Matter Lesson 3: Gases and Pressure

  2. How does a gas behave? Kinetic Molecular Theory (KMT)- Describes an “ideal” gas. We imagine how it would behave. It would have five properties: • Be made of particles with negligible volume • Particles move in random, straight-lines • Completely elastic collisions • No intermolecular attractive forces • Speed of particles is directly proportional to Kelvin temperature

  3. Ideal is not Real Real gases violate some/all of the KMT But- Only when the particles are moving slow and are squeezed together. Low Temperature & High Pressure = Non-Ideal Behavior When would this happen?

  4. Avogadro’s Hypothesis Equal numbers of gas particles occupy equal volumes of space under the same conditions of temperature and pressure. They all contain equal numbers of molecules!!! Amedeo Avogadro (1776 – 1856) How can this be?!?

  5. Standard Temperature and Pressure (STP) Because things happen differently at different temperatures and pressures (particularly for gasses), we have to set a standard reference point. Standard Temperature: 0° C = 273 K Standard Pressure: 1.000 atm = 101.3 kPa = 760 mmHg (torr) These are in Reference Table A.

  6. What is this “Pressure” of which we speak Pressure = Force exerted over an area. Anything with mass can exert a force. This includes the atmosphere. Standard Pressure: 1 atmosphere of pressure (at sea level)= 14.7 pounds per square inch (psi).

  7. Brief notes on Torr. Torr = millimeters of mercury (mmHg) Refers to the column of mercury in a barometer. 760 torr = Standard pressure Why do we use mercury? Evangelista Torricelli (1608 – 1647)

  8. Pressure conversions 1.000 atm = 14.7 psi = 101.3 kPa = 760.0 mmHg Convert 2.35 atm to kPa: Convert 1.234 kPa to atm:

  9. Vapor Pressure When a liquid in a sealed container is at vapor-liquid equilibrium, the vapor exerts a pressure (like any gas). Stronger IMAF = Lower vapor pressure. Higher vapor pressure = faster rate of evaporation. Volatile= Substances that evaporate quickly.

  10. Why do things boil? Boiling happens when the vapor pressure of a liquid is greater than the atmospheric pressure the liquid is under. Boiling Point = Vapor pressure = atmospheric pressure. How Can you increase vapor pressure?

  11. Normal Boiling Point The boiling point of a liquid at Standard Atmospheric Pressure. What happens to boiling point if atmospheric pressure increases? Decreases?

  12. Reference Table H

  13. Use Method A! Problem: What is the vapor pressure of ___ at ___°C?

  14. Use Method B! Problem: What is the boiling point of ___ at a pressure of ___kPa?

  15. Use Method C! Problem: What is the normal boiling point of ___?

  16. Equal numbers of chemistry students, occupying equal volumes of classrooms do not possess equal numbers of questions....You? Amedeo Avogadro (1776 – 1856) What now?

  17. Things To Do Now:

  18. Passing Gases Unit 3: Phases of Matter Lesson 4: Partial Pressure and Effusion

  19. Dalton’s Law of Partial Pressures The total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas in the mixture. NOT in your Reference Tables (memorize!) Ptotal = PgasA + PgasB + PgasC + ... John Dalton (1766 – 1844)

  20. Practice Helps Us Learn! 1) What is the total pressure of a mixture of O2 (g), N2 (g) and NH3 (g) if the pressure of the O2 (g) is 20. kPa, N2 (g) is 60. kPa and the NH3 (g) is 15 kPa?

  21. 2) A mixture of 1 mole of O2 and 2 moles of N2 exerts a pressure of 150. kPa. What is the partial pressure of each gas?

  22. 3) A mixture of 30.0% He and 70.0% Ar exerts a pressure of 150 kPa at 25oC. What is the partial pressure of each gas?

  23. 4) A sample of NH3 (g) is decomposed into its component elements. If the pressure of the nitrogen gas produced equals 40.0 kPa, what would the pressure of the hydrogen gas?

  24. The total questions asked by a class of chemistry students equals the sum of the questions asked by each student in the class. Any Questions? John Dalton (1766 – 1844) What now?

