Gases
From MyMCAT
Contents |
Introduction
Unlike solids and liquids, gases are an extremely loose collection of weakly attracted molecules moving very rapidly in random directions. In fact, they are so loosely organized that the molecules of the gas typically make up less than 0.1% of the total volume of their container! (Liquids are generally around 70%)
These properties mean that gas particles are free to move in almost all directions and that all gases are miscible with each other regardless of polarity differences and thus will form homogenous mixtures (unlike fluids in which an oil will never mix with an aqueous solution). Only under very low temperatures will a heavier gas tend to settle below a lighter gas.
Kinetic Molecular Theory
Scientists often simply the characteristics of gases by assuming they act as an ideal gas in which:
- the gas molecules have zero volume (thus we can neglect their size and pretend they are points)
- the gas molecules exert no forces (thus the gases neither repel or attract each other)
- the gas molecules collide following elastic collision kinetics (no energy is lost and momentum is conserved)
- the average kinetic energy of the gas molecule is directly proportional to the temperature of the gas (thus when the temperature increases so do the velocities of the particles!)
When such assumptions are made, the characteristics of the gas can be described using the following formula:
where P is the pressure in atmospheres, V is the volume in litters, n is the number of moles of gas, T is the temperature in kelvin (NOT celcius), and R is the universal gas constant (8.213 JK-1mol-1).
This formula can be used for all calculations, however three notable special cases of the ideal gas law (Charle's law, Boyle's law, and Avogadro's law) are worth discussing.
Charles' law
Charle's law states that at constant pressure, the volume of a gas is proportional to temperature. One can visualize this if they imagine all the particles bouncing and colliding off the walls of a container. In this example, if the temperature goes down, there will be a decrease in the number of collisions against the walls of the container and so the pressure would have to drop but to maintain a constant pressure we can reduce the size of the container such that the particles are more "cramped" and a proportional number of collisions occur again.
