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print "Handouts" instead of "Slides" in the print setup. Also,

turn off the backgrounds (Tools>Options>Print>UNcheck

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GASESGASES

22

Importance of GasesImportance of Gases

• Airbags fill with NAirbags fill with N22 gas in an gas in an accident. accident.

• Gas is generated by the Gas is generated by the decomposition of sodium decomposition of sodium azide, NaNazide, NaN33..

• 2 NaN2 NaN33 2 Na + 3 N 2 Na + 3 N22

• Air Bag Video

33

THREE STATES OF THREE STATES OF MATTERMATTER

THREE STATES OF THREE STATES OF MATTERMATTER

44General General Properties of Properties of

GasesGases• There is a lot of “free” There is a lot of “free”

space in a gas.space in a gas.

• Gases can be expanded Gases can be expanded infinitely.infinitely.

• Gases fill containers Gases fill containers uniformly and completely.uniformly and completely.

• Gases diffuse and mix Gases diffuse and mix rapidly.rapidly.

55

Properties of Properties of GasesGases

Gas properties can be modeled Gas properties can be modeled using math. Model depends using math. Model depends on—on—

• V = volume of the gas (L)V = volume of the gas (L)• T = temperature (K)T = temperature (K)

–ALL temperatures in the ALL temperatures in the entire chapter MUST be in entire chapter MUST be in Kelvin!!! No Exceptions!Kelvin!!! No Exceptions!

• n = amount (moles)n = amount (moles)• P = pressureP = pressure

(atmospheres) (atmospheres)

66

PressurPressureePressure of air is Pressure of air is

measured with a measured with a BAROMETERBAROMETER (developed by (developed by Torricelli in 1643)Torricelli in 1643)

77PressurPressuree

Hg rises in tube until Hg rises in tube until force of Hg (down) force of Hg (down) balances the force balances the force of atmosphere of atmosphere (pushing up). (Just (pushing up). (Just like a straw in a soft like a straw in a soft drink)drink)

P of Hg pushing down P of Hg pushing down related to related to

• Hg densityHg density• column heightcolumn height

88PressurPressureeColumn height measures Column height measures Pressure of atmospherePressure of atmosphere

• 1 standard atmosphere 1 standard atmosphere (atm) *(atm) *

= 760 mm Hg (or torr) *= 760 mm Hg (or torr) *= 29.92 inches *= 29.92 inches *= 14.7 pounds/in= 14.7 pounds/in2 2 (psi) *(psi) *= about 34 feet of water= about 34 feet of water= 101.3 kPa * (SI unit is = 101.3 kPa * (SI unit is

PASCAL) PASCAL)

* Memorize these! * Memorize these! (Honors Chemistry)(Honors Chemistry)

1111Boyle’s Boyle’s LawLawP P αα 1/V 1/VThis means Pressure This means Pressure

and Volume are and Volume are INVERSELY INVERSELY PROPORTIONAL if PROPORTIONAL if moles and moles and temperature are temperature are constant (do not constant (do not change). For change). For example, P goes up example, P goes up as V goes down.as V goes down.

PP11VV11 = P = P22 V V22

Robert Boyle Robert Boyle (1627-1691). (1627-1691). Son of Earl of Son of Earl of Cork, Ireland.Cork, Ireland.

1212

Boyle’s Law and Boyle’s Law and Kinetic Molecular Kinetic Molecular

TheoryTheory

Boyle’s Law and Boyle’s Law and Kinetic Molecular Kinetic Molecular

TheoryTheory

P proportional to 1/VP proportional to 1/V

1313

Boyle’s LawBoyle’s LawBoyle’s LawBoyle’s Law

A bicycle pump is a A bicycle pump is a good example of good example of Boyle’s law. Boyle’s law.

As the volume of the As the volume of the air trapped in the air trapped in the pump is reduced, its pump is reduced, its pressure goes up, pressure goes up, and air is forced into and air is forced into the tire.the tire.

1515

Charles’s Charles’s LawLaw

If n and P are If n and P are constant, constant, then V then V αα T T

V and T are directly V and T are directly proportional.proportional.

VV11 V V22

==

TT11 T T22

• If one temperature goes If one temperature goes

up, the volume goes up!up, the volume goes up!

Jacques Charles (1746-Jacques Charles (1746-1823). Isolated boron 1823). Isolated boron and studied gases. and studied gases. Balloonist.Balloonist.

1616

Charles’s original balloonCharles’s original balloon

Modern long-distance balloonModern long-distance balloon

1818Gas Pressure, Gas Pressure, Temperature, and Temperature, and Kinetic Molecular Kinetic Molecular

TheoryTheory

Gas Pressure, Gas Pressure, Temperature, and Temperature, and Kinetic Molecular Kinetic Molecular

TheoryTheory

P proportional to TP proportional to T

1919

Gay-Lussac’s LawGay-Lussac’s Law

If n and V are If n and V are constant, constant, then P then P αα T T

P and T are directly P and T are directly proportional.proportional.

