Chemistry 20

Chapter 3

PowerPoint presentation by R. Schultz

robert.schultz@ei.educ.ab.ca

Preparation

• Density:

mD

V 3 solids, liquidsg g

mLcm

FP

A 2 2, 1000 1

N NPa kPa

m m

Worksheet BLM 3.0.1

Preparation

DATE: NAME: CLASS:

CHAPTER 3

What Do You Know About Gases?

BLM 3.0.5

ASSESSMENT

Read each statement and decide if it is true (T) or false (F). Write NS if you are not sure.

Before Unit After unit completion

1. The volume of a gas expands when heated.

2. At constant temperature and pressure, the volume of one mole of oxygen is greater than the volume of one mole of helium.

3. At 25 ºC, the average kinetic energy of oxygen and helium are equal.

4. When a balloon is compressed, the molecules get smaller.

5. The density of liquid water is greater than the density of gaseous water.

6. A gas molecule travels in straight lines until it collides with another gas molecule or the walls of the container.

7. Substances that mix completely with each other are homogenized.

8. The volume of a tire on a loaded truck decreases because air is pushed out.

9. If two variables, A and B, exhibit an inverse relationship, the graph of the data will appear as follows:

10. If you wish to examine how volume changes with temperature

in an experiment while holding pressure constant, temperature will be the responding variable.

11. Brick A exerts more pressure on the table than brick B.

12. If the pressure of a gas is increased from 50 kPa to 100 kPa,

and the temperature rises from 100 ºC to 200 ºC, the volume will not change.

13. Four litres of oxygen combine with 8.00 L of nitrogen to form 6.00 L of mixed gases.

A

B

A B

3.1 Gases and Kinetic Molecular Theory

• Gases take shape of their container and completely fill it. Liquids ……

• Properties:• compressible

• expand as temp at constant pressure

• low viscosity• low density• completely miscible with each other

macroscopic properties

3.1 Gases and Kinetic Molecular Theory

• Gas technologies: pneumatics

SCUBA

hot air balloons

3.1 Gases and Kinetic Molecular Theory

• Gases defined theoretically using concept of an “ideal gas”

• ideal gases don’t really exist but gas theories based on gases being ideal

an ideal gas has molecules that:

- are point masses (they have no size)

- collide with walls and each other, with perfect elastic collisions (no attraction between molecules or molecules and wall)

3.1 Gases and Kinetic Molecular Theory

• ideal gases never condense; no matter how low the temp or how high the pressure

• Which real gas is most ideal?

• Why?

He

smallest “molecules” and weakest “intermolecular” attractions (LDF: 2 eˉ)

helium boils at -269ºC 4 degrees above absolute zero

hydrogen is almost as ideal, boiling at -253ºC

3.1 Gases and Kinetic Molecular Theory

• Under room temperature/pressure conditions most real gases are very close to ideal

• When are real gases least ideal?

low temperature and high pressure

no need to memorize this – Why?

just think about conditions most likely to make a gas condense

3.1 Gases and Kinetic Molecular Theory

• Gas particles are in constant random motion

sample of ideal gas particles

random path of a single ideal gas particlefig 3.3, page 100

kE T the higher the temperature, the higher the average Ek of the particles

3.1 Gases and Kinetic Molecular Theory

• Do 3.1 Review, page 101 – questions 1, 3, 5, 7, 8, 9

3.2 Gases and Pressure

• Pressure is force per unit area F

PA

2

101325101325 101.325

1NF

P Pa kPaA m

Galileo and water pumps – recall from Science 10?

figure 3.4, page 102

=101 325 N

area = 1 m2

3.2 Gases and Pressure

3.2 Gases and Pressure• Mercury barometer - Toricelli

figure 3.6, page 103

sealed end

open end

petri dish:half-filled with Hg

glass tube completely filled with Hg

cover open end and insert into dish; remove cover

a similar device made with water would have a height ??? times as tall?

vacuum

13.6 times: 10.3 m

3.2 Gases and Pressure

• Standard atmospheric pressure = 760 mmHg• Pressure Units:

1 atm = 760 mmHg = 101 325 Pa = 101.325 kPa = 1.01325 bar

page 104 – I will give this to you on all tests and quizzes related to gases

Torr

psi"Hg

pressure demos*

according to the kinetic

molecular theory, pressure is

due to?

