Gas Laws

Chapter 13

Elements that exist as gases at 250C and 1 atmosphere

Observed GAS physical propertiesObserved GAS physical propertiesGases have massGases have massGases can compress (springy)Gases can compress (springy)Gases FILL containers completely Gases FILL containers completely Gases diffuse through each other (bad Gases diffuse through each other (bad

smells).smells).Gases exert pressureGases exert pressurePressure depends on temperature.Pressure depends on temperature.These properties led to the Kinetic These properties led to the Kinetic

molecular theory (KMT)molecular theory (KMT)

The KMT says that gases….….The KMT says that gases….….Consist of small particles having massConsist of small particles having massDistance between particles large (particle Distance between particles large (particle

size close to 0)size close to 0)Particles always move at random speeds Particles always move at random speeds

in random directions.in random directions.Perfectly elastic collisions (no energy lost)Perfectly elastic collisions (no energy lost)Average KE of particles proportional to T.Average KE of particles proportional to T.Forces between particles is zero.Forces between particles is zero.

Measuring GasesMeasuring Gases

n, moles of gas = n, moles of gas = Mass___ Mass___

Molar MassMolar Mass• Volume in L . (1000cmVolume in L . (1000cm3 3 = 1L)= 1L)• Temperature, K . (K = °C + 273.15)Temperature, K . (K = °C + 273.15)• Pressure , Pascals (Pa)Pressure , Pascals (Pa)

Sea level 1 atm

4 miles 0.5 atm

10 miles 0.2 atm

Atmospheric PressureAtmospheric PressureAbove every square inch of the earth’s Above every square inch of the earth’s

surface is about 15 Lbs of air. It is like surface is about 15 Lbs of air. It is like being at the bottom of a gas sea.being at the bottom of a gas sea.

Higher up, ex.Denver there is ___ Higher up, ex.Denver there is ___ pressurepressure

Think of aircraft altimeters, Think of aircraft altimeters, Ears, Cooking, weather,Sports.Ears, Cooking, weather,Sports.Barometer measures atmospheric Barometer measures atmospheric

pressure.pressure.

Units of Pressure1 pascal (Pa) = 1 N/m2

1 atm =760mmHg=760 torr

1 atm = 101,325 Pa

Barometer

Pressure = ForceArea

Open

ended

ClosedEnd

Using a manometerUsing a manometer Manometer measures pressure differences. Manometer measures pressure differences.

We are only using open-ended ones in the We are only using open-ended ones in the problems (P13)problems (P13)Pressure in connected container (mmHg) = Pressure in connected container (mmHg) =

height difference + atmospheric pressure.height difference + atmospheric pressure.

The height diff. is (+) when the open end is higher The height diff. is (+) when the open end is higher than the closed end.than the closed end.

The height diff. is (-) when the open end is ___er The height diff. is (-) when the open end is ___er than the closed end.than the closed end.

ALWAYS DRAW A PICTURE FOR MANOMETER ALWAYS DRAW A PICTURE FOR MANOMETER PROBLEMS!!!!!!!!PROBLEMS!!!!!!!!

You must be able to convert You must be able to convert between units of pressure……between units of pressure……

Use the following conversion factors:Use the following conversion factors:1atm = 101,325 Pa1atm = 101,325 Pa1atm = 760mm Hg1atm = 760mm Hg1 atm = 14.70 Lb/in1 atm = 14.70 Lb/in22

1 bar = 100,000 Pa1 bar = 100,000 PaLet’s do some problems on the Let’s do some problems on the

board…..board…..

How many mmHg is 3.5atm?How many mmHg is 3.5atm?Convert 35psi to atm.Convert 35psi to atm.How many Pa is 6.4 atm?How many Pa is 6.4 atm?Convert 5000Pa to atm.Convert 5000Pa to atm.The next ones must got to atm first and The next ones must got to atm first and

then to the desired unitthen to the desired unitHow many psi is 50.0 Pa?How many psi is 50.0 Pa?How many mmHg is 200,000Pa?How many mmHg is 200,000Pa?How many psi is 2500Pa? How many psi is 2500Pa?

Section 13.3Section 13.3What happens the volume of a gas when the What happens the volume of a gas when the

pressure increases?pressure increases?This is described by This is described by Boyles LawBoyles Law : : ““At a constant temperature, the volume of a gas At a constant temperature, the volume of a gas

is inversely proportional to the pressure”is inversely proportional to the pressure”This is expressed byThis is expressed by

PP11VV1 1 = P= P22VV22

P is pressure, V is Volume (L), 1 means initial, 2 P is pressure, V is Volume (L), 1 means initial, 2 means final means final

Some examples of Boyles LawSome examples of Boyles LawBag of chips taken from sea level to Denver. Bag of chips taken from sea level to Denver.

What happens the bag?What happens the bag?Take a helium balloon from top of Sears tower Take a helium balloon from top of Sears tower

to street level. What happens?to street level. What happens?What happens your eardrum as you go up in a What happens your eardrum as you go up in a

plane?plane?

Charles LawCharles Law ““At a constant pressure, the volume of a gas is At a constant pressure, the volume of a gas is

proportional to the temperature”proportional to the temperature”

VV11TT22 = V = V22TT11

Examples:Examples:Hot air balloonHot air balloon Internal combustion engineInternal combustion engineExplosionExplosionWhat would happen if you took a balloon and What would happen if you took a balloon and

put it in the freezer?put it in the freezer?