PP11 P P22

==

TT11 T T22

• If one temperature goes If one temperature goes

up, the pressure goes up!up, the pressure goes up!

Joseph Louis Gay-Joseph Louis Gay-Lussac (1778-1850)Lussac (1778-1850)

2020

Combined Gas Law

• The good news is that you don’t have to remember all three gas laws! Since they are all related to each other, we can combine them into a single equation. BE SURE YOU KNOW THIS EQUATION!

P1 V1 P2 V2

=

T1 T2

No, it’s not related to R2D2

2121

Combined Gas Law

If you should only need one of the other gas laws, you can cover up the item that is constant and you will get that gas law!

= P1 V1

T1

P2 V2

T2

Boyle’s Law

Charles’ Law

Gay-Lussac’s Law

2222

Combined Gas Law ProblemA sample of helium gas has a volume of 0.180 L, a pressure of 0.800 atm and a temperature of 29°C. What is the new temperature(°C) of the gas at a volume of 90.0 mL and a pressure of 3.20 atm?

Given:

P1 = 0.800 atm V1 = 0.180 L T1 = 302 K

P2 = 3.20 atm V2= 0.090 L T2 = ??

Find:

Equation:

Solve:

2323

CalculationP1 = 0.800 atm V1 = 180 mL T1 = 302 KP2 = 3.20 atm V2= 90 mL T2 = ??

P1 V1 P2 V2

= P1 V1 T2 = P2 V2 T1

T1 T2

T2 = P2 V2 T1

P1 V1

T2 = 3.20 atm x 90.0 mL x 302 K

0.800 atm x 180.0 mL

T2 = 604 K - 273 = 331 °C

= 604 K

2424

Learning Check A gas has a volume of 675 mL at 35°C and 0.850

atm pressure. What is the temperature in °C when the gas has a volume of 0.315 L and a pressure of 8.02 atm?

G:

F:

E:

2525

One More Practice Problem

A balloon has a volume of 785 mL on a fall day when the temperature is 21°C. In the winter, the gas cools to 0°C. What is the new volume of the balloon?

G:

F:

E:

2626And now, we pause for this commercial message from STP

OK, so it’s really not THIS kind of STP…

STP in chemistry stands for Standard Temperature and

Pressure

Standard Pressure = 1 atm (or an equivalent)

Standard Temperature = 0 oC

(273 K)

STP allows us to compare amounts of

gases between different pressures and temperatures

STP allows us to compare amounts of

gases between different pressures and temperatures

2727

Try This One

A sample of neon gas used in a neon sign has a volume of 15 L at STP. What is the volume (L) of the neon gas at 2.0 atm and –25°C?

2828

Avogadro’s Avogadro’s HypothesisHypothesis

Equal volumes of gases at the same Equal volumes of gases at the same T and P have the same number of T and P have the same number of molecules.molecules.

V and n are directly related.V and n are directly related.

twice as many twice as many moleculesmolecules

2929

Avogadro’s Avogadro’s HypothesisHypothesisAvogadro’s Avogadro’s HypothesisHypothesis

The gases in this experiment are all The gases in this experiment are all measured at the same T and P.measured at the same T and P.

Animation of Avogadro’s Hypothesis

3030

IDEAL GAS LAWIDEAL GAS LAW

Brings together gas Brings together gas properties.properties.

Can be derived from Can be derived from experiment and theory.experiment and theory.

BE SURE YOU KNOW BE SURE YOU KNOW THIS EQUATION!THIS EQUATION!

P V = n R TP V = n R T

3131

Using PV = nRTUsing PV = nRTP = PressureP = Pressure

V = VolumeV = Volume

T = TemperatureT = Temperature

n = number of molesn = number of moles

R is a constant, called the R is a constant, called the Ideal Gas ConstantIdeal Gas Constant

Instead of learning a different value for R for all the Instead of learning a different value for R for all the possible unit combinations, we can just possible unit combinations, we can just useuse oneone value and value and convert the units to match R.convert the units to match R.

R = 0.0821R = 0.0821L • atm

mol • K

3232

Practice ProblemPractice ProblemHow much NHow much N22 is required to fill a small room is required to fill a small room

with a volume of 960 cubic feet (27,000 L) with a volume of 960 cubic feet (27,000 L) to 0.980 atm at 25 to 0.980 atm at 25 ooC?C?

Given:Given:

V = 27,000 LV = 27,000 L

T = 25T = 25ooC C

P = 0.980 atmP = 0.980 atm

Make sure data is in proper units!Make sure data is in proper units!

