1

Chapter 4 - Gases

• Movie – phase diagrams• Kinetic Theory• Pressure Measurement

– Units– Unit Conversions

• Boyle & Charles• Combined Gas Law

2

Movie – Phase Diagrams

• Take home message:– how gases move around– distances between molecules

• Other– There is more information in the

movie – it’s all chemistry.– Much of it is “common sense” with

scientific labels.

3

Kinetic Theory

• Define Temperature• Define Source of Pressure• Relate Molecular Motion to Gas

Behavior.

4

Kinetic Theory – Define Temperature

• Temperature relates to the speed of molecular motion.

• Fast moving molecules = higher temp.– Solids vibrate– Liquids wiggle (movie)– Gases move in straight lines

• Actually, temp. relates to kinetic energy which is mass times speed.– Fat molecules move slower at same temp.– Skinny molecules move faster– (Geo Metro vs. GMC Suburban)

5

Kinetic Theory - Source of Pressure

• Pressure is the force of gas atoms bouncing off the walls of a container.

POW

BIFF

6

Kinetic Theory: Molecular Motion Gas Behavior

• Hit the wall = pressure• Gas collisions are perfect bounces.

• T = Moving faster P__• T = moving slower P__• More molecules P__• Smaller Bottle P__

7

How do we measure pressure?

• Definition: P = force area

• Wall Barometer (Fig. 4.8)– air pushes on a diaphram against a

spring– calibrate the scale

• Manometer– Air pushes on a mercury column.

8

9

10

Pressure Units

Name Source Unit Normal Day

Pascals SI (like in lab) kPa 102 kPa

Pounds-per-Square-Inch

Life psi 14.7 psi

Atmospheres Lab atm 1.00 atm

Torr Lab/Barometer

torrmmHg

760 mmHg

11

Pressure Unit Conversions• We always get to convert these!

• 1 atm 760 torr = 760 mmHg = 14.7 psi = 101.3 kPa – Forget about “bar”

psi

kPa

psi

torr

psi

mmHg

kPa

atm

psi

atm

torr

atm

mmHg

atm

7.14

3.101,

7.14

760,

7.14

760

3.101

1,

7.14

1,

760

1,

760

1

12

Vacuum

• A vacuum is a truly empty space.

• No gas molecules.

• Pressure = 0.0000000 atm.– Real experiments can only get to

0.000 000 000 000 000 01 atm or so.

13

What is vacuum?• Vacuum- literally translated, the word describes

the volume from which all the air has been totally removed.

• However, so far nobody has succeeded in achieving such a condition. Scientifically the highest vacuum yet reached is in the range of 10-14 torr. This result has been achieved during a test by IBM Thomas J. Watson Research Center, New York, USA, October 1976, in a cryogene system where the temperature dropped to minus 269 °C.

• Even at this low pressure thousands of gas molecules were detected per cm3. Also far out in space the presence of gas has been measured.

http://www.busch.com.au/whatisvacuum.htm

14

Absolute vs. Gauge

• Bike Tires? 45 psi• Car Tire?

• The psi values we use are usually relative to atmospheric pressure.

• “Gauge” pressure means 1 atm + the reading.• “Absolute” means 0.000 atm + the reading• Actual pressure in bike tire is 45 psi + 14.7 psi

= 59.7 psi!• Chemistry lab experiments use absolute

pressure.

15

Pressure - Volume

• Boyles’ Law– Change the volume of a gas sample– See what the pressure does (graph

it).

• Avogadro’s Law– Trap different volumes of gas in the

syringe.– See what the pressure does.

16

Pressure – Volume Math

• Boyle’s Law says that: PV = k, P= k/V

• If you do 2 experiments: – 1st Experiment: 1.0 atm, 5.0 mL– 2nd Experiment: ? atm, 25.0 mL

atmmL

mLatm

V

VPPforSolve

VPVPsokVPkVP

20.00.25

0.50.1

,

2

112

22112211

17

18

Pressure - Temperature

• Charles’ Law says that:

Pressure Temperature– P T– P T– Double temperature double pressure

• Careful:– You have to use Kelvin units for

temperature.

19

20

21

22

Pressure – Temperature Math

• Charles’ Law says that: P = kT, k = P/T

• If you do 2 experiments: – 1st Experiment: 1.0 atm, 0 °C– 2nd Experiment: ? atm, 100 °C

atmKelvin

Kelvinatm

T

TPPforSolve

T

P

T

Psok

T

Pk

T

P

4.115.273

15.3730.1

,

1

212

2

2

1

1

2

2

1

1

23

Combined Gas Law

• We can shove Boyle & Charles together:

2

22

1

11

2

22

1

11 ,T

VP

T

VPsok

T

VPk

T

VP

24

Combined Gas Law - Math• These problems are all about setting

them up right• At 56°C and 868 torr, a gas takes up

4.30 L. What will the pressure be if the volume is changed to 6.36 L and cooled to 12 °C?

Case 1 Case 2P _868 torr_ _???_____V _4.30 L___ _6.36 L_T _56°C=329K_ _12°C=285K_

25

Combined Gas Law - Math• Once you make the table, fill into the

big equation.

• Answer: 508.4 torr

SolveSimplify

L

LP

K

Ltorr

T

VP

T

VP

&

285

36.6

329

30.4868 2

2

22

1

11

26

Combined Gas Law - Math• Is out answer good?

– P went down.– Sample was cooled.– Volume went up.

• Sounds ok by kinetic theory…