The Nature of Gases

1. The Nature of Gases Regardless of their chemical identity, all gases display similar physical behavior

2. Examples of gases • A particle of gas can be: • A single atom • He, Ne, Ar • A diatomic molecule • H2,I2, O2 • A polyatomic molecule • CO2, CH4

3. Scientists use a model to explain the behavior of gases • Kinetic Molecular Theory • Describes the behavior of the sub-microscopic particles that make up a gas • Gases are composed of tiny particles, either atoms or molecules, each of which has a mass. • Gas particles are separated by large distances relative to their size. The particles are so small and the distances between them so great that the volume of the particles themselves is negligible.

4. The gas particles are in constant, rapid, random motion. • Gases exert pressure because their particles frequently collide with the walls of the container and with each other. Between collisions the particles travel in straight lines.

5. Physical properties of gases • Gases have mass • A basketball filled with air weighs more than an empty basketball. • It is easy to compress a gas • Squeeze a balloon. • Gases fill their containers completely. • A balloon fills evenly…all the gas does not concentrate on one side.

6. Physical properties of gases • Different gases move through each other quite rapidly in a process called diffusion. • The smell of cooking spreads. • Gases exert pressure. • Ears pop going up or down hills. • The pressure of a gas depends on its temperature. • Check tire pressure on hot or cold days!

7. Physical properties of gases • Because gas particles keep colliding without slowing down, the collisions are elastic (no energy or motion is lost) • The average kinetic energy of gases depends only on the temperature of the gases. • KE = ½ mv2 • The higher the temperature, the higher the kinetic energy. Mass is constant, so particles speed up as the temperature increases. • Gas particles exert no force on each other.

8. How do we measure gases? • Four variables • Amount of gas n • Volume of gas V • Temperature of gas T • Pressure of gas P

9. Amount of gas • The amount of a gas is measured in moles • The variable used to represent this is n= number of moles • If a problem expresses the amount in grams, convert to moles using the molar mass. n = mole = mass (g) / molar mass (g/mol)

10. Volume of gas • The volume of a gas is often measured in liters • The variable used to represent this is V = volume = liters = L • If a problem expresses the amount in other units you may convert to liters, but you don’t always need to.

11. Temperature of a gas • The temperature of a gas is expressed in degrees centigrade (or Celsius) and converted to Kelvin • The variable used to represent temperature is T = temperature = 0C = K The Kelvin scale uses the same size degrees, but it starts at absolute zero, -273.2 0C. • To convert 0C to K, add 273.2 T (K) = 0C + 273.2

12. Pressure of a gas • The variable used to express pressure is P = pressure = different units are used • Pressure exerted by air in the atmosphere is atmospheric pressure. The mass of air is attracted by the gravity of the earth. • The force exerted on one square unit of area, say one meter, equals atmospheric pressure.

13. Pressure of a gas • Pressure is calculated in units of force per unit area. • SI unit of force is the newton • One newton of force exerted on one square meter of area = pascal = Pa • P = force (newton)/ area (square meter) One pascal is small, so we often use kilopasals, kPa

14. Measure Pressure • Barometers measure pressure by noting the change in the level of mercury in a tube caused by the force of the atmosphere • Manometers are used to measure the pressure of a gas in a closed containeds

15. Common Units of Pressure • 1 pascal (Pa) = 1 newton of force / 1 square meter of area • 1 kiloPascal (kPA)= 1000 pascal • 1 atmosphere (atm) = 101,325 pascal (Pa) • 1 atm = 760 mm Hg = 760 torr • 1 atm = 14.70 lb/sq in • 1 bar = 100,000 Pa = .9869 atm