Pop TestPop Test

Please take out FIVE blank sheets of Please take out FIVE blank sheets of paper and your calculators.paper and your calculators.

Solve the following 20 problems, showing Solve the following 20 problems, showing all work using the G.U.E.S.S. method.all work using the G.U.E.S.S. method.

1) Find the new Volume of a hedgehog 1) Find the new Volume of a hedgehog travelling at 0.5m/s with a body travelling at 0.5m/s with a body temperature of 5K going north. The mass temperature of 5K going north. The mass of the hedgehog is 1.5kg. of the hedgehog is 1.5kg.

2) What date is it today?2) What date is it today?Got Ya !!!!!Got Ya !!!!!

Dalton’s Law of Partial PressuresDalton’s Law of Partial PressuresIf there is more than one gas present, the If there is more than one gas present, the

total Pressure = sum of pressures of each total Pressure = sum of pressures of each gasgas

PPt t = P= Pa a + P + Pbb + …….. + P + …….. + Pzz

Let’s do a simple exampleLet’s do a simple example. .

Avogadro’s LawAvogadro’s LawThe volume of a gas at STP is The volume of a gas at STP is

proportional to the number of moles proportional to the number of moles present.present.

V = k nV = k n

V , Volume in LV , Volume in Lk , k molar constant (22.4L/mol)k , k molar constant (22.4L/mol)n , molesn , moles

Ideal gas EquationIdeal gas EquationMost gases act as ideal gases except Most gases act as ideal gases except

when close to 0K and/or under extreme when close to 0K and/or under extreme pressure.pressure.

Boyles Law, Charles Law and Avogadro’s Boyles Law, Charles Law and Avogadro’s Law can be combined into an all-purpose Law can be combined into an all-purpose Law allowing any “gas problem” to be Law allowing any “gas problem” to be solved (kind of like Beano) called the Ideal solved (kind of like Beano) called the Ideal gas Lawgas Law

Ideal Gas LawIdeal Gas LawPV = nRTPV = nRT

n , moles of gasn , moles of gasV, Volume (L)V, Volume (L)P , pressure (atm)P , pressure (atm)T , temperature (K) T , temperature (K) R , Ideal gas constant (0.0821 atm-L/mol-K)R , Ideal gas constant (0.0821 atm-L/mol-K)For problems with initial and final For problems with initial and final

conditions, use…..conditions, use…..PP11VV1 1 / T/ T1 1 = P= P22VV2 2 / T/ T22

There are lots of examples:There are lots of examples:What happens the temperature and pressure of What happens the temperature and pressure of

COCO22 gas in a paintball gun when it fires? gas in a paintball gun when it fires?What happens the temperature and pressure of What happens the temperature and pressure of

wind as it blows up a slope?wind as it blows up a slope?What happens the temperature and pressure of What happens the temperature and pressure of

wind as it blows down a slope?wind as it blows down a slope?What happens the temperature and pressure of What happens the temperature and pressure of

air in a pump?air in a pump?The above example explains how a diesel The above example explains how a diesel

engine works. Can you think of any other engine works. Can you think of any other examples? examples?

Some things to know about gases – CH13.5Some things to know about gases – CH13.5Lifting power of gasesLifting power of gases::All objects in a gas have a buoyant force on All objects in a gas have a buoyant force on

them. This force is in the upward direction. them. This force is in the upward direction. It depends on the size of the object.It depends on the size of the object.

Weight is a force in the downward direction.Weight is a force in the downward direction.A balloon with a light gas in it has a smaller A balloon with a light gas in it has a smaller

weight than one with a heavier gas. Can you weight than one with a heavier gas. Can you think of some light gases?think of some light gases?

If buoyant force on the balloon > weight, If buoyant force on the balloon > weight, balloon rises. If buoyant force<weight, it balloon rises. If buoyant force<weight, it descends. descends.

A way to make a gas less dense is by heating.A way to make a gas less dense is by heating.A more expensive way is to use He gas or HA more expensive way is to use He gas or H22..What would be an advantage of each? What would be an advantage of each?

The ozone hole – causes/effectsThe ozone hole – causes/effects

Ozone (OOzone (O33) is formed by the action of sunlight ) is formed by the action of sunlight on Oon O22 in the upper stratosphere. in the upper stratosphere.

Ozone filters out most harmful U.V. rays.Ozone filters out most harmful U.V. rays.Some manmade substances – Some manmade substances –

chlorofluorocarbons (CFC’s) in particular cause chlorofluorocarbons (CFC’s) in particular cause this reaction to be reversed, destroying Ozone.this reaction to be reversed, destroying Ozone.

CFC’s are used in refrigerators, air conditioners CFC’s are used in refrigerators, air conditioners and some plastic foam products.and some plastic foam products.

An ozone hole developed over the Arctic and An ozone hole developed over the Arctic and Antarctic with reduced levels of ozone Antarctic with reduced levels of ozone worldwide. worldwide.

Since most CFC production has been banned Since most CFC production has been banned since 1980, the problem has improved since 1980, the problem has improved somewhat. somewhat.