1 Chapter 4 - Gases Movie – phase diagrams Kinetic Theory Pressure Measurement –Units –Unit Conversions Boyle & Charles Combined Gas Law

2 Movie – Phase Diagrams Take home message: –how gases move around –distances between molecules Other –There is more information in the movie – it’s all chemistry. –Much of it is “common sense” with scientific labels.

3 Kinetic Theory Define Temperature Define Source of Pressure Relate Molecular Motion to Gas Behavior.

4 Kinetic Theory – Define Temperature Temperature relates to the speed of molecular motion. Fast moving molecules = higher temp. –Solids vibrate –Liquids wiggle (movie) –Gases move in straight lines Actually, temp. relates to kinetic energy which is mass times speed. –Fat moleculesmove slower at same temp. –Skinny moleculesmove faster –(Geo Metro vs. GMC Suburban)

5 Kinetic Theory - Source of Pressure Pressure is the force of gas atoms bouncing off the walls of a container. POW BIFF

6 Kinetic Theory: Molecular Motion  Gas Behavior Hit the wall = pressure Gas collisions are perfect bounces. T  = Moving faster  P__ T  = moving slower  P__ More molecules  P__ Smaller Bottle  P__

7 How do we measure pressure? Definition: P = force area Wall Barometer (Fig. 4.8) –air pushes on a diaphram against a spring –calibrate the scale Manometer –Air pushes on a mercury column.

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10 Pressure Units NameSourceUnitNormal Day PascalsSI (like in lab)kPa102 kPa Pounds-per- Square-Inch Lifepsi14.7 psi AtmospheresLabatm1.00 atm TorrLab/Barometertorr mmHg 760 mmHg

11 Pressure Unit Conversions We always get to convert these! 1 atm  760 torr = 760 mmHg = 14.7 psi = kPa –Forget about “bar”

12 Vacuum A vacuum is a truly empty space. No gas molecules. Pressure = atm. –Real experiments can only get to atm or so.

13 What is vacuum? Vacuum- literally translated, the word describes the volume from which all the air has been totally removed. However, so far nobody has succeeded in achieving such a condition. Scientifically the highest vacuum yet reached is in the range of torr. This result has been achieved during a test by IBM Thomas J. Watson Research Center, New York, USA, October 1976, in a cryogene system where the temperature dropped to minus 269 °C. Even at this low pressure thousands of gas molecules were detected per cm3. Also far out in space the presence of gas has been measured.

14 Absolute vs. Gauge Bike Tires? 45 psi Car Tire? The psi values we use are usually relative to atmospheric pressure. “Gauge” pressure means 1 atm + the reading. “Absolute” means atm + the reading Actual pressure in bike tire is 45 psi psi = 59.7 psi! Chemistry lab experiments use absolute pressure.

15 Pressure - Volume Boyles’ Law –Change the volume of a gas sample –See what the pressure does (graph it). Avogadro’s Law –Trap different volumes of gas in the syringe. –See what the pressure does.

16 Pressure – Volume Math Boyle’s Law says that: PV = k, P= k/V If you do 2 experiments: –1 st Experiment: 1.0 atm, 5.0 mL –2 nd Experiment: ? atm, 25.0 mL

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18 Pressure - Temperature Charles’ Law says that: Pressure  Temperature –P   T  –P   T  –Double temperature  double pressure Careful: –You have to use Kelvin units for temperature.

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22 Pressure – Temperature Math Charles’ Law says that: P = kT, k = P/T If you do 2 experiments: –1 st Experiment: 1.0 atm, 0 °C –2 nd Experiment: ? atm, 100 °C

23 Combined Gas Law We can shove Boyle & Charles together:

24 Combined Gas Law - Math These problems are all about setting them up right At 56°C and 868 torr, a gas takes up 4.30 L. What will the pressure be if the volume is changed to 6.36 L and cooled to 12 °C? Case 1Case 2 P_868 torr__???_____ V_4.30 L____6.36 L_ T_56°C=329K__12°C=285K_

25 Combined Gas Law - Math Once you make the table, fill into the big equation. Answer: torr

26 Combined Gas Law - Math Is out answer good? –P went down. –Sample was cooled. –Volume went up. Sounds ok by kinetic theory…