# Section 3.5—Gas Behavior Objectives:

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Section 3.5—Gas Behavior 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 How does the behavior of gases affect airbags?

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.

Pressure and Number of Molecules
If the number of gas particles in the container were to increase, 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 Volume If the number of gas particles in the container were to remain the same, 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.

What is “Temperature”? All particles of matter are in motion. Particles that are at a higher temperature move faster. Temperature and speed of motion are proportional. Temperature – indication of the average kinetic energy of the particles Energy due to motion Because they have more energy, the motion of the particles increases As temperature increases, particles gain more energy.

Pressure and Temperature
As temperature increases, gas particles move faster. This means, the number of collisions between the particles and container will increase. Pressure increases 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.

As altitude increases, atmospheric pressure decreases.
As one moves away from the Earth’s surface, the number of air particle decreases. Climb in altitude Less layers of air Lower atmospheric pressure As altitude increases, atmospheric pressure decreases.

Pressure Inside Versus Pressure Outside
A container will expand or contract until the pressure inside = atmospheric pressure outside A bag of chips is bagged at sea level. What happens if the bag is then brought up to the top of a mountain. Example: 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
Rigid containers cannot expand!! An aerosol can is left in a car trunk in the summer. What happens? Example: The temperature inside the can begins to rise. As temperature increases, pressure increases. The internal pressure is higher than the external pressure. The can is rigid—it cannot expand, it explodes!

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

Kinetic Molecular Theory
What is the 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.

Assumptions of the KMT All gases are made of very small particles (atoms or molecules). 1 2 Gas particles are in constant, rapid, random motion. The temperature of a gas is proportional to the average kinetic energy of the particles. 3 Gas particles are not attracted to nor repelled from one another. 4 All gas particle collisions are perfectly elastic (no kinetic energy is lost). 5 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. 6

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. 4 Gas particles are not attracted to nor repelled from one another. Gas particles DO have attractions and repulsions towards one another! 6 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 take up space—thereby reducing the space available for other particles to be!!

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
As the mass of gas particles increase, Slower particles take more time to find the hole or to go across a room. The rate (speed) of effusion and diffusion is lower. the particles move slower. As mass of the particles increases, rate of effusion and diffusion is lowered.