Gases-Intro

1. Gases-Intro • A state of matter, not a “kind” of matter or substance • Operational Definition: no fixed volume or shape (assumes those of the container in which it is in) • Conceptual Definition: nanoscopic “units” (“particles”) are completely separated from one another

2. Gases-Intro • A state of matter, not a “kind” of matter or substance • e.g., Many say “CO2” is a gas; “He” is a gas….BUT • Under certain conditions, nearly all substances can exist as a gas! • Flipside of that is this:All gases can be turned into a liquid or solid if the conditions are adjusted! • What conditions favor the gaseous state? • Consider H2O! It tends to be a gas at high _________ • Consider a refrigeration cycle in an air-conditioning system? • Refrigerant turns from liquid to gas after “expansion”; at low ___________ T (temperature) P (pressure)

3. Compression and Expansion (“Decompression”) of refrigerant occurs in a refrigerator! Chapter 09

4. Gases are favored at “High T” and “Low P” • BUT specific T’s and P’s depend on the substance • Water: • turns to a gas at 100C at 1 atm pressure; • if the pressure is about 2 atm (as in a pressure cooker), won’t turn to a gas until about 120C • O2 and N2 and CO2: • Are gases at room temperature and 1 atm pressure • Need to be cooled to much lower T’s to turn to a liquid at 1 atm. (or compressed greatly to turn to a liquid at moderate T’s) Chapter 09

5. Gases’ properties (with respect to T, P, V, and n) are independent of the type of particle (substance) • As long as a substance is under “favorable conditions” to be in the gaseous state (appropriate T and P for that substance) , it turns out that it will behave the same way(with respect to certain variables) as any other substance “acting like a gas under its favorable conditions”! • --i.e., it doesn’t matter if the “gas” is water, methane, carbon dioxide. The size or mass of molecules doesn’t change the way it behaves! (Pretty amazing!) • --under such ”ideal” conditions, a substance is said to behave “ideally” as a gas.  Called an “ideal gas” • --the behaviors of “ideal gases are described by “gas laws” Chapter 09

6. Gas Pressure 02 • Units of pressure: atmosphere (atm) • Pa (N/m2, 101,325 Pa = 1 atm) Torr (760 Torr = 1 atm) • bar (1.01325 bar = 1 atm) • mm Hg (760 mm Hg = 1 atm) • lb/in2 (14.696 lb/in2 = 1 atm) • in Hg (29.921 in Hg = 1 atm) Chapter 09

7. How Can Pressure be Measured? • Importance of mechanical equilibrium idea • If something is not moving, and it stays not moving, there is no net force on it • “equal but opposite” forces • Consider a syringe with gas in it. • If the only external pressure (force/area) on it is the air in the room (Pbar), then Pgas = Pbar • If Pgas were to immediately increase (say, because you raised T), it would not stay that way because the barrel would move! • Vgas would increase, decreasing P until it was equal to Pbar again

8. Using a Manometer to Determine Pgas • Uses “mechanical equilibrium” idea • liquid not moving and stays not moving • levels equal => Pgas = Pbar • left level higher => Pgas + P”extra” = Pbar • right level higher => Pgas = Pbar + P”extra” • If liquid in manometer is Hg(l), then the height difference, in mm, is the P”extra”in units of mmHg (or torr)

9. Pressure–Volume Law (Boyle’s Law): Boyle’s Law 01 Chapter 09

10. http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html Chapter 09

11. Boyle’s Law 02 • Pressure–Volume Law (Boyle’s Law): • The volume of a fixed amount of gas maintained at constant temperature is inversely proportional to the gas pressure. Chapter 09

12. http://www.grc.nasa.gov/WWW/K-12/airplane/aglussac.html Chapter 09

13. Charles’ Law 01 • Temperature–Volume Law (Charles’ Law): Chapter 09

14. V T µ V 1 = k T 1 1 Charles’ Law 01 • Temperature–Volume Law (Charles’ Law): • The volume of a fixed amount of gas at constant pressure is directly proportional to the Kelvin temperature of the gas. (?, ? const) V1 = k1T1  (?, ? const) Chapter 09

15. Avogadro’s Law 01 • The Volume–Amount Law (Avogadro’s Law): T What variable is being held constant here? ___ Chapter 09

16. Avogadro’s Law 01 • The Volume–Amount Law (Avogadro’s Law): • At constant pressure and temperature, the volume of a gas is directly proportional to the number of moles of the gas present. • Avogadro’s Hypothesis: Equal volumes of different gases at the same T and P contain equal numbers of gas particles. (?, ? const) V1 = k1n1  (?, ? const) Chapter 09

17. Gases Worksheet-I Practice • Proportional relationships / calculations • If A is proportional to B then: • If B doubles, then A doubles! • If B triples, then A triples! • If B becomes 3.21 times what it was, then A becomes ____ times what it was. • If B becomes x times what it was, then A becomes ___ times what it was. 3.21 x

18. Gases Worksheet-I Practice If A is inversely proportional to B then: If B doubles, then A becomes ½ of what it was! If B triples, then A becomes 1/3 of what it was! If B becomes 3.21 times what it was, then A becomes ____ times what it was. If B becomes x times what it was, then A becomes ___ times what it was. 1/3.21 1/x