Physical PropertiesPhysical

Properties

Unit 5: Gases

StandardsStandards

4a. Students know the random motion of molecules and their collisions with a surface create the observable pressure on the surface

4b. Students know the random motion of molecules explains the diffusion of gases

A. Kinetic Molecular TheoryA. Kinetic Molecular Theory

Used to predict and explain the behavior of a theoretical gas or ‘ideal gas’

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

‘Ideal gases’- are elastic (do not lose energy upon collision)

Cannot be compressed given a change in temperature

Can be measured using the eq. KE= 1/2mv2

B. Real GasesB. Real Gases

Particles in a REAL gas…• have their own volume• attract each other

Gas behavior is most ideal…• at low pressures• at high temperatures• in nonpolar atoms/molecules

C. Characteristics of GasesC. Characteristics of Gases

Gases expand to fill any container• Random constant motion, no

attractionvery low densities

C. Characteristics of GasesC. Characteristics of Gases

can be compressed given a change in Temp/Pressure

State Changes

DiffusionDiffusion

The movement of one material through another.

The rate depends on the mass of the particles

Lighter = rapid diffusion

EffusionEffusion

When a gas escapes through a tiny opening

Rate of effusion can be calculated according to Graham’s law of effusion:

Rate of effusion = 1/SQRT MM

EffusionEffusion

Using Graham’s Law, you can also set up a proportion to compare the diffusion rates for two gases

** see eq on board.

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 GasesD. Describing Gases

Gases can be described by their:

• Temperature

• Pressure

• Volume

• Number of molecules/moles

• K

• atm

• L

• #

E. TemperatureE. Temperature

ºF

ºC

K

-459 32 212

-273 0 100

0 273 373

32FC 95 K = ºC + 273

Always use absolute temperature (Kelvin) when working with gases!

F. PressureF. Pressure

area

forcepressure

Which shoes create the most pressure?

F. PressureF. Pressure

Barometer• measures atmospheric pressure

exact height of the Hg depends on atmospheric pressure

usually measured in mm Hg

F. PressureF. Pressure

Manometer• measures contained gas pressure

Difference in height in two arms of U-tube is measure of pressure of gas sample

measured in various different units

F. PressureF. Pressure

2m

NPa

KEY EQUIVALENT UNITS

101.325 kPa (kilopascal)

1 atm

760 mm Hg

760 torr

14.7 psi

G. STPG. STP

Standard Temperature & Pressure

0°C 273 K

1 atm 101.325 kPa-OR-

STP

H. Pressure Problem 1H. Pressure Problem 1

The average pressure in Denver, Colorado, is 0.830 atm. Express this in (a) mm Hg and (b) kPa.

(a) 0.830 atm

1 atm

760 mm Hg= 631

mm Hg

(b) 0.830 atm

1 atm

101.325 kPa= 84.1

kPa

H. Pressure Problem 2H. Pressure Problem 2

Convert a pressure of 1.75 atm to kPa and mm Hg.

(a) 1.75 atm

1 atm

101.325 kPa= 177

kPa

(b) 1.75 atm

1 atm

760 mm Hg= 1330

mm Hg

H. Pressure Problem 3H. Pressure Problem 3

Convert a pressure of 570. torr to atmospheres and kPa.

(a) 570 torr

760 torr

1 atm= .750

atm

(b) 570 torr

760 torr

101.325 kPa= 76.0

kPa