why balloons float (and why they don’t) unit 3: phases of matter lesson 3: gases and pressure 1

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Why Balloons Float(and why they don’t)

Unit 3: Phases of MatterLesson 3: Gases and Pressure

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How does a gas behave?

Kinetic Molecular Theory (KMT)-Describes an “ideal” gas.

We imagine how it would behave.It would have five properties:1. Be made of particles with negligible volume2. Particles move in random, straight-lines3. Completely elastic collisions4. No intermolecular attractive forces5. Speed of particles is directly proportional to

Kelvin temperature

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Ideal is not Real

Real gases violate some/all of the KMTBut-

Only when the particles are moving slow and are squeezed together.

Low Temperature & High Pressure = Non-Ideal Behavior

When would this happen?

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They all contain equal numbers of molecules!!!

Amedeo Avogadro(1776 – 1856)

Avogadro’s Hypothesis

How can this be?!?

Equal numbers of gas particles occupy equal volumes of space under the same conditions of temperature and pressure.

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Standard Temperature and Pressure (STP)Because things happen differently at different

temperatures and pressures (particularly for gasses), we have to set a standard reference point.

Standard Temperature:0° C = 273 K

Standard Pressure:1.000 atm = 101.3 kPa = 760 mmHg (torr)

These are in Reference Table A.

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What is this “Pressure” of which we speak

Pressure = Force exerted over an area.

Anything with mass can exert a force.This includes the atmosphere.

Standard Pressure:1 atmosphere of pressure (at sea level)=

14.7 pounds per square inch (psi).

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Brief notes on Torr.

Torr = millimeters of mercury (mmHg)Refers to the column of mercury in a

barometer. 760 torr = Standard pressure

Why do we use

mercury?

Evangelista Torricelli(1608 – 1647)

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Pressure conversions1.000 atm = 14.7 psi = 101.3 kPa = 760.0 mmHgConvert 2.35 atm to kPa:

Convert 1.234 kPa to atm:

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Vapor Pressure

When a liquid in a sealed container is at vapor-liquid equilibrium, the vapor exerts a pressure (like any gas).

Stronger IMAF = Lower vapor pressure.

Higher vapor pressure = faster rate of evaporation.

Volatile=Substances that evaporate quickly.

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Why do things boil?

Boiling happens when the vapor pressure of a liquid is greater than the atmospheric pressure the liquid is under.

Boiling Point =Vapor pressure = atmospheric pressure.

How Can you increase vapor

pressure?

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Normal Boiling Point

The boiling point of a liquid at Standard Atmospheric Pressure.

What happens to boiling point if atmospheric pressure increases? Decreases?

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Reference Table H

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Problem: What is the vapor pressure of ___ at ___°C?

Use Method A!

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Problem: What is the boiling point of ___ at a pressure of ___kPa?

Use Method B!

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Problem: What is the normal boiling point of ___?

Use Method C!

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What now?Amedeo Avogadro

(1776 – 1856)

Equal numbers of chemistry students, occupying equal volumes of classrooms do

not possess equal numbers of questions....You?

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Things To Do Now:

Unit 3: Phases of MatterLesson 4: Partial Pressure and Effusion

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Passing Gases

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Dalton’s Law of Partial Pressures

NOT in your Reference Tables (memorize!)

Ptotal = PgasA + PgasB + PgasC + ...

John Dalton(1766 – 1844)

The total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas in the mixture.

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Practice Helps Us Learn!

1) What is the total pressure of a mixture of O2 (g), N2 (g) and NH3 (g) if the pressure of the O2 (g) is 20. kPa, N2 (g) is 60. kPa and the NH3 (g) is 15 kPa?

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2) A mixture of 1 mole of O2 and 2 moles of N2 exerts a pressure of 150. kPa. What is the partial pressure of each gas?

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3) A mixture of 30.0% He and 70.0% Ar exerts a pressure of 150 kPa at 25oC. What is the partial pressure of each gas?

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4) A sample of NH3 (g) is decomposed into its component elements. If the pressure of the nitrogen gas produced equals 40.0 kPa, what would the pressure of the hydrogen gas?

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What now?

John Dalton(1766 – 1844)

The total questions asked by a class of chemistry students equals the sum of the questions asked by each student in the class. Any Questions?

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Things To Do Now:

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Imagine a Piston...

Unit 3: Phases of MatterLesson 5: The Gas Laws

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Gases Obey Physical Laws

This should not surprise you.

The behavior of gases can be predicted and expressed according to mathematical relationships.

We will look at relationships of Pressure, Volume, Temperature and the # of molecules (aka moles) of a gas.

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A Brief Note on Units

We will use the following units:

Pressure- Atmospheres(atm) & KiloPascals(kPa)

Volume-Liters(L) and milliliters(ml)

Temperature- Kelvin(K)

# of molecules-Moles(mol)

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The Beginning: Avogadro’s Hypothesis

All of the gas laws stem from Avogadro’s Hypothesis:

Amedeo Avogadro(1776 – 1856)

Equal numbers of gas particles occupy equal volumes of space under the same conditions of temperature and pressure.

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2 Illustrative Problems to Consider

1. Consider two 4.00 L containers, each at 298 K and 1.00 atm. Container A holds nitrogen gas, Container B holds carbon dioxide gas. If container A holds 2.00 moles of nitrogen gas, how many moles of carbon dioxide must be present in container B?

2. Do equal volumes of gases under the same conditions of temperature and pressure have the same MASS? Why or why not?

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How To Solve Any Gas Law Problem

1. Get rid of the words! Read the problem and pick out the variables. Make a list of them.Make sure your units are acceptable and agree.

2. Write down the particular Gas Law you need.3. Rearrange to isolate the variable you’re

solving.4. Plug in your numbers.5. Solve for unknown

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Boyle’s Law: Pressure & Volume

Temperature must be constant

Robert Boyle(1627 - 1691)

As Pressure Increases, Volume Decreases

P1V1 = P2V2

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A sample of gas occupies a volume of 2.00 L at STP. If the pressure is increased to 2.00 atm at constant temperature, what is the new volume of the gas?

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Charles’ Law: Temperature & Volume

Jacques Charles(1746 - 1823)

As Temperature Increases, Volume Increases

V1/T1 = V2/T2

Pressure must be constant

TEMP MUST BE KELVIN

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A sample of gas occupies a volume of 5.00 L at 300. K. If the temperature is doubled under constant pressure, what will the new volume of the gas be?

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Gay-Lussac’s Law: Temperature & Pressure

Joseph-Luis Gay-Lussac(1778 – 1850)

Volume must be constantTEMP MUST BE KELVIN

As Temperature Increases, Pressure Increases

P1/T1 = P1/T2

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A 10.0 L sample of gas in a rigid container at 1.00 atm and 200. K is heated to 800. K. Assuming that the volume remains constant, what is the new pressure of the gas?

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The Combined Gas LawPuts all three gas laws together.

Any variable being held constant, can be ignored.

On Reference Table T!

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A 2.00 L sample of gas at STP is heated to 500. K and compressed to 200. kPa. What is the new volume of the gas?

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A 2.00 L sample of gas at 1.00 atm and 300. K is heated to 500.K and compressed to a volume of 1.00 L. What is the new pressure of the gas?

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A 2.00 L sample of gas at 300. K and a pressure of 80.0 kPa is placed into a 1.00 L container at a pressure of 240. kPa. What is the new temperature of the gas?

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The Ideal Gas Law

Relates the number of moles of a gas to it’s pressure, volume and temperature:

NOT on your Reference Tables (MEMORIZE!)

Comes from the observation that 1 mole of any gas occupies a volume of 22.4L at STP.

PV = nRTn = # of molesR = Gas constant

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What is the pressure exerted by 3.00 moles of gas at a temperature of 300. K in a 4.00 L container?

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What is the volume of a sample of gas if 5.00 moles if it exerts a pressure of 0.500 atm at 200. K?

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A sample of gas is contained in a cylinder with a volume of 10.0 L. At what temperature will 2.50 moles of contained gas exert 20.0 atm of pressure on the container?

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A sample of gas contained in a cylinder of 5.00 L exerts a pressure of 3.00 atm at 300. K. How many moles of gas are trapped in the cylinder?

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R!Any Questions?