Chapter 13 NotesKinetic Molecular Theory

Kinetic Theory and Gases

• Kinetic Energy—Energy that an object has due to motion.

• The Kinetic Theory states: Tiny particles form all matter, and they are constantly in motion.

Kinetic Theory and Gases

1) A gas is composed of particles that are small, hard spheres with insignificant volume and no particle interaction.

Kinetic Theory and Gases

2) Particles in a gas are in constant motion—they travel straight paths unless they collide with another particle or their container.

Kinetic Theory and Gases

3) All collisions are considered elastic—no energy is lost to friction.

Kinetic Theory and Gases

4.) No kinetic energy is lost when gas particles collide

– elastic collisions occur w/ other gas particles or with the wall of the container

– energy can be transferred in collision but the total kinetic energy of the 2 particles does not change

Kinetic Theory and Gases

5.) All gases have same average kinetic energy at the same temperature

• kinetic energy of motion (molecules are always moving)

• Temperature is a measure of the average kinetic energy of the particles in a sample of matter (at a given temp., all gases have the same avg. KE)

Temperature

↓ in temp = ↓ in K.E. (molecules slow down)

Theoretically, if you could lower the temp enough motion would cease

This temperature is called Absolute Zero

Gas Pressure

• Kinetic theory explains the existence of gas pressure.

• Gas pressure—the force exerted by a gas per unit surface area.

Gas Pressure

• The force of one molecule hitting an object is relatively small, but the result of billions of particles of air hitting a surface at once is significant.

Gas Pressure

pressure = force / unit area

To increase pressure (force/area):

1. more particles per unit area

a. decrease volume of container (↓ area)

b. add more particles

2. increase temp: ↑ speed of particles causing ↑ collisions

What happens as you increase altitude (climb a mountain)?

Gravity pulls air particles in toward earth.

The air at higher altitudes has less air above pushing down and fewer air molecules in a given space.

Atmospheric pressure decreases as you gain altitude. Pilots gauge their altitude by measuring pressure.

What happens as you increase altitude (climb a mountain)?

Atmospheric Pressure

A barometer measures atmospheric pressure.

The SI unit for pressure is the pascal (Pa). Atmospheric pressure at sea level is about 101.3 kilopascals (kPa). Other units of measurement are atmospheres (atm), mm Hg, and pounds per square inch (psi).

1 atm = 101.3 kPa = 760 mm Hg = 14.7 psi

Comparison of Pressure Units

Units of Pressure

1 atm = the average atmospheric pressure at sea level

kilopascal 1 atm = 101.3 kPa

Torricelli 1 atm = 760 torr

mm mercury 1 atm = 760 mm Hg

inches mercury 1 atm = 29.9 in Hg

pounds / in2 1 atm = 14.7 psi

Pressure conversion problems

1. Convert 190 mm Hg to atm

2. The pressure at the top of Mt Everest is 4.89 psi. How many mm of Hg is this? in. of Hg? How many atm?

What is an absence of particles called?

• A vacuum!

• No particles = no pressure

• Atmospheric pressure is the amount of pressure from the particles in the atmosphere colliding with objects.

STP

STP = Standard Temperature and Pressure

Since temperature and air pressure may vary form place to place it is necessary to have standard reference conditions for testing purposes

STP is commonly used to define standard conditions for temperature and pressure

0oC or 273K and 1 atm or 760 mm