Physical Properties

Unit 5: Gases

Standards4a. 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 TheoryUsed 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 GasesParticles 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 GasesGases expand to fill any

container• Random constant motion, no

attractionvery low densities

C. Characteristics of Gasescan be compressed given a change

in Temp/Pressure

State Changes

DiffusionThe movement of one material

through another.The rate depends on the mass of

the particlesLighter = rapid diffusion

EffusionWhen a gas escapes through a tiny

openingRate of effusion can be calculated

according to Graham’s law of effusion:

Rate of effusion = 1/SQRT MM

EffusionUsing 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 GasesGases can be described by their:

• Temperature• Pressure• Volume• Number of molecules/moles

• K• atm• L• #

E. 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. Pressure

areaforcepressure

Which shoes create the most pressure?

F. PressureBarometer

• measures atmospheric pressure

exact height of the Hg depends on atmospheric pressure

usually measured in mm Hg

F. PressureManometer

• 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. Pressure

2mNPa

KEY EQUIVALENT UNITS 101.325 kPa (kilopascal)

1 atm

760 mm Hg

760 torr

14.7 psi

G. STP

Standard Temperature & Pressure

0°C 273 K

1 atm 101.325 kPa-OR-

STP

H. Pressure Problem 1The average pressure in Denver,

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

(a) 0.830 atm1 atm

760 mm Hg = 631 mm Hg

(b) 0.830 atm1 atm

101.325 kPa= 84.1 kPa

H. Pressure Problem 2Convert a pressure of 1.75 atm to

kPa and mm Hg.

(a) 1.75 atm1 atm

101.325 kPa= 177 kPa

(b) 1.75 atm1 atm

760 mm Hg = 1330 mm Hg

H. Pressure Problem 3Convert a pressure of 570. torr to

atmospheres and kPa.

(a) 570 torr760 torr

1 atm = .750 atm

(b) 570 torr760 torr

101.325 kPa= 76.0 kPa