1 Physical Characteristics of Gases Chapter 10

2 Kinetic-molecular theory Particles of matter are always in motion

3 Ideal gas An imaginary gas that perfectly fits all the assumptions of the kinetic- molecular theory. We can often treat real gases as ideal gases and still get good results.

4 Assumptions of KMT of gases 1.Large numbers of tiny particles that are far apart compared to their size Low density Easily compressed 2.Elastic collisions No kinetic energy is lost when gas particles collide with each other or their container It can be transferred between particles, but the total kinetic energy remains the same

5 3.Gas particles are in continuous, rapid, random motion 4.There are no attractive or repulsive forces between gas particles When they hit, they don’t stick together 5.The average kinetic energy of gas particles depends on the temperature of the gas Direct relationship

6 Expansion Gases have indefinite shape and volume. They completely fill any container they are in. They also take the shape of that container. Because they move rapidly in all directions and don’t stick together.

7 Fluidity Gas particles slide past each other. They can flow –Fluid: something that can flow (can be gas or liquid)

8 Diffusion Spontaneous mixing of the particles of two substances caused by their random motion. Gas particles spread out to fill their new container

9 Effusion When gas particles pass through a small opening Particles leak out of the container

10 Real gases Do not completely follow kinetic- molecular theory Especially deviant at high pressures and low temperatures Noble gases are closest to ideal Very polar gases are farthest from ideal

11 Discuss Describe the conditions under which a real gas is most likely to behave ideally. Explain the following properties of gases using the kinetic-molecular theory: expansion, fluidity, low density, compressibility, and diffusion.

12 Describing gases Needed: –Volume –Temperature –Number of molecules –Pressure They are mathematically related.

13 Pressure balloon The force per unit area on a surface.

14 Force A push or a pull Measured in newtons (N). At the Earth’s surface, 1 kg of mass exerts 9.8 N of force due to gravity.

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16 Pressure of gases Gases exert pressure on any surface with which they collide. –Depends on volume, temperature, and number of molecules

17 Atmospheric pressure Air around Earth exerts a pressure on it’s surface and everything on it. –Like the weight of all the molecules pressing down.

18 Barometer Used to measure atmospheric pressure. Height of liquid (usually mercury) in tube can be used to express atmospheric pressure. At sea level, the average is 760 mm Hg. h

19 Manometer Used to measure the pressure of gases. The height difference between the two arms is the pressure.

20 Pressure Units

21 STP Standard temperature and pressure. 0 °C and 1 atm Used to compare volumes of gases.

22 Example A weather report gives a current atmospheric pressure of mm Hg. Convert this to –Atmospheres atm –Torr torr –Kilopascals kPa

23 Discuss Define pressure What is STP? Convert kPa to atmospheres – atm

24 Boyle’s Law Fixed: mass and temperature The volume varies inversely with pressure –Less volume, means the particles hit the walls more often. –This increases the pressure

25 Boyle’s Law Mathematically: Each sample of gas has its own k.

26 Example A helium-filled balloon contains 125 mL of gas at a pressure of atm. What volume will the gas occupy at standard pressure, assuming constant temperature? 122 mL

27 You try A weather balloon with a volume of L is released from Earth’s surface at sea level. What volume will the balloon occupy at an altitude of 20.0 km, where the air pressure is 10.0 kPa, assuming constant temperature? 13.9 L

28 Charles’s Law Fixed: mass and pressure Volume varies directly with temperature. –As temperature goes up, the particles have more energy, so they hit the walls more often and with more force –This pushes the walls outward.

29 Charles’s Law Mathematically

30 Kelvin Scale Charles’s law works more elegantly on the Kelvin Scale than the Celsius Scale. –If you double the temperature, the volume doubles. Not true with Celsius We must use Kelvin for Charles’s Law.

31 Kelvin Scale Absolute zero: lowest possible temperature –All particle motion stops –0 K, °C Often rounded to 273

32 Example A balloon filled with oxygen gas occupies a volume of 5.5 L at 25 °C. What volume will the gas occupy at 100. °C, assuming constant pressure? 6.9 L

33 You try A sample of nitrogen gas is contained in a piston with a freely moving cylinder. At 0.0 °C, the volume of the gas is 375 mL. To what temperature must the gas be heated to occupy a volume of 500. mL, assuming constant pressure? 91 °C

34 Gay-Lussac’s Law Fixed: mass and volume Pressure varies directly with temperature (in Kelvin) –As temperature goes up, energy of particles goes up. –They go faster and hit the walls harder. –If the walls can’t move, the pressure goes up.

35 Gay-Lussac’s Law Mathematically:

36 You try The temperature within an automobile tire at the beginning of a long trip is 25 °C. At the conclusion of the trip, the tire has a pressure of 1.80 atm. What is the final Celsius temperature within the tire if its original pressure was 1.75 atm? Assume constant volume. 34 °C

37 Combined gas law Expresses the relationship between pressure, volume, and temperature of a fixed amount of gas.

38 You try The volume of a gas at 27.0 °C and atm is 80.0 mL. What volume will the same gas sample occupy at standard conditions? 14.6 mL

39 Dalton’s Law The total pressure in a container is the sum of the partial pressures of all the gases in the container.

40 Application We can collect gases by displacing water. When we do this, Read P atm from the barometer. Look up P water in table A-8 in the appendix.

41 Example A student has stored mL of neon gas over water on a day when the temperature is 27.0 °C. If the barometer in the room reads mm Hg, what is the pressure of the neon gas in its container? mm Hg

42 You try A sample of nitrogen gas is collected over water at a temperature of 23.0 °C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mm Hg? 764 mm Hg