Gas Laws

Molecules In The Air

How do we know there are molecules in the air?

Has anyone ever had wind burn? How about smelled a hamburger

when you drove by McDonalds? These examples demonstrate that

air is made up of constantly moving molecules.

Kinetic Molecular Theory Kinetic energy is perfectly maintained in

“elastic” molecular collisions.

Molecules are in constant Motion.– Vibrational motion.– Translational motion.

Tiny particles called “molecules” make up all matter.

Kinetic Molecular Theory

Describes only “Ideal Gases”– Ideal gases follow the gas laws in all

conditions of pressure and temperature.

Ideal gases assume particles of gas have no volume or intermolecular attraction.

Ideal gases have perfectly elastic molecular collisions.

In REALITY!

Real gases have volume.

Real gases have some intermolecular attraction.

Real gases can be liquefied and solidified by cooling and applying pressure.

In reality, experimentation shows us that real gases do not follow “Ideal Behavior”

Gases Are All Around Us!

Many gases are invisible, but some we can see.

Iodine Vapor is Pink. Chlorine Gas is Yellow-Green. Smoke, clouds, and fog are NOT

gases!!

Gases Are All Around Us!

Some gases we can smell.

Hydrogen Sulfide smells like rotten eggs.

In natural gas, a smell has been added for our protection.

Other gases we can’t smell at all.

Gases Are All Around Us!

Gases have mass.

Balloons that are filled with air weigh more than those that are un-filled.

When air moves it can do work. For example tornadoes or windmills.

Gases Are All Around Us!

Gases occupy space, or have volume.

Examples:– Inflated Balloon– Our Lungs– Scuba Tanks

Gas Behaviors Compressibility

– The volume of a gas can be decreased by increasing the pressure.

Permeability– The mixing of molecules in a container.

Diffusion– The ability to spread from a high

concentration to a lower concentration. Expansibility

– The ability of a gas to expand and fill a container of any size.

Under Pressure

What causes pressure? Pressure is caused by the collision

of gas molecules. The molecules can collide with

each other or with the walls of their container.

Atmospheric Pressure

Air exerts pressure on earth.– Gravity holds air molecules in the

earth’s atmosphere. Atmospheric Pressure occurs when

those air molecules collide with each other or other objects.

Atmospheric pressure decreases as you move to a higher elevation.

Gas Laws

There are several different gas law equations.

Variables:– V– P– T– n– R– d

Variables

V= volume in liters. T= temp in Kelvins. P= pressure in atm,

mmHg, or kPa. n= number of moles

of gas. R= the universal gas

constant.

d= gas density in g/L.

= molar mass of a gas in g/mol.

Units of Pressure

Pressure is defined as a force over a specific area.– P=F/A

English system:– lbs/in2 = PSI

Metric system:– N/M2=Pascal

Boyle’s Law

Robert Boyle (1627-1691)

At a constant temperature, volume and pressure of a gas are inversely proportional.

Boyle’s Law

As pressure increases, volume decreases, and vice versa.

V1P1=V2P2

V œ 1/P

Remember the volume of a gas can NOT be squeezed down to zero!

Charles’s Law

Jacques Charles (1746-1823)

At constant pressure, the volume of a gas and temperature are directly proportional.

Charles’s Law

As temperature increases, volume increase, and vice versa.

V1/T1=V2/T2

V œ T Gases expand as they get warm ,

and contract as they cool.

Gay-Lussac’s Law

Joseph Gay-Lussac (1778-1850)

At constant volume, gas temperature and pressure are directly proportional.

Gay-Lussac’s Law

As temperature increases, pressure increases, and vice versa.

P1/T1=P2/T2 T œ P This is why aerosol cans say “Do

not incinerate!”

Combined Gas

To simplify life, Boyle’s Law, Charles’s Law and Gay-Lussac’s Law have been put together into one mathematical expression.

Combined Gas Law V1P1 = V2 P2

T1 T2