Gases Judy Hugh

Useful Units to Remember P: Pressure - Atmospheres (atm), torr, mmHg V: Volume - Liters (L) n: Amount of gas - moles (mol) T: Temperature - Kelvin (K) R: Ideal gas constant = L-atm/mol-K = L-torr/mol-K = L-mmHg/mol-K

Properties of Gases Gases are easy to compress Gases expand to fill their containers, so volume of gas = volume of its container Gases occupy far more space than the liquids or solids from which they form The pressure of a gas is directly proportional to the amount of gas in the container Ideal conditions are High Temp & Low Pressure Real conditions are Low Temp & High Pressure

Kinetic Molecular Theory Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion These particles move in a straight line until they collide with another particle or the walls of the container These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space There is no force of attraction between gas particles or between the particles and the walls of the container

Kinetic Molecular Theory Collisions between gas particles are perfectly elastic –– None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container The average kinetic energy of a collection of gas particles depends on the temperature of the gas –– if temperature increases, then the average kinetic energy increases Kinetic Energy is defined by KE = ½ mv 2

Graham’s Law According to Graham's law, the rate at which gases effuse is dependent on their molecular weight  Therefore gases with a lower molecular weight will effuse more quickly than gases with a higher molecular weight For two gases at the same temperature (and having the same specific heat), and thus having the same kinetic energy, the average molecular speed of each gas can be found using the equation E = ½ mv 2 Thus, lighter molecules will have a higher speed  more molecules will pass through the hole per unit time

Dalton’s Law of Partial Pressures Each gas in a mixture creates pressure as if the other gases were not present The pressure each gas exerts in mixture is called its partial pressure The total pressure is the sum of the pressures created by the gases in the mixture  P total = P 1 + P 2 + P P n n is the total number of gases in the mixture