collisions of gas particles with

walls of container

Other pressure units: discuss

*egg, pop bottle, can

3.2 Gases and Pressure

• pressure unit conversion examples:• use conversion box from page 104

• Example 2 b page 112

is still the key

850 mmHg = ? kPa

Try 2 a, c and 3, page 112

1 atm = 760 mmHg = 101 325 Pa = 101.325 kPa = 1.01325 bar

looking forgiven

101.325

850 113760

kPammHg kPa

mmHg

3.2 Gases and Pressure

• Do worksheet BLM 3.2.7 questions 1 and 2 only

3.2 Gases and Pressure

The relationship between pressure and volume for a fixed quantity of gas at constant temperature

Pressure can be varied by adding weights to plate on top of syringe plunger

Volume can be read by reading scale on syringe

diagram page 106

3.2 Gases and Pressure• Here are some data from a similar experiment –

weights have been converted to pressures for you

• Can you find a mathematical relationship between P and V?

• Example: Try , do it for each line – does it give a near constant value?

P (kPa) V (mL)

61.7 32.4

76.6 26.1

92.8 21.6

194.2 10.3

408 4.9

2PV

117

225

399

3663

33972

2PV

No. Try something else

3.2 Gases and Pressure

• Answer:• This is Boyle’s Law

P (kPa) V (mL)

61.7 32.4

76.6 26.1

92.8 21.6

194.2 10.3

408 4.9

2.00 x 103

2.00 x 103

2.00 x 103

2.00 x 103

2.0 x 103

P V

(a constant)P V k

3.2 Gases and Pressure

• Statements of Boyle’s Law:

• Volume of a gas is inversely proportional to (varies inversely as) applied pressure

• Mathematically:

• and

• You should know Boyle’s Law in these ways but you won’t use it to calculate – it will be part of other formulas

P V k

1 1 2 2P V P V

(at constant temperature)

Fig 3.11A page 109

3.2 Gases and Pressure

• Kinetic Molecular Theory and Boyle’s Law

• Pressure due to collision of particles with walls of container

• As volume , more collisions with walls → pressure

3.3 Gases and Temperature

• Read page 113

• Discover relationship – like before

T (ºC) V (L)

10.0 32.0

30.0 34.2

50.0 36.5

70.0 38.8

90.0 41.0

Nothing simple works

3.3 Gases and Temperature

• Problem: Zero on Celsius (and Fahrenheit) temp scales is not really zero! Need to change temperature to absolute temperature in Kelvins (where 0 is 0!)

t (ºC) V (L)

10.0 32.0

30.0 34.2

50.0 36.5

70.0 38.8

90.0 41.0

T (K)

283

303

323

343

363

0.113

0.113

0.113

0.113

0.113

( )V

T in Kelvins

3.3 Gases and Temperature

• Absolute (Kelvin) temperature scale based upon 0 being absolute zero, (-273.15ºC)

• To change from Celsius to Kelvins, add 273.15

if t = temp (ºC) and T = temp (K),

273.15T t 273.15t T

Do questions 7 and 8, page 119 – add or subtract 273.15, but round to correct # of significant digits

3.3 Gases and Temperature

• Statements of Charles’ Law:

• Volume is directly proportional to (or varies directly as) absolute temperature

• *T(K)

• *T(K)1 2

1 2* *V VT T

*V

kT

As with Boyle’s Law, you will do no direct calculations with Charles’ Law – you will use as part of a more complete law

3.3 Gases and Temperature

• Finding Absolute Zero Experiment

3.3 Gases and Temperature

• Kinetic molecular theory and Charles’ Law:• As temp , particles gain kinetic energy and travel

faster – they would therefore exert greater pressure on the walls of the container

• If walls of the container are flexible or movable they will expand to make Pinside= Poutside

Theories and Laws

• Theories and Laws:

What’s the difference?

Law

A statement about observations that seems to be true.

Theory

An explanation, often at the atomic level, to explain why laws work

Example: Boyle’s Law:

P V k

Example: Kinetic Molecular Theory – explains why Boyle’s Law works

3.2 Gases and Pressure

• Kinetic Molecular Theory and Boyle’s Law

• Pressure due to collision of particles with walls of container

• As volume , more collisions with walls → pressure

3.3 Gases and Temperature