The Gas State and Kinetic Molecular Theory

Lesson 1Unit 5 Gases and Atmospheric Chemistry

Particle Theory1. All Matter is made up of extremely tiny particles

2. Each Pure substance has its own kind of particles, different from the particles of other pure substances

3. Particles are always moving. Particles at a higher temperature are generally moving faster, on average than particles at a lower temperature.

4. Particles attract each other

There are three states of matter.Gas, Liquid, and Solid. All three of these

states are physical states of matter. There is also a forth, it is called a plasma which has charged particles that can conduct electricity and are influenced by magnetic fields. It is similar to a gas in its properties

Phase Structure Movement of Particles

Shape of Sample

Density Compressibility

Solid - Particles closely packed - Particles in fixed positions

- Vibration

-Definite shape -Volume of its own

-Usually more dense than liquid or gas

-Cannot be compressed significantly

Phase Structure Movement of Particles

Shape of Sample

Density Compressibility

Liquid

-Particles Closely packed - Particles not fixed in position

- Vibration - Rotation - Translation (slower)

- Takes shape of container - Volume of its own

- Usually less dense than solid

- Cannot be compressed significantly

Phase Structure Movement of Particles

Shape of Sample

Density Compressibility

Gas - Particles widely spaced - Particles not fixed in position

- Vibration - Rotation, - Translation (fast)

- Occupies entire volume of container

- Much less dense than solid or liquid

- Can be easily compressed

Two properties of matter that are particularly important to the study of fluids are density and compressibility.

Density – The mass per unit volume of a substance; it is a scalar quantity with SI units of kg/m3

Laws vs Theories

Laws are empirical generalizations which describes the results of several experiments.

It only describes results; it does not explain why the results have been explained.

Theories are descriptions that explain results of experiments.

The kinetic molecular theory is used to help explain the following laws: Boyle, Charles, and Dalton.

Kinetic Molecular Theory of GasesThere are 4 postulates

1. A gas consists of molecules in constant random motion

Kinetic Molecular Theory of Gases2. Gas molecules influence each other only by

collision; they exert no other forces on each other.

3. All collisions between gas molecules are perfectly elastic; all kinetic energy is conserved.

4. The volume actually occupied by the molecules of a gas is negligibly small; the vast majority of the volume of the gas is empty space through which the gas molecules are moving.

These 4 postulates correspond to a physical model of gas that we can think of like billiard balls on a billiards table for ideal gasses.

Gasses at room temperatures and pressures at or below normal atmospheric pressures tend to behave as described by the postulates and the empirical gas laws in a quantitative way.

Gas PressuresIn the KMT, pressure is the force exerted

against the wall of a container by the continual collision of molecules against it.  Newton's second law of motion tells us that the

force exerted on a wall by a single gas molecule is equal to the mass of the molecule multiplied by the velocity of the molecule. 

The molecule rebounds elastically and no kinetic energy is lost in a collision.  All the molecules in a gas do not have the same velocity. The average velocity is used to describe the

overall energy in a container of gas. 

The pressure volume product of the ideal gas equation is directly proportional to the average velocity of the gas molecules.

It has been necessary to use the average velocity of the molecules of a gas because the actual velocities are distributed over a very wide range.   This distribution, can be described by Maxwell's law of distribution of velocities.  The root mean square velocity of a molecule can be calculated using the formula below.

Where M is molecular mass divided by 1000, R is the molar gas constant (8.314 J/mol K) and Temperature in K. Molecules can be traveling easily around 2000km/h.

If gasses traveled this speed in a room we would smell things instantly, we don’t because the molecules only travel a short distance before colliding with other molecules and changing direction.

Read 416 – 422 # Practice 1- 4, section 9.1 # 1-5.