The Gas LawsThe Gas LawsThe Gas LawsThe Gas Laws

Learning about the special Learning about the special behavior of gasesbehavior of gases

The States of MatterA) What are the 3 states of matter

that chemists work with? Solids, liquids, and gasesB) We will explain the behavior of

gases using the Kinetic Molecular Theory

The Kinetic Molecular Theory

1. What is Kinetic Energy- the energy of motion

K.E. = ½(mass)(velocity)2

• Molecules are in motion!– Solids - a bit– Liquids - a bit more– Gases - a whole bunch!

The Kinetic Molecular Theory

2. Define the Kinetic Molecular Theory as it pertains to gases: atoms and molecules of gases are in constant, random motion.

We will focus on gases only and use the kinetic theory to explain their behavior.

3 Basic Assumptions of the Kinetic Theory

a. A gas is composed of particles, usually molecules or atoms.– The atoms occupy a negligible

volume compared to their container!– No attractive or repulsive forces

between molecules.

Second Assumptionb. The particles in a gas move rapidly

in constant random motion.– The molecules travel in straight lines and

move independently of each other.– They change direction only when they

rebound from collisions with one another or with other objects.

Third Assumptionc. All collisions are perfectly elastic

– ‘perfectly elastic’ means that energy is transferred from one particle to another during collisions, but the total energy remains the same.

4. Question to Ponder:• If we opened a perfume bottle in

Washington D.C., the gas molecules should reach Mexico City in about 90 minutes…

• Why don’t they ever make it?They collide with too many other

things and never make the trip.

5. Define random walk -

• The aimless path molecules take as they diffuse from areas of higher concentration to areas of lower concentration.

• Review the definition of diffuse with your neighbor.

Gas Pressure pg. 268A. Define Gas Pressure - the result of

simultaneous collisions of billions and billions of gas particles on an object.

B. Define Vacuum - absence of particles, no pressure– empty space caused by removing gas

molecules

C. Define Atmospheric Pressure - results from the collisions of air molecules with objects

D. What is a Barometer - a device used to measure air pressure

Units of Pressure• Pressure can be measured in 3

different units: Pascal - the SI unit of pressure (kiloPascals)

• 101.3 kPa is Standard Pressure

mmHg - (millimeters of mercury) - one mm of Hg is the pressure needed to support a column of mercury 1 mm high.

• 760 mm Hg is standard pressure

atm- atmosphere - the average atmospheric pressure at sea level.

• 1 atm is standard pressure

Standard Conditions pg. 2F. Standard Pressure and Temperature

defined:• Standard Pressure:

1 ATM = 760 mmHg = 101.3 kPa

• Standard Temperature 0o Celsius or 273 Kelvin

To find Kelvin…. Kelvin = 273 + Celsius

Practice converting• Example 1: A gas is at a pressure

of 1.5 atm. Covert this pressure to– kPa

– mm Hg

Practice converting• Example 1: A gas is at a pressure

of 1.5 atm. Covert this pressure to– kPa

1.5 atm 1 – mm Hg

( )

Practice converting• Example 1: A gas is at a pressure of

1.5 atm. Covert this pressure to– kPa

1.5 atm 101.3 kPa = 151.95 kPa 1 1 atm– mm Hg

( ) ( )

Practice converting• Example 1: A gas is at a pressure of

1.5 atm. Covert this pressure to– kPa

1.5 atm 101.3 kPa = 151.95 kPa 1 1 atm– mm Hg

1.5 atm 760 mmHg = 1,140 mmHg

1 1 atm

( ) ( )

( ) ( )

Practice Converting• Example 2: What pressure, in

kilopascals and in atmospheres, does a gas exert at 385 mm Hg?

Please work with you neighbor

Practice Converting• Example 2: What pressure, in

kilopascals and in atmospheres, does a gas exert at 385 mm Hg?

385 mmHg 101.3 kPa = 51.3 kPa

1 760 mmHg

385 mmHg 1 atm = 0.51 atm

1 760 mmHg

( ) ( )( )( )

Practice Converting• Example 3: The Pressure at the

top of Mount Everest is 33.7 kPa. Is that pressure greater or less than 0.25 atm?

Practice Converting• Example 3: The Pressure at the top of

Mount Everest is 33.7 kPa. Is that pressure greater or less than 0.25 atm?

33.7 kPa 1 atm = 0.33 atm 1 101.3 kPa

A: Yes, 0.33 atm is greater than 0.25 atm

( ) ( )

Kinetic Energy and Kelvin Temperature pg. 269-271

A. Why do gas molecules contain Kinetic Energy?– They are in constant motion

B. What happens to the amount of kinetic energy a object contains as it is heated?– The Kinetic Energy increases as the Kelvin

temperature increases. (Directly related)

Important Relationship C. The average kinetic energy of the

particles of a substance is proportional to the temperature of the substance.

D. Particles of all substances at the same temperature have the same average kinetic energy.

Important Relationship E. Theoretically, there is no upper

limit to which a substance’s temperature can be raised.

• By contrast, there is a lower limit…

Absolute Zero

F. What is the term used to describe the temperature at which the motion of particles ceases ~ no more Kinetic Energy?

G. The values of Absolute Zero…

0 Kelvin or -273 Celsius

H. THE KELVIN SCALE IS USED TO DIRECTLY MEASURE THE KINETIC ENERGY OF AN OBJECT, NOT THE CELSIUS SCALE!

Ch. 12 The Behavior of Gases (Notepack pg. 3)

• How does adding molecules into a container, keeping volume and temperature constant, effect the pressure inside the container?

• More molecules results in an increased pressure BECAUSE they will strike the side of the container more often = higher pressure

In contrast…

• How does removing particles, keeping volume and temp constant, affect the pressure?

• Lowers it because fewer particles = fewer collisions against the side of the container

The effect of Changing the Size of the Container:

• What happens to the pressure inside of a container when we reduce the volume, keeping temp constant?– Increases because shorter distance

traveled before a collision against the side• What happens if we increase the volume?

– Lower pressure because greater distance traveled means fewer collisions

The Effect of Heating or Cooling a Gas

• How does raising the temperature affect pressure if volume is constant?– INCREASE = increased kinetic energy =

molecules move faster and strike the side more often.

• How does lowering the temperature affect pressure if volume is constant?– DECREASE - lowered KE = move slower

=fewer collisions

HOMEWORKPlease spend some time reading

over these notes, mentally engaging in the content.