Gases

I. Pressure
A. A gas exerts pressure on its surroundings. The barometer is a device that is used to measure pressure. The standard units of pressure are mm Hg, torr and atm. The SI unit of pressure is Pascals.

II. The Laws of Gases
A. Boyles law states that the pressure and the volume of a gas vary inversely. Charles law states that the temperature and the volume of a gas vary directly. Avogadro's law states that the number of particles in a gas varies directly with the volume.
B. These three laws are combined into one law called the ideal gas law. PV=nRT.

III. Gas Stoichiometry
A. 22.42 liters is the molar volume of an ideal gas. STP is at 273K or 0C and 1 atm.

IV. Dalton's Law of Partial Pressures
A. The total pressure of a mixture is the sum of the partial pressures of the gases. The moles of the gas determines the partial pressure of the gas.
B. Mole fraction is the ration of moles of a gas to the total moles of the mixture. The mole fraction is equal to the partial pressure of a gas divided by the total pressure of the mixture.
C. When a reaction is completed over water there is always a vapor pressure of the water that must be subtracted from the total pressure to find the pressure of the product.

V. The Kinetic Molecular Theory of Gases
A. The postulates of the kinetic molecular theory of gases are as follows. The particles are so small compared with the distances between them that the volume of the individual particles can be assumed to be negligible (zero). The particles are in constant motion. The collisions of the particles with the walls of the container are the cause of the pressure exerted by the gas. The particles are assumed to exert no forces on each other; they are assumed neither to attract nor to repel each other. The average kinetic energy of a collection of gas particles is assumed to be directly proportional to the Kelvin temperature of the gas.
B. The Kevin temperature is an index of the random motions of the particle of a gas, with higher temperatures meaning greater motion. (KE)avg=3/2RT. The root mean square velocity is the average velocity of a gas. Urms=square root of 3RT/M.

VI. Effusion and Diffusion
A. Diffusion is the term used to describe the mixing of gases. Effusion is the term used to describe the passage of a gas through a tiny orifice into an evacuated chamber. The rate of effusion of a gas is inversely proportional to the square root of the mass of its particles. Rate of effusion for gas 1/Rate of effusion of gas 2=square root of M1/square root of M2.

VII. Real Gases
A. Most gases do not follow the ideal gas law at normal temperatures and pressures. They only exhibit ideal behavior at low pressures and high Temperatