GASES

Unit 1

Chapter 1

Motion of particles:

In solids the particles:

are moving relatively slowly.

have low kinetic energy

In liquids the particles:

molecules move faster.

have higher kinetic energy.

In gases, the particles:

move fastest,

have high kinetic energy.

Kinetic Theory Model of States

SolidParticles vibrate but

don’t “flow”. Strong

molecular

attractions keep

them in place.

LiquidParticles vibrate, rotate,

tumble and “flow”, but

cohesion (molecular

attraction) keeps them

close together.

GasParticles move freely through

container. The wide spacing

means molecular attraction is

negligible.

4

Particles can have 3 types of motion:

Vibrational kinetic energy (vibrating)

Rotational kinetic energy (tumbling)

Translational kinetic energy (flying around)

5

Kinetic Theory of s, l & g. When it is cold, molecules move slowly

In solids, they move so slowly that they are held in

place (only vibrational energy)

In liquids they move a bit faster, they can tumble

and flow, but they don’t escape from the

intermolecular attraction with other molecules

(mostly rotational energy, some vibration & translation)

In gases they move so quickly that can overcome

the intermolecular attractions and leave the

container (more translational energy, with a little bit

of rotation & vibration).

6

Plasma, the “Fourth State”

When strongly heated, or exposed to high

voltage or radiation, gas atoms may lose some

of their electrons. As they capture new

electrons, the atoms emit light—they glow. This

glowing, gas-like substance is called “plasma”

7

Properties of gases:Gases:

can be atoms or molecules

I Have No Bright Or Clever Friends

I2 H2 N2 Br2 O2 Cl2 F2 are all diatomic gases,

have mass,

no definite volume or shape,

are compressible & can expand.

Therefore properties of gasses can only be compared

under specific conditions.

Gas particles are very spaced out!

Find the properties of some common

gases: Finish for homework!

Read pages 38-43 from your textbook.

For N2, O2, CO2, radon & methane gas:

List their:

Abundance

General use (Do we breath it? Do plants use it etc)

Technological applications

For O3 how is it a useful and harmful gas?

Fun Gases (of no real importance)

Nitrous Oxide (N2O) AKA: Laughing gas, Happy gas, Nitro, NOS

Uses

anaesthetic in dentist offices, this sweet-smelling gas reduces pain sensitivity and causes euphoric sensations.

It is an excellent oxidizer, reigniting a glowing splint much like oxygen would.

It is used in racing where it is injected into the carburetor to temporarily increase an engine’s horsepower.

Sulfur Hexafluoride

One of the densest gases in common use. Fun with Sulfur hexafluoride

10

Matchthe gas with the problem it causes

Gas Problem

Carbon Dioxide Ozone layer depletion

CFCs Climate Change

Methane Toxic poisoning

Carbon monoxide Noxious smell

Sulfur dioxide Acid Rain

Last class: We talked about the motion of molecules:

Vibrational

Rotational

Translational

Gasses have mostly translational motion.

As you increase temperature their motion also

increases.

Increase in translational movement

= increase in velocity

Pressure in gases: A force applied over a unit of area.

For a gas, pressure results from gas molecules

colliding with the wall of its container.

Measured in Pascals (Pa) or kiloPascals (kPa)

Adding a gas:

Adds more gas molecules

More collisions

Increased net pressure

Ex. Double # of molecules =

double pressure

If container is not

strong enough

walls can rupture

Removed a Gas:

Removes gas molecules

Less collisions

Pressure decreasesIf container is not

strong enough

walls can collapse

Change Size of Container:

Decrease container size

Decreases space for molecules to move

Increases collisions

Increases pressure

Change Size of Container:

Increase container size

Increases space for molecules to move

Decreases collisions

Decreases pressure

Heating a Gas:

Gas molecules absorb heat

Molecules move more rapidly

Increase collisions

Increase pressure

Cooling a Gas:

Gas molecules release heat

Molecules move more slowly

Decrease collisions

Decrease pressure

Gases exert a pressure as they collide with the

walls of containers. The total pressure is

dependent on magnitude & quantity of collisions.

Concentration:

Add more gas Pressure

Remove gas Pressure

Container size:

decrease Pressure

increase Pressure

Temperature:

Increase Temp Pressure

Decrease Temp Pressure

Kinetic Molecular Theory (K.M.T):

1. A gas is composed of particles

2. Gas particles move rapidly & are in constant

random motion

3. All collisions are perfectly elastic

4. Kinetic energy is proportional to

temperature

Textbook questions

Do Page 62 # 3,4,5,6,

To be done for next class

Kinetic energy & temperature: As temperature increases molecules move faster

& have a greater KE.

Not all molecules are moving at the same speed.

The KE of moving objects is expressed by:

This shows that the KE of molecules is

dependent on both their mass & velocity.

2

2

1mvEk

“Slow”

molecules

The range of kinetic energies can be

represented as a “bell curve.”

Maxwell’s Velocity Distribution Curve.

Increasing kinetic energyAverage

kinetic energy

Incre

asin

g #

mole

cule

s

Most molecules

mo

de

me

an

“Average”

molecules

The mean & mode can

help establish “average”

molecules

“Fast”

Molecules

Distribution of Particles Around Average

Kinetic Energies.

Kinetic Energy of molecules

(proportional to velocity of molecules)

Num

be

r o

f m

ole

cu

les

Ave

rag

e k

ine

tic

en

erg

y o

f m

ole

cu

les

Ave

rag

e k

ine

tic

en

erg

y o

f w

arm

er

mo

lec

ule

s

Faster

than

average molecules

Slower

than

average moleculesAve

rag

e k

ine

tic

en

erg

y o

f c

old

er

mo

lec

ule

sConclusion:

As temperature increases.

Curve broadens.

Average KE increases.

Two different gases at the same temperature will:

Have the same AVERAGE EK

Lighter molecules will move faster.

Heavier molecules will move slower.

Fun Fact

The average speed of oxygen molecules at 20°C is 1656km/h.

At that speed an oxygen molecule could travel from Montreal to Vancouver in three hours…If it travelled in a straight line.

= ½ mv2

Observing gases As scientists observed gases, they saw

mathematical relationships that very closely, but

not perfectly, described the behaviour of many

gases.

They have developed theories & mathematical

laws that describe a hypothetical gas, called

“ideal gas.”

It is an approximation that helps us model and

predict the behavior of real gases.

Kinetic Theory for ideal gases.1. Particles of a gas are infinitely small.

Explains effusion & compressibility.

2. Particles of a gas are in constant motion, and move in straight lines.

Until they run into another particle or wall.

Explains diffusion.

3. The particles do not attract or repel each other.Explains why gases expand to fill a space.

4. The average kinetic energy of the particles are proportional to the absolute temperature.

Explains observed changes in pressure.

28

Fun fact Each air molecule has about ten billion collisions per second

Thomas Graham (1805-1869) And in my spare time

I invented dialysis,

which has saved the

lives of thousands of

kidney patients

m = mass (kg)

v = velocity (m/s)

Graham studied the speed of diffusion &

effusion.

Diffusion is when gas molecules spread throughout

a container until they are evenly distributed

Effusion is when gas molecules pass through tiny

opening in container.

He derived his law from Ek = ½ mv2

we will use Ek= ½Mv2

M = molar mass (g/mol)

Write this!

Graham’s Law

Rate of diffusion of a gas is inversely related to the square root of its molar mass.

The equation shows the ratio of Gas 1’s speed to Gas 2’s speed.

v = velocity M = molar mass

(Leave a space for variants)

1

2

M

M

2

1

v

v

1

2

2

1

M

M

v

v Same

as

Ex. 1

Determine the relative rate of diffusion for

krypton and bromine.

1.381

Ans: Kr diffuses 1.381 times faster than Br2.

Kr

Br

Br

Kr

M

M

v

v2

2

1

2

2

1

M

M

v

v

g/mol83.80

g/mol159.80

Relative rate means find the ratio “v1/v2”!

C. Johannesson

Ex. 2Oxygen gas molecules have an average speed of 12.3 m/s at a given temp and pressure. What is the average speed of hydrogen molecules under the same conditions?

2

2

2

2

H

O

O

H

M

M

v

v

g/mol 2.02

g/mol32.00

m/s 12.3

vH2

3.980m/s 12.3

vH2

m/s49.0 vH 2

1

2

2

1

M

M

v

v

Derive an equation that allows

you to solve for.

Distance, if time is kept constant

Time, if distance is kept constant

Graham’s Law Version #2, Effusion Time

It can be easier to measure the time it takes for a gas to effuse

completely, rather than the speed.

2

1

2

1

M

M

t

t

Add the equations!

It can be useful to know the distance that a gas would spread

to in a certain amount of time.

1

2

2

1

M

M

d

d

C. Johannesson

Ex. 3

An unknown gas diffuses 4.0 times faster than O2.

Find its molar mass.

XM

g/mol32.00 16

X

O

O

X

M

M

v

v2

2

XM

g/mol32.00 4.0

2

2

16

g/mol32.00 M X g/mol2.0

The ratio “v1/v2” is 4.0.

Square both

sides to get rid

of the square

root sign.1

2

2

1

M

M

v

v