And we always know R= 0.0821 And we always know R= 0.0821 L atmL atm mol Kmol K

T = 25 T = 25 ooC + 273 = 298 KC + 273 = 298 K

3333

Using PV = nRTUsing PV = nRTFind:Find:

How much NHow much N22 = moles = n = moles = n

Equation:Equation:

Now solve for n and then plug in those given valuesNow solve for n and then plug in those given values

PV = PV = nnRTRT

n = (0.98 atm)(2.7 x 10 4 L)

(0.0821 L • atm/K • mol)(298 K)n =

(0.98 atm)(2.7 x 10 4 L)

(0.0821 L • atm/K • mol)(298 K)

n = 1081.5 moles Nn = 1081.5 moles N22

RT RTRT RT

3434

Learning Check

Dinitrogen monoxide (N2O), laughing gas, is used by dentists as an anesthetic. If 2.86 mol of gas occupies a 20.0 L tank at 23°C, what is the pressure (in atm) in the tank in the dentist office?

3.48 atm

G:

F:

E:

3535

Learning Check

A 5.0 L cylinder contains oxygen gas at 20.0°C and 0.967. How many moles of oxygen are in the cylinder?

G:

F:

E:

0.20 mol O2

3636Deviations from Deviations from Ideal Gas LawIdeal Gas Law

• Real molecules have

volume.

The ideal gas consumes the entire amount of available volume. It does not account for the volume of the molecules themselves.

• There are intermolecular forces.

An ideal gas assumes there are no attractions between molecules. Attractions slow down the molecules and reduce the amount of collisions.

– Otherwise a gas could not condense to become a liquid.

3737

Gases in the AirThe % of gases in air Partial pressure (STP)

78.08% N2 0.781 atm

20.95% O2 0.209 atm

0.94% Ar 0.009 atm

0.03% CO2 0.0003 atm

PAIR = PN + PO + PAr + PCO = 1.00 atm 2 2 2

Total Pressure 1.00 atm

3838

Dalton’s Dalton’s LawLaw

John DaltonJohn Dalton1766-18441766-1844

3939Dalton’s Law of Partial Dalton’s Law of Partial PressuresPressures

What is the total pressure in the flask?What is the total pressure in the flask?

PPtotaltotal in gas mixture = P in gas mixture = PAA + P + PBB + ... + ...Therefore, Therefore,

PPtotaltotal = P = PHH22OO + P + POO22 = 0.48 atm = 0.48 atm

Dalton’s Law: The total pressure is Dalton’s Law: The total pressure is sum ofsum of ALL PARTIALALL PARTIAL pressures.pressures.

2 H2 H22OO2 2 (l) (l) 2 H 2 H22O (g) + OO (g) + O2 2 (g)(g)

0.32 atm 0.32 atm 0.16 0.16 atmatm

4040Collecting a gas “over water”

• Gases, since they mix with other gases readily, must be collected in an environment where mixing can not occur. The easiest way to do this is under water because water displaces the air. So when a gas is collected “over water”, that means the container is filled with water and the gas is bubbled through the water into the container. Thus, the pressure inside the container is from the gas AND the water vapor. This is where Dalton’s Law of Partial Pressures becomes useful.

4343

GAS DENSITYGAS DENSITYGAS DENSITYGAS DENSITY

HighHigh densitydensity

Low Low densitydensity

22.4 L of ANY gas AT STP = 1 mole

4444Health NoteWhen a scuba diver is several hundred feet under water, the high pressures cause N2 from

the tank air to dissolve in the blood. If the diver rises too fast, the dissolved N2 will form

bubbles in the blood, a dangerous and painful condition called "the bends". Helium, which is inert, less dense, and does not dissolve in the blood, is mixed with O2 in

scuba tanks used for deep descents.

5050

GAS DIFFUSION AND GAS DIFFUSION AND EFFUSIONEFFUSION

• diffusiondiffusion is the is the gradual mixing of gradual mixing of molecules of molecules of different gases.different gases.

• effusioneffusion is the is the movement of movement of molecules through a molecules through a small hole into an small hole into an empty container.empty container.

5252

GAS DIFFUSION AND GAS DIFFUSION AND EFFUSIONEFFUSION

Molecules effuse thru holes in a Molecules effuse thru holes in a rubber balloon, for example, at a rubber balloon, for example, at a rate (= moles/time) that israte (= moles/time) that is

• directly proportional to Timedirectly proportional to Time

• inversely proportional to M.inversely proportional to M.

Therefore, He effuses more rapidly Therefore, He effuses more rapidly than Othan O22 at same T. at same T.

HeHe

5353

Gas DiffusionGas Diffusionrelation of mass to rate of relation of mass to rate of

diffusiondiffusion

Gas DiffusionGas Diffusionrelation of mass to rate of relation of mass to rate of

diffusiondiffusion

• HCl and NH3 diffuse from opposite ends of tube.

• Gases meet to form NH4Cl

• HCl heavier than NH3

• Therefore, NH4Cl forms closer to HCl end of tube.

• HCl and NH3 diffuse from opposite ends of tube.

• Gases meet to form NH4Cl

• HCl heavier than NH3

• Therefore, NH4Cl forms closer to HCl end of tube.