Physical Properties Unit 5: Gases Unit 5: Gases

StandardsStandards b 4a. Students know the random motion of molecules and their collisions with a surface create the observable pressure on the surface b 4b. Students know the random motion of molecules explains the diffusion of gases

A. Kinetic Molecular Theory b Used to predict and explain the behavior of a theoretical gas or ‘ideal gas’ b Particles in an ideal gas… have no volume or elastic collisions in constant, rapid, random, straight-line motion don’t attract or repel each other

b ‘Ideal gases’- are elastic (do not lose energy upon collision) b Cannot be compressed given a change in temperature b Can be measured using the eq. KE= 1/2mv 2

B. Real Gases b Particles in a REAL gas… have their own volume attract each other b Gas behavior is most ideal… at low pressures at high temperatures in nonpolar atoms/molecules

C. Characteristics of Gases b Gases expand to fill any container Random constant motion, no attraction b very low densities

C. Characteristics of Gases b can be compressed given a change in Temp/Pressure State Changes

DiffusionDiffusion b The movement of one material through another. b The rate depends on the mass of the particles b Lighter = rapid diffusion

EffusionEffusion b When a gas escapes through a tiny opening b Rate of effusion can be calculated according to Graham’s law of effusion: Rate of effusion = 1/SQRT MM

EffusionEffusion b Using Graham’s Law, you can also set up a proportion to compare the diffusion rates for two gases b ** see eq on board.

b Ammonia has a molar mass of 17.0 g/mol; hydrogen chloride has a molar mass of 36.5 g/mol. What is the ratio of their diffusion?

D. Describing Gases b Gases can be described by their: Temperature Pressure Volume Number of molecules/moles K atm L #

E. Temperature ºF ºC K K = ºC b Always use absolute temperature (Kelvin) when working with gases!

F. Pressure Which shoes create the most pressure?

F. Pressure b Barometer measures atmospheric pressure b exact height of the Hg depends on atmospheric pressure b usually measured in mm Hg

F. Pressure b Manometer measures contained gas pressure b Difference in height in two arms of U-tube is measure of pressure of gas sample b measured in various different units

F. Pressure b KEY EQUIVALENT UNITS kPa (kilopascal) 1 atm 760 mm Hg 760 torr 14.7 psi

G. STP Standard Temperature & Pressure 0°C 273 K 1 atm kPa -OR- STP

H. Pressure Problem 1 b The average pressure in Denver, Colorado, is atm. Express this in (a) mm Hg and (b) kPa. (a) atm 1 atm 760 mm Hg = 631 mm Hg (b) atm 1 atm kPa = 84.1 kPa

H. Pressure Problem 2 b Convert a pressure of 1.75 atm to kPa and mm Hg. (a) 1.75 atm 1 atm kPa = 177 kPa (b) 1.75 atm 1 atm 760 mm Hg = 1330 mm Hg

H. Pressure Problem 3 b Convert a pressure of 570. torr to atmospheres and kPa. (a) 570 torr 760 torr 1 atm =.750 atm (b) 570 torr 760 torr kPa = 76.0 kPa