the gas laws the density of a gas decreases as its temperature increases

The Gas LawsThe density of a gas decreases as its temperature increases.

Chemical Properties Produce Gases

• Chemists harness Chemists harness chemical properties to chemical properties to produce a desired gas produce a desired gas through chemical through chemical reactions. Such as the reactions. Such as the reaction of zinc and reaction of zinc and hydrochloric acid.hydrochloric acid.

Physical Properties of Gases

Gases are compressible and that they Gases are compressible and that they assume the shape and volume of any assume the shape and volume of any container. Gases are all infinitely container. Gases are all infinitely soluble in one another. Each of these soluble in one another. Each of these characteristics can be explained by the characteristics can be explained by the distances between the molecules (or distances between the molecules (or atoms) in a gaseous sample. atoms) in a gaseous sample.

Physical Properties of Gases are affected by temperature and pressure

States of matter simulation.

Collisions of Gas Particles

Kinetic Molecular Theory explains why gases behave as they do

deals with “ideal” gas particles…

Kinetic Theory

Kinetic Molecular TheoryPostulates of the Kinetic Molecular Theory of Gases

1. Gases consist of tiny particles (atoms or molecules)

2. These particles are so small, compared with the distances between them, that the volume (size) of the individual particles can be assumed to be negligible (zero).

3. The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas.

4. The particles are assumed not to attract or to repel each other.

5. The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas.

Kinetic Molecular TheoryPostulates Evidence

1. Gases are tiny molecules in mostly empty space.

The compressibility of gases.

2. There are no attractive forces between molecules.

Gases do not clump.

3. The molecules move in constant, rapid, random, straight-line motion.

Gases mix rapidly.

4. The molecules collide classically with container walls and one another.

Gases exert pressure that does not diminish over time.

5. The average kinetic energy of the molecules is proportional to the Kelvin temperature of the sample.

Charles’ Law

Newton’s First Law of Motion (Law of Inertia)

Object at rest tends to stay at rest, and object in motion tends to stay in motion at constant velocity unless object is acted upon by an unbalanced, external force.

mass inertia

Elastic vs. Inelastic Collisions

8

3

8

Elastic vs. Inelastic Collisions

8v1

elastic collision

inelastic collision

POW v2

v3 v4

8

Elastic Collision

8v1

before

v2

after

Model Gas Behavior

• All collisions must be All collisions must be elasticelastic • Take one step per beat of the Take one step per beat of the

metronome metronome • ContainerContainer

– Class stands outside tape boxClass stands outside tape box

• Higher temperature Higher temperature – Faster beats of metronomeFaster beats of metronome

• Decreased volumeDecreased volume– Divide box in halfDivide box in half

• More Moles More Moles – More students are inside boxMore students are inside box

Mark area of container Mark area of container with tape on ground. with tape on ground.

Add only a few molecules Add only a few molecules of of inertinert gas gas

Increase temperatureIncrease temperature Decrease volumeDecrease volume Add more gasAdd more gas Effect of diffusionEffect of diffusion Effect of effusion Effect of effusion

(opening size)(opening size)

Kinetic Molecular Theory• Particles in an ideal gas…Particles in an ideal gas…

– have no volume.have no volume.

– have elastic collisions. have elastic collisions.

– are in constant, random, straight-line motion.are in constant, random, straight-line motion.

– don’t attract or repel each other.don’t attract or repel each other.

– have an avg. KE directly related to Kelvin temperature.have an avg. KE directly related to Kelvin temperature.

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Real Gases

• Particles in a REAL gas…Particles in a REAL gas…– have their own volumehave their own volume– attract each otherattract each other

• Gas behavior is most ideal…Gas behavior is most ideal…– at low pressuresat low pressures– at high temperaturesat high temperatures– in nonpolar atoms/moleculesin nonpolar atoms/molecules


Characteristics of GasesGases expand to fill any container.Gases expand to fill any container.

– random motion, no attractionrandom motion, no attraction

Gases are fluids (like liquids).Gases are fluids (like liquids).– no attractionno attraction

Gases have very low densities.Gases have very low densities.– no volume = lots of empty spaceno volume = lots of empty space


Characteristics of Gases• Gases can be compressed.Gases can be compressed.

– no volume = lots of empty spaceno volume = lots of empty space

• Gases undergo diffusion & effusion.Gases undergo diffusion & effusion.– random motionrandom motion


Pressure

• Pressure is defined as Pressure is defined as force divided by the force divided by the area.area.

