Physical Characteristics of Gases Chapter 10. Section 1: The Kinetic-Molecular Theory of Matter

Physical Characteristics of Gases

Chapter 10

Section 1:

The Kinetic-Molecular

Theory of Matter

Section 1: Kinetic-Molecular Theory of Matter

Kinetic-molecular theory is based on…

All matter is made of particles that are in constant motion.

Is used to explain properties of solids, liquids, & gases.


Solids, liquids, and gases vary due to the energy of the particles and the forces that act upon them.

This chapter will study gases.


Theory pertains to ideal gases - not real gases.

An ideal gas is an imaginary gas that perfectly fits all the assumptions of the k-m theory.


5 assumptions:1.Gases consist of large

numbers of tiny particles that are far apart relative to their size

Much farther apart than liquids & solids so they can be compressed.


5 assumptions (continued):

2.Collisions are elastic - there is no net loss of kinetic energy. Kinetic energy is completely transferred during collisions.



Kinetic energy is constant (if at same temperature)



3.Gas particles are in constant, rapid, random motion. They have kinetic energy.



4. No forces of attraction act on gas particles.



5. The average kinetic energy of gas particles depends on the temperature of the gas.

KE= mv2

2


Consider: KE= mv2

2

What does KE depend on if gases are the same kind?

What does KE depend on if gases are at same temperature but are different kinds of gases?

Physical Properties of Gases

Expansion – Completely fill container & take its shape


Fluidity – Gases have the ability to flow. Particles can glide past each other.


Low density: Gas densities are about 1/1000 that of the liquid or solid phase.

Compressibility: Steel canisters contain about 100 times the number of particles than at normal pressure.


Diffusion: Gases randomly move and mix by random motion of their particles.

The rate of diffusion depends on the mass of the particles. Heavier ones move more slowly.

Animation of diffusion


Effusion: Gases under pressure spread out when released from a small opening.

Deviations of Real Gases

Real gases do not behave according to all the assumptions of the K-M theory.

Real gases deviate most when they are under very high pressures and very cold temperatures.

Reason behind this…

Both high pressure and colder temperatures force the atoms or molecules of a gas closer together.

When real gases get closer together they experience intermolecular attractions and then they condense to form liquids.

Deviations of Real Gases

Noble gases behave more like ideal gases than any others.

More polar gases behave less like ideal gases.

Section 2:

Pressure

Section 2: Pressure

To describe a gas you must state 4 quantities:1.Volume2.Temperature3.Number of molecules4.Pressure

Section 2: Pressure

Pressure is defined as the amount of force per unit of area.

P = force

area

Section 2: Pressure

P = force

area

Force unit is Newtons

Area unit is square meters

Pressure = N/m2

Section 2: Pressure

Open canClosed Can

Air Pumped

Out

Measuring Pressure

What tool do we use to measure atmospheric pressure?

Barometer!First built by

Evangelista Torricelli (1600’s)

Measuring Pressure

Torricelli noticed that pumps could raise water only 34 feet high.

He compared density of mercury to density of water (14x greater)

Predicted height that mercury could be raised (1/14 of 34 ft or about 30 inches).

Units of Pressure

Several different units are used for pressure: inches of mercury (ex. 30.4 and

rising) mm of Hg atm (atmospheres) torrs Kilopascals (1 Kpa= 1N/m2)

Units of Pressure

Standard pressure taken at sea level: 29.9 inches of mercury 760 mm of Hg 1 atm (atmospheres) 760 torrs 101.3 Kilopascals

Section 3

The Gas LawsMathematical relationships between volume, temperature, pressure and quantity of gases

Section 3: The Gas Laws

Boyle’s LawCharles’ LawDalton’s Law of Partial

PressuresGay-Lussac’s LawCombined Gas Law


Boyle’s Law Relates gas volume to pressure Has an inverse relationship

P ↑ V ↓


Formula for Boyle’s Law

P1V1 = P2V2

What would be a gas’s volume if the pressure reduced from 98 kPa down to 60 kPa if its original volume was 300 Liters?

Simulation of Boyle's Law


P1V1 = P2V2

98 (300) = 60 (V2)

V2 = 490 L


Charles’ Law

Relates temperature to gas volume.Directly proportional.Is based on absolute zero.


In 1787, Jacques Charles found that as he decreased the temperature of a gas 1 degree then the volume decreased 1/273.


The volume of a fixed mass of gas at constant pressure varies directly with the Kelvin temperature.


Formula for Charles’ Law (temperature must be in Kelvin)

V1 V2

=T1 T2

Charles’ Law Graph

V

T

Problem:

If the temperature of a gas increases from 25 degrees Celsius up to 80 degree Celsius and the original volume was 10 liters of gas, then what would the final volume be????

10 X

(25 + 273) (80 + 273)

X = 11.8 liters

Gay-Lussac’s Law

Relates pressure and temperature (assumes constant volume)

P1 P2

=T1 T2

Combined Gas Law

This law is used when 2 variables change – pressure, temperature, or volume.

P1V1 = P2V2

T1 T2

Dalton’s Law of Partial Pressure

If there is no chemical reaction occurring then the pressure of a mixture of gases will be equal to all the combined pressures of each gas.

Equation:

PT = P1 + P2 + P3 + …

Collecting Gases by Water Displacement

Water is commonly collected by bubbling it through water.

The pressure then in the container is a combination of both the pressure of the gas as well as a small amount of water vapor.

Gas Collection

Collecting Gases by Water Displacement

To determine the pressure of the gas you must subtract the pressure of the water vapor.

The pressure of the water varies according to the temperature.

Use the chart for H2O pressure:

Vapor Pressure of Water

Temp. (0C)

Vapor Pressure (torr)

Temp (0C)

Vapor Pressure (torr)

18 15.5 24 22.4

19 16.5 25 23.8

20 17.5 26 25.2

21 18.6 27 26.7

22 19.8 28 28.3

23 21.2 29 30.3

Sample Problem:

Oxygen was collected under water at 20°C. If the combined pressure was 731 torr, then what is the pressure of the gas within?

H2O pressure at 20 C = 17.5 torrTotal press: 731

Minus H2O - 17.5Gas press: 713.5 torr

Assignment

Page 329: questions 39-43

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Physical Characteristics of Gases Chapter 10. Section 1: The Kinetic-Molecular Theory of Matter