Chapter 5

GASSES

Gas PressureGas Pressure• Gas pressure is the force exerted by a gas per unit

surface area of an object.– Relate this to ice-skating, i.e. a 150 pound person

whose weight is distributed on the thin blades of ice skates. (Why do you “cut” into the ice when you skate?)

• Vacuum is the space created when there are no molecules to collide with one another, therefore no pressure. (Nature abhors a vacuum.)

• Atmospheric pressure is the result of molecules colliding in the atmosphere. (A barometer measures atmospheric pressure.)

Gas pressure continued

• Pressure is measured in pascals(Pa) Example: Atmospheric pressure at sea level is 101.3 kiloPascals

• Standard Atmosphere (atm) is the pressure required to support 760 mm of mercury in a mercury barometer at Zero degrees Celsius.

• It is important to be able to relate measured gas to standards. Standard temperature and pressure (STP) is a temp. of 0 degrees Celsius (273K) and a pressure of 101.3 or 760 mm Hg or 1 atm.

• The air on Earth exerts pressure because of gravity. There is Atmospheric pressureAtmospheric pressure, as well, it is the collision of air molecules with objects around them.

• As you climb up, atmospheric pressure decreases and vice versa.

• Air moves from HIGH to LOW pressure. Remember nature abhors a vacuum.

• Gas Pressure is the force exerted by a gas per unit surface area of the object.

• Without any particles there is no pressure, thus a vacuum.

Boyle’s Law

Englishman Robert Boyle

PRESSURE AND VOLUME RELATIONSHIP

howstuffworks

Boyle’s Law is a Pressure/Volume relationship

As Pressure increases the volume of a gas decreases…inverse relationship

FORMULAV1P1=V2P2

25 cm3 at 75 psi changes to 20 cm3 at what new pressure?

200 mls. @100 kpa will occupy what volume at 60 kpa (kilopascals)

Boyles Law

• Pressure1 x Volume1 = Pressure2 x Volume2

• For a given mass of gas at a constant temperature the volume of the gas varies inversely with the pressure.

• In an inverse relationship the product of the two variable quantities is constant.

Examples

• Deep sea diving:– Boyle’s Law– Nitrous Oxide forms in

the body when breathing high-pressured air

– The Bends: is when ascending out of water and nitrogen is diffusing out of the blood. Bubbles form.

• Breathing– Inhaling, the

diaphragm goes downward. Lungs are an area of low pressure allowing air to come in.

– Exhaling, diaphragm goes upward.High pressure in lungs, air goes out.

http://science.howstuffworks.com/question101.htmThe Bends Web Site =

http://science.howstuffworks.com/lung1.htm

Charles’s Law

Frenchman Jacque Charles

The Kelvin Temperature Scale• The Kelvin scale, named after Lord Kelvin, is based on the concept

that temperature is the result of particle movement.• At ABSOLUTE ZERO, 0K, there is NO movement of atoms including

the electrons around the nucleus. • The size of a Kelvin degree and a Celsius degree are the same; the

starting point is simply different.• Conversion to Celsius is simple:

degrees Celsius + 273 = degrees K• M.P. water = 273K• B.P. water = 373K• The Kelvin scale is used because there are no negative values on

the scale.

Charles’s Law is a Temperature and Volume Relationship

As temperature increases volume increases as well

Charles’s law states That the volume of a Fixed mass of gas isDirectly proportional To its Kelvin temp. if The pressure is kept Constant.

Charles Law

• Volume1 = Volume2

Temperature1 Temperature2

• The volume of a fixed mass of gas is directly proportional to its Kelvin temperature, if the pressure is kept constant.

• In a direct relationship the ratio of the two quantities that change is constant.

Examples

• Hot air balloon:– Charles’s Law– As the heat source

increases the volume of the balloon increases allowing it to become less dense and rise.

• Thermals:– Charles’s Law– Hot air pockets which have

increased volume when heated which decrease their density. Therefore the hot air can rise.

– Thermals condense over the ocean causing cool wind to form new thermals.

– Birds use them and hot air balloons don’t like them.

Hot Air Balloons

http://travel.howstuffworks.com/hot-air-balloon2.htm

Thermals and Migration

• How Migrating Animals Take Advantage of Thermals and UpdraftsBirds, butterflies and dragonflies all take advantage of rising currents to migrate. Think about the shape of the wings of a soaring eagle, crane, hummingbird, and monarch butterfly. All these animals are so light in comparison to their large wings that even gently rising air can push them up.

• Cumulous clouds are tell-tale signs of thermals

A thermal is a column of rising air, caused by uneven heating of the

earth

Thermal Bubbles

Thermals can cause hot air balloons to rise very rapidly.

Gay-Lussac’s Law

• The pressure of a gas is directly proportional to the Kelvin temperature if the volume remains constant.

P1 / T1 = P2 / T2

Frenchman Joseph Gay Lussac

Aerosol Cans follow Gay Lussac’s Law

Do NOT incinerate!

• P1 x V1 = P2 x V2

T1 T2

• Pressure times Volume divided by Temperature the new Pressure times Volume divided by Temperature

Combined Gas Law

Ideal Gas Law: PV = nRT

Ideal Gas Constant (R)= Pressure x Volume

Moles x Temperature

n = moles of gas

This law is simply a rearrangement of the Combined Law taking into account the amount (moles) of gas.

Daltons LawDaltons Law

• The total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of the component gases.

• Ptotal = PA + PB + PC….

• Problem: Find the partial pressure of Nitrogen, Oxygen, other gases in the room…use today’s barometer reading.

Grahams Law

• The rate of effusion of a gas is inversely proportional to the square route of the gas’s molar mass. This relationship is also true for the diffusion of gasses.

Graham’s Law: The diffusion rate of 2 gases is inverselyProportional to the square route of the molecular gases.

Compare gas A to gas BRate of diffusion Gas A = square root Gas B Mass

Gas B Gas A MassSmaller masses diffuse fasterTo solve: Put larger mass on top, the answer = how many times faster the smaller gas diffuses.

Example: Hydrogen diffuses 4.36 times faster than fluorine.

Cars and Gas Lawshttp://auto.howstuffworks.com/engine3.htm

Weather and Pressure• The pressure is high at the surface where air is slowly descending -

much too slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally - but not quite always - associated with good weather.

• The air that descends in high pressure areas, has to get to high altitudes in some way, and that way is by rising in areas where the pressure at the surface is low.

• As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there is enough water, or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.

Weather maps

Mt. Everest

• Physiology on Mt. Everest

• 10.67 kPa of O2 needed by humans

• Pressure on summit: 1/3 STP

Pilots and mountaineers must have supplemental oxygen when they are working in high altitudes, where oxygen is scarce.