Collisions of Gas Particles

Collisions of Gas Particles

Kinetic Theory

Kinetic Molecular TheoryPostulates of the Kinetic Molecular Theory of Gases

1. Gases consist of tiny particles (atoms or molecules)

2. These particles are so small, compared with the distances betweenthem, that the volume (size) of the individual particles can be assumedto be negligible (zero).

3. The particles are in constant random motion, colliding with the walls ofthe container. These collisions with the walls cause the pressure exertedby the gas.

4. The particles are assumed not to attract or to repel each other.

5. The average kinetic energy of the gas particles is directly proportionalto the Kelvin temperature of the gas

Kinetic Molecular Theory (KMT)

1. …are so small that they are assumed to have zero volume

2.…are in constant, straight-line motion

3.…experience elastic collisions in which no energy is lost

4.…have no attractive or repulsive forces toward each other

5.…have an average kinetic energy (KE) that is proportional to the absolute temp. of gas (i.e., Kelvin temp.)

AS TEMP. , KE

explains why gases behave as they do

deals w/“ideal” gas particles…

Elastic vs. Inelastic Collisions

8

3

8

Elastic vs. Inelastic Collisions

8v1

elastic collision

inelastic collision

POW v2

v3 v4

8

Elastic Collision

8v1

before

v2

after

Model Gas Behavior

• All collisions must be elastic • Take one step per beat of the

metronome • Container

– Class stands outside tape box

• Higher temperature – Faster beats of metronome

• Decreased volume– Divide box in half

• More Moles – More students are inside box

Mark area of container with tape on ground.

Add only a few molecules of inert gas

Increase temperature Decrease volume Add more gas Effect of diffusion Effect of effusion

(opening size)

Kinetic Molecular Theory

• Particles in an ideal gas…– have no volume.– have elastic collisions. – are in constant, random, straight-line motion.– don’t attract or repel each other.– have an avg. KE directly related to Kelvin temperature.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Molecular Velocities

speed

Fra

ctio

ns o

f pa

rtic

les

the Maxwell speed distribution

http://antoine.frostburg.edu/chem/senese/101/gases/slides/sld016.htm

molecules sorted by speed

many different molecular speeds

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

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Characteristics of GasesGases expand to fill any container.

– random motion, no attraction

Gases are fluids (like liquids).– no attraction

Gases have very low densities.– no volume = lots of empty space

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Characteristics of Gases• Gases can be compressed.

– no volume = lots of empty space

• Gases undergo diffusion & effusion.– random motion

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Properties of Gases

V = volume of the gas (liters, L)

T = temperature (Kelvin, K)

P = pressure (atmospheres, atm)

n = amount (moles, mol)

Gas properties can be modeled using math.Model depends on:

Pressure - Temperature - Volume Relationship

P T V P T V

Gay-Lussac’s P T

Charles V T

P T

V

P T

V P T V P T V

Boyle’s P 1V ___

Pressure - Temperature - Volume Relationship

P T V P T V

Gay-Lussac’s P T

Charles V T

Boyle’s P 1V ___

P n V P n V

Pressure and Balloons

A

B = pressure exerted ON balloonA = pressure exerted BY balloon

BWhen balloon is being filled:

PA > PB

When balloon is filled and tied:PA = PB

When balloon deflates:PA < PB

When the balloons are untied,will the large balloon (A) inflatethe small balloon (B); will they end up the same size or will the small balloon inflate the large balloon?

Why?

Balloon Riddle

A

B

C

Kinetic Theory and the Gas Laws

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 323 (newer book)

original temperatureoriginal pressureoriginal volume

increased temperatureincreased pressureoriginal volume

increased temperatureoriginal pressureincreased volume

(a) (b) (c)

10 10 10

10