Section 3.5—Gas Behavior

How does the behavior of gases affect airbags?

Objectives:•Define pressure and atmospheric pressure•Qualitatively relate properties of gases: volume, temperature, pressure, and number of particles•Define Kinetic Molecular Theory•Distinguish between real and ideal gases

What is gas pressure?

Pressure is created by the force of gas particles running into a surface.

For example, gas particles exert pressure when they collide with the walls of their container.

there would be more particles to collide with the walls of the container.

This means there would be more collisions.

The pressure would increase.

As # of molecules increases, pressure increases.

Pressure and Number of Molecules

If the number of gas particles in the container were to increase,

Pressure and Volume

but the volume (space) in the container increased, the gas particles would have to travel farther before hitting the wall.

This means, collisions between the particles and the walls of the container would decrease.

Pressure would, therefore, decrease.

As volume increases, pressure decreases.

If the number of gas particles in the container were to remain the same,

What is “Temperature”?

Temperature – indication of the average kinetic energy of the particles

Energy due to motion

As temperature increases, particles gain more energy.

Because they have more energy, the motion of the particles increases

All particles of matter are in motion. Particles that are at a higher temperature move faster. Temperature and speed of motion are proportional.

Pressure increases

Pressure and Temperature

As temperature increases, gas particles move faster.

This means, the number of collisions between the particles and container will increase.

As temperature increases, pressure increases

What is Atmospheric Pressure?

Atmospheric Pressure – Pressure due to the layers of air in the atmosphere.

Atmospheric (or air) pressure is measured with an instrument called a barometer.

Atmospheric Pressure

As one moves away from the Earth’s surface, the number of air particle decreases.

Less layers of air

Lower atmospheric pressure

Climb in altitude

As altitude increases, atmospheric pressure decreases.

Pressure Inside Versus Pressure Outside

Example: A bag of chips is bagged at sea level. What happens if the bag is then brought up to the top of a mountain.

A container will expand or contract until the pressure inside = atmospheric pressure outside

•The atmospheric pressure at sea level is greater than the pressure on a mountain.•Therefore, at the top of the mountain, the pressure IN the bag (i.e. the same pressure of sea level) is greater than OUTSIDE the bag.•In order to reduce the internal pressure of the bag, the gas particles move farther from each other to increase the volume of the bag.•The bag will expand!

When Expansion Isn’t Possible

Example: An aerosol can is left in a car trunk in the summer. What happens?

Rigid containers cannot expand!!

The internal pressure is higher than the external pressure.

The can is rigid—it cannot expand, it explodes!

The temperature inside the can begins to rise.

As temperature increases, pressure increases.

Figure It Out

A balloon is removed from a warm store to a cold car. What happens to the balloon? WHY??

Kinetic Molecular Theory

Definition

Theory – An attempt to explain why or how behavior or properties are as they are. They are based on empirical (or experimental) evidence.

Kinetic Molecular Theory (KMT) – An attempt to explain gas behavior based upon the motion of the gas particles.

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Assumptions of the KMTAll gases are made of very small particles (atoms or molecules).

Gas particles are in constant, rapid, random motion.

The temperature of a gas is proportional to the average kinetic energy of the particles.

Gas particles are not attracted to nor repelled from one another.

All gas particle collisions are perfectly elastic (no kinetic energy is lost).

The volume of a gas particle is so small compared to the space between the particles, that the volume of the particle itself is insignificant.

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Ideal Gases

Ideal gases follow ALL the assumptions of the KMT.

Ideal gases take up no space, neither attract or repel other gas particles, move in constant, random motion, and never lose any energy in a collision.

Real Gases

No gas is truly ideal.Real Gas – several of the assumptions of the Kinetic Molecular Theory are not valid.Gas particles are not attracted to nor repelled from one another.

The volume of gas particles is so small compared to the space between the particles, that the volume of the particle itself is insignificant.

Gas particles DO have attractions and repulsions towards one another!

Gas particles do take up space—thereby reducing the space available for other particles to be!!

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Properties of Gases

Effusion

Effusion –the escape of gas from a tiny hole in a container

Effusion is why balloons deflate over time!

Diffusion

Diffusion –movement of gas particles across a space.

Diffusion is the reason we can smell perfume across a room.

Effusion, Diffusion & Particle Mass

the particles move slower.

Slower particles take more time to

find the hole or to go across a

room.

The rate (speed) of

effusion and diffusion is

lower.

As mass of the particles increases, rate of effusion and diffusion is lowered.

As the mass of gas particles increase,