The kinetic theory of gases and the gas laws

1. The kinetic theory of gases and the gas laws

2. Kinetic theory/ideal gas We can understand the behaviour of gases using a very simple model, that of an “ideal” gas. The model makes a few simple assumptions;

3. Ideal gas assumptions • The particles of gas (atoms or molecules) obey Newton’s laws of motion. You should know these by now!

4. Ideal gas assumptions • The particles in a gas move with a range of speeds

5. Ideal gas assumptions • The volume of the individual gas particles is very small compared to the volume of the gas

6. Ideal gas assumptions • The collisions between the particles and the walls of the container and between the particles themselves are elastic (no kinetic energy lost)

7. Ideal gas assumptions • There are no forces between the particles (except when colliding). This means that the particles only have kinetic energy (no potential) Do you remember what internal energy is?

8. Ideal gas assumptions • The duration of a collision is small compared to the time between collisions.

9. Pressure – A reminder Pressure is defined as the normal (perpendiculr) force per unit area P = F/A It is measured in Pascals, Pa (N.m-2)

10. Pressure – A reminder What is origin of the pressure of a gas?

11. Pressure – A reminder Collisions of the gas particles with the side of a container give rise to a force, which averaged of billions of collisions per second macroscopically is measured as the pressure of the gas Change of momentum

12. Explaining the behaviour of gaseshttp://phet.colorado.edu/sims/ideal-gas/gas-properties.jnlp When we heat a gas at constant volume, the pressure increases. Why?

13. Explaining the behaviour of gases When we heat a gas at constant volume, the pressure increases. Why? Increased average kinetic energy of the particles means there are more collisions with the container walls in a period of time and the collisions involve a greater change in momentum.

14. Explaing the behaviour of gases When we heat a gas a constant pressure, the volume increases. Why?

15. Explaing the behaviour of gases When we heat a gas a constant pressure, the volume increases. Why? Increasing the volume reduces the chance of particles colliding with the container walls, opposing the effect of the particles increased kinetic energy.

16. Explaing the behaviour of gases When we compress (reduce the volume) a gas at constant temperature, the pressure increases. Why?

17. Explaing the behaviour of gases When we compress (reduce the volume) a gas at constant temperature, the pressure increases. Why? A smaller volume increases the likelihood of a particle colliding with the container walls.

18. Explaing the behaviour of gases In this way we are explaining the macroscopic behaviour of a gas (the quantities that can be measured like temperature, pressure and volume) by looking at its microscopic behaviour (how the individual particles move)

19. The Gas Laws There are mathematical relationships between pressure, volume and temperature of a gas (at a fixed mass). We will study these next year at higher level!