GasesPhysical Characteristics of Gases:

The Kinetic Theory (a model for gases): 1. Gases consist of a large number of tiny particles with

insignificant volume2. The particles are in constant, random motion.3. The collisions between particles and walls are elastic.4. There are no forces of attraction or repulsion between

molecules5. The average kinetic energy is directly proportional to

temperature in Kelvin ( __°C + 273).

Diffusion: spontaneous mixing due to random motion (molecules moving from high

concentration towards low concentration)Effusion: gas moving through a small hole

Real Gas – a gas that does not completely behave according to the kinetic theory

Due to: 1. occupy space 2. exert attractive forces on each

other

SOLID LIQUID GASHas its own shape

Takes shape of container

Fills container

Highest density

Middle density

Lowest density

Not compressible

Not compressible

Compressible

Little movement

Some movement

Rapid movement

• Pressure (P): force that a gas exerts on a given area

• 1 atm = 760 torr = 760 mmHg

• Volume (V): space occupied by gas• 1 L = 1000 mL = 1000 cm3

• Temperature (T): measure of the average kinetic energy of the gas

• MUST be in Kelvin!• K = ˚C + 273

• Number of moles (n): quantity of gas molecules

Properties (P,V,T,n)

What is Pressure?

• Changing the force or area will change the pressure (shoes!)

• Atmospheric (air) pressure is measured by a barometer:

• 1 atm = 760 torr = 760 mmHg =1.013 x 105 Pa =101.3 kPa

• STP: standard temperature and

pressure. 1 atm and 0ºC

2cm

Newtons

area

forcepressure

atmosphericpressure

vacuum

mercury

Boyle’s Law2211 VPVP

When T is constant: inverse relationship between P and V (one goes up… the other goes down)

A sample of oxygen gas occupies a volume of 20 mL at 2.0 atm. At what pressure will it occupy 55 mL?

2

112 V

VPP

mL 55

20mLatm 2.0P2

atm 0.73P2

Charles’s Law

2

2

1

1

T

V

T

V

When P is constant: direct relationship between V and T (one goes up… the other goes up)

2121 V TT V

You get a 1.7 L balloon inside at a temperature of 23˚C. At what temperature will the volume drop to 1.5 L ?

Convert initial temperature to K

T1 = ˚C + 273

= 23 + 273= 296 K

Rearrange equation

Solve for final temperature

How cold is that?!?!

2

2

1

1

T

V

T

V

2

2

1

1

V

T

V

T 2

1

21 TV

VT

2TL 1.7

L 1.5K 296

K 126T2

Gay-Lussac’s Law

When V is constant: direct relationship between P and T (one goes up… the other goes up)

Higher T: more collisions in same area.

A fire extinguisher has CO2 at 22ºC and 20 atm. What is the pressure at 30ºC?

2

2

1

1

T

P

T

P

303K27330T

295K27322T

2

1

?P

atm 02P

2

1

2

2

1

1

T

P

T

P 2

1

21 PT

TP

2PK 295

K 303 x atm 20

21atmP2

2121 PTTP

Practice!

• Pages 419-425

• #7-12,15-17 &20

Combined Gas Law

• True when moles are constant• Use to remember

– Boyle’s law:– Charles’s law:– Gay-Lussac’s law:

• A balloon has a volume of 20.0 L at 23ºC and 770 torr. What will its volume be at 685 torr and 25ºC?

2

22

1

11

T

VP

T

VP

2

22

1

11

T

VP

T

VP 2

12

121 VTP

VTP

685torrP

770torrP

2

1

298K27325T

296K27323T

2

1

?V

L 20.0V

2

1

L 32K) torr)(296(685

L) K)(20.0 torr)(298(770V2

Ideal Gas Law

PV=nRT• R is the gas constant. It will

always have the same values.• You must know which one to

use• R = 8.314 kPa L K-1 mol-1 if

Pressure is in kilopascals(kPa), Volume is in liters(L), Temperature is in Kelvin(K)R = 0.0821 L atm K-1 mol-1 if Pressure is in atmospheres(atm), Volume is in litrers(L), Temperature is in Kelvin(K)

Ideal Gas Law• What volume is needed to

store 0.050 moles of helium gas at 202.6 kPa and 400 K?

• What pressure will be exerted by 20.16 g hydrogen gas in a 7.5 L cylinder at 20ºC?

• A 50 L cylinder is filled with argon gas to a pressure of 10130.0 kPa at 30ºC. How many moles of argon gas are in the cylinder?

• To what temperature does a 250 mL cylinder containing 0.40 g helium gas need to be cooled in order for the pressure to be 253.25 kPa?

Dalton’s Law of Partial Pressure

• The total pressure of a gas sample is equal to the sum of the partial pressures of individual gases.

• Example: Earth’s atmosphere

...PPPP 321Total

Gas Pressure (torr)

N2 593.5

O2 159.2

Ar 7.1

CO2 0.23

Total 760

93.4 kPa

3.3 kPa

Gases You Know N2

The most common gas in our atmosphere (78%)

Not reactive O2

20% of the atmosphere Supports combustion

CO2 Greenhouse gas More dense than air Used in fire extinguishers

H2 Very low density Explosive if mixed with O2

TP V

TP V

Boyle’s Law

Charles’s Law

Vapor Pressure and Boiling

• Vapor Pressure – the pressure exerted by a vapor in equilibrium with its liquid state.

• Liquid molecules at the surface escape into the gas phase.

• These gas particles create pressure above the liquid in a closed container.

The condition in which two opposing processes are occurring simultaneously at equal rates is called a dynamic equilibrium. A liquid and its vapor are in equilibrium when evaporation and condensation occur at equal rates. This can only be obtained in a closed container.

• Vapor Pressure increases with increasing temperature.

20oC 80oC

• As temperature increases, the amount of vapor generated by a liquid in a closed container increases. 

• This occurs because as the liquid gains kinetic energy, the molecules can overcome the intermolecular forces of attraction that are prevalent in the liquid phase.

Evaporation vs Boiling

• Evaporation - when a liquid gas at any temperature

• Vaporization – When a liquid gas when heat is applied or at the boiling temperature

• Boiling – occurs when the vapor pressure above the liquid equals the atmospheric pressure.

Vapor Pressure Curves

Graph shows how boiling points change with change in vapor pressure.

Boiling Points change with pressure changes.• Less pressure = lower boiling point (example = water boils at lower temperatures at high elevations- water boils at 76oC on Mt. Everest).

• Higher Pressure = higher boiling point (example = pressure cooker – cooks faster because you cook at a higher temperature.)