KINETIC MOLECULAR THEORY AND PRESSURE13.3: pgs. 474 – 478 & 13.1: pgs. 442 - 445

Kinetic Molecular Theory

A model to explain the behavior of an ideal gas.

Composed of 5 Assumptions Gases consist of tiny particles (atoms or

molecules) These particles are so small we say their size is

zero The particles are in constant random motion.

When they hit the container, we get pressure. Particles do not attract or repel one another. Average kinetic energy of the gas particles is

directly proportional to the Kelvin temperature of the gas

Temperature

Aussumption 5- Kelvin temperature is directly proportional to average kinetic energy of gas particles Temperature is a measure of the

movement of the gas particles At high temperatures, move fast, hit the walls

more frequently How about low temps??http://www.chm.davidson.edu/ChemistryApplets/KineticMolecularTheory/Maxwell.html

Relationship between Temperature and Pressure

The pressure of a gas is measured is due to collisions with the walls of a container

What happens to pressure as we heat a gas? (volume not allowed to change) Pressure increase as temperature

increases How about when temp decreases?

Relationship between Volume and Temperature Imagine a container that can change size

The pressure of the gas (Pgas) is equal to the pressure of the exterior (Pext) surroundings.

What happens if we heat the gas to a higher temperature? http://www.youtube.com/watch?v=08ezkSrQ

5lc

Real Gases

KMT is for ideal gases- those don’t exist Ideal gas- particles have no

attractions, zero volume Why don’t these exist??

Real gases will behave like ideal gases under certain conditions Low pressures – 1 atm or

lower Moderate temperatures – 0oC

or higher

Gases have Pressure

What causes gas pressure? Pop can demo It’s a measure of the

collisions a gas has with a surface

Atmospheric pressure is measured in a barometer

Mercury Barometer

The pressure exerted by the atmospheric gasses on the surface of the mercury in the dish keep the mercury in the tube. At sea level the height of

the column of mercury averages 760 mm Hg

The level of Hg will change as the air pressure changes

Units of Pressure

Mercury barometers measure the height of the mercury column: mm Hg or torr

Standard atmosphere or atm 1 standard atmosphere = 1.000 atm = 760.0 mm Hg =

760.0 torr SI unit: pascal, Pa

1 atm = 101,325 Pa Engineering: pound per square inch, psi

1.000 atm = 14.69 psi Can use these units to perform conversions

Atmospheric Pressure

Where does it come from? Gravity pulling air down towards the earth Influenced by weather conditions: lows and

highs Varies with altitude

DALTON’S LAW OF PARTIAL PRESSURES

13.1: Pgs. 445 - 457

Mixtures of Gases

Many important gases contain a mixture of components. Examples

Air Helium and Oxygen

(Scuba divers’ tanks) Studies of gaseous

mixtures show that each component behaves independently of the others.

John Dalton

Known as Dalton’s Law of Partial Pressures For a mixture of gases in a container, the total

pressure exerted is the sum of the partial pressures of the gases present.

The partial pressure of a gas is the pressure that the gas would exert if it were alone in the container.

DOES NOT DEPEND ON THE TYPE of gas, just the NUMBER of particles present!!

Equation

For a mixture of 3 gases,

Subscripts refer to the individual gases (gas 1, gas 2, and gas 3)

Ptotal = P1 + P2 + P3

Example

A gas mixture containing oxygen, nitrogen and carbon dioxide has a total pressure of 32.9 kPa. If PO2=6.6kPa and PN2 = 23.0 kPa, what is PCO2?