  25. Things To Do Now:

  26. Imagine a Piston... Unit 3: Phases of Matter Lesson 5: The Gas Laws

  27. Gases Obey Physical Laws This should not surprise you. The behavior of gases can be predicted and expressed according to mathematical relationships. We will look at relationships of Pressure, Volume, Temperature and the # of molecules (aka moles) of a gas.

  28. A Brief Note on Units We will use the following units: Pressure- Atmospheres(atm) & KiloPascals(kPa) Volume-Liters(L) and milliliters(ml) Temperature-Kelvin(K) # of molecules-Moles(mol)

  29. The Beginning: Avogadro’s Hypothesis All of the gas laws stem from Avogadro’s Hypothesis: Equal numbers of gas particles occupy equal volumes of space under the same conditions of temperature and pressure. Amedeo Avogadro (1776 – 1856)

  30. 2 Illustrative Problems to Consider • Consider two 4.00 L containers, each at 298 K and 1.00 atm. Container A holds nitrogen gas, Container B holds carbon dioxide gas. If container A holds 2.00 moles of nitrogen gas, how many moles of carbon dioxide must be present in container B? • Do equal volumes of gases under the same conditions of temperature and pressure have the same MASS? Why or why not?

  31. How To Solve Any Gas Law Problem • Get rid of the words! Read the problem and pick out the variables. Make a list of them. Make sure your units are acceptable and agree. • Write down the particular Gas Law you need. • Rearrange to isolate the variable you’re solving. • Plug in your numbers. • Solve for unknown

  32. Boyle’s Law: Pressure & Volume As Pressure Increases, Volume Decreases P1V1 = P2V2 Temperature must be constant Robert Boyle (1627 - 1691)

  33. A sample of gas occupies a volume of 2.00 L at STP. If the pressure is increased to 2.00 atm at constant temperature, what is the new volume of the gas?

  34. Charles’ Law: Temperature & Volume As Temperature Increases, Volume Increases V1/T1 = V2/T2 Jacques Charles (1746 - 1823) Pressure must be constant TEMP MUST BE KELVIN

  35. A sample of gas occupies a volume of 5.00 L at 300. K. If the temperature is doubled under constant pressure, what will the new volume of the gas be?

  36. Gay-Lussac’s Law: Temperature & Pressure As Temperature Increases, Pressure Increases P1/T1 = P1/T2 Joseph-Luis Gay-Lussac (1778 – 1850) Volume must be constant TEMP MUST BE KELVIN

  37. A 10.0 L sample of gas in a rigid container at 1.00 atm and 200. K is heated to 800. K. Assuming that the volume remains constant, what is the new pressure of the gas?

  38. The Combined Gas Law Puts all three gas laws together. Any variable being held constant, can be ignored. On Reference Table T!

  39. A 2.00 L sample of gas at STP is heated to 500. K and compressed to 200. kPa. What is the new volume of the gas?

  40. A 2.00 L sample of gas at 1.00 atm and 300. K is heated to 500.K and compressed to a volume of 1.00 L. What is the new pressure of the gas?

  41. A 2.00 L sample of gas at 300. K and a pressure of 80.0 kPa is placed into a 1.00 L container at a pressure of 240. kPa. What is the new temperature of the gas?

  42. The Ideal Gas Law Relates the number of moles of a gas to it’s pressure, volume and temperature: NOT on your Reference Tables (MEMORIZE!) Comes from the observation that 1 mole of any gas occupies a volume of 22.4L at STP. PV = nRT n = # of moles R = Gas constant

  43. What is the pressure exerted by 3.00 moles of gas at a temperature of 300. K in a 4.00 L container?

  44. What is the volume of a sample of gas if 5.00 moles if it exerts a pressure of 0.500 atm at 200. K?

  45. A sample of gas is contained in a cylinder with a volume of 10.0 L. At what temperature will 2.50 moles of contained gas exert 20.0 atm of pressure on the container?

  46. A sample of gas contained in a cylinder of 5.00 L exerts a pressure of 3.00 atm at 300. K. How many moles of gas are trapped in the cylinder?

  47. R! Any Questions?

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