FP

A

Pressure

• The mercury in the inverted tube is The mercury in the inverted tube is pushed upward by the force of pushed upward by the force of atmospheric pressure pushing down on atmospheric pressure pushing down on the surface of the mercury in the dish. the surface of the mercury in the dish. The height of the mercury in the tube The height of the mercury in the tube changes with changing atmospheric changes with changing atmospheric pressure. Under conditions of pressure. Under conditions of standard standard atmospheric pressureatmospheric pressure, the height of the , the height of the mercury in the tube is 760 mm. (1 atm = mercury in the tube is 760 mm. (1 atm = 760 mm Hg = 760 torr = 1.01325 760 mm Hg = 760 torr = 1.01325 kPa) kPa)

Collisions cause Pressure• The pressure of a gas is The pressure of a gas is

caused by the collision of caused by the collision of molecules against the molecules against the sides of the container. sides of the container. The force of the collision The force of the collision against the container can against the container can be calculated by be calculated by Newton’s Second Law of Newton’s Second Law of Motion: F=ma. The “F” Motion: F=ma. The “F” = force, “m”=mass in kg = force, “m”=mass in kg and “a” is the and “a” is the acceleration in m/sacceleration in m/s22..

Low Pressure vs. High Pressure inside a System

The number of collisions The number of collisions of gas molecules against of gas molecules against the wall of the container the wall of the container determines the pressure determines the pressure in the container. Notice in the container. Notice the difference in the the difference in the number of collisions. number of collisions. Figure (a) would have a Figure (a) would have a lower pressure than lower pressure than Figure (b).Figure (b).

PressurePressure

Is caused by the collisions of molecules with the walls of a container

is equal to force/unit area

SI units = Newton/meter2 = 1 Pascal (Pa)

1 standard atmosphere = 101,325 Pa

1 standard atmosphere = 1 atm =

760 mm Hg = 760 torr

Pressure

KEY UNITS AT SEA LEVELKEY UNITS AT SEA LEVEL

101.325 kPa (kilopascal)101.325 kPa (kilopascal)

1 atm 1 atm

760 mm Hg760 mm Hg

760 torr 760 torr

14.7 psi14.7 psi

1 bar = 100 kPa1 bar = 100 kPaCourtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

2m

NkPa

Sea level

The first device for measuring atmosphericpressure was developed by Evangelista Torricelli during the 17th century.

The device was called a “barometer”

Baro = weight Meter = measure

Evangelista Torricelli, circa 1644

Measuring PressureMeasuring Pressure

“We live submerged at the bottom of an ocean of air.”

Barometer

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

Empty space(a vacuum)

Hg

Weight of themercury inthe column

Weight of theatmosphere(atmosphericpressure)

Barometer

• Mercury filledMercury filled760 mm = 1 atm760 mm = 1 atm

• Water filledWater filled10400 mm = 1 atm10400 mm = 1 atm

The barometer measuresair pressure

Water column(34.0 ft. highor 10.4 m)

Atmosphericpressure

Mercury column(30.0 in. highor 76 cm)

Barometers

Mount Everest

Sea level On top of Mount EverestSea level

fraction of 1 atm

average altitude

(m) (ft)

1 0 0

1/2 5,486 18,000

1/3 8,376 27,480

1/10 16,132 52,926

1/100 30,901 101,381

1/1000 48,467 159,013

1/10000 69,464 227,899

1/100000 96,282 283,076

Pressure Practice

Convert the Convert the following:following:

1.1. 145 mm Hg into 145 mm Hg into barsbars

2.2. 450 psi into kPa450 psi into kPa3.3. 900 mm Hg into 900 mm Hg into

torrstorrs4.4. 4580 Pa into kPa4580 Pa into kPa

5.5. 25 psi into atm 25 psi into atm6.6. 150 atm into Pa 150 atm into Pa

7.7. 109 kPa into atm109 kPa into atm

8.8. 76.9 mm Hg into 76.9 mm Hg into barsbars

9.9. 98.6 torr into kPa98.6 torr into kPa

10.10. 3 atm into kPa3 atm into kPa

Answers

1)1) 0.19 bars0.19 bars

2)2) 3102.4 kPa3102.4 kPa

3)3) 900 torr900 torr

4)4) 4.58 kPa4.58 kPa

5)5) 1.7 at5m1.7 at5m

6)6) 15199108.32 Pa15199108.32 Pa

7)7) 0.10 bars0.10 bars

8)8) 13.14 kPa13.14 kPa

9)9) 303.98 kPa303.98 kPa

10)10) 1.08 atm1.08 atm

Temperature

ºF

ºC

K

-459 32 212

-273 0 100

0 273 373

32FC 95 K = ºC + 273

Always use absolute temperature (Kelvin) Always use absolute temperature (Kelvin) when working with gases.when working with gases.


STP

0°C0°C

1 atm1 atm - OR -

STP


Standard Temperature & PressureStandard Temperature & Pressure

273 K

101.325 kPa760 mm Hg

the gas laws the density of a gas decreases as its temperature increases

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