The Behavior of gases Section 14.1: Properties of Gases
Why do soccer balls explode if you over pump them?
What is meant by the term compressibility? Compressibility is a measure of how much the volume of matter decreases under pressure Example - airbags Airbags are much more compressible than the dashboard The particles in the airbag are able to move together, absorbing much of the impacts energy Under increased pressure, particles in a gas are forced together, or compressed
Modeling air at different pressures Which piston is at the higher pressure?
What variables are used to describe a gas? Pressure Directly related to the number of molecules (number of moles) More particles in container = more pressure Used to make predictions as in, maximum possible pressure If pressure in sealed container is lower than atmospheric pressure air will rush
What variables are used to describe a gas? Volume Pressure can be increased by decreasing the volume of the container Temperature Higher temperature means that particles move faster, and therefore collide with side of object with more energy
The Gas Laws Section 14.2
How does a hot air balloon work?
Boyle s law There is an inverse relationship between pressure and volume (assuming a constant temperature) Practice problems - page 429
Charles s law The volume of a gas is directly proportional to its kelvin temperature if pressure remains constant Temperature must be in Kelvins! Practice problems page 432
Guy - Lussac s law The pressure of a gas is directly proportional to the Kelvin temperature, assuming that pressure remains constant Knowing this, how could you fix this?
The combined gas law All three gas laws can be combined to determine what occurs when pressure, temperature and volume all vary
Ideal Gases Section 14.3
The amount of gas... Whilst discussing the gas laws in section 14.2, we have not really spoken amount the 4th variable, the number of moles The ideal gas law describes the physical behavior of an ideal gas - the relationship of pressure, volume, temperature and number of moles The ideal gas constant is a set value of 8.31 (L.kPa/K.mol).
What is the difference between ideal and real gases? In reality ideal gases do not exist, BUT most gases sometimes can behave like an ideal gas In real gases, particles have a volume and can be attracted to each other Question Ideal Gas Real Gas Does it exist No Yes Does it obey all gas laws? Yes Sometimes, mainly at High T and Low P How big are the particles? Are there attractive forces between particles So small they are insignificant No Small, but when compressed, size is important Yes, but due to particle speed often insignificant. Become important at lower temperatures
Gases: Mixtures and Movements Section 14.4
What is meant by the term partial pressure? In a mixture of gases, the partial pressure is the contribution each gas makes to the total pressure
Dalton s law This law states that the total pressure of a mixture of gases equals the sum of the partial pressure of the gases in the mixture Essentially the number of particles is important for calculating pressure - the type of particle is not significant This law assumes that volume and temperature are constant AND that each gas s pressure is not affected by the pressures of other gases in the mixture
Percent composition and partial pressure If you know the total pressure of a system, and the percent composition, you can calculate the partial pressure of a component gas
Diffusion vs Effusion Diffusion: The tendency for particles to move from areas of high concentration to areas of low concentration until the the concentration is uniform throughout Effusion: The process that occurs when a gas escapes through a tiny hole in a container
How fast can particles diffuse or effuse? The answer depends on the mass of the particle The lower the molar mass, the faster the rate of diffusion or effusion - Graham s Law This is based on the idea that if two objects with different masses have the same kinetic energy, the lighter object will move faster This explains why helium blimps must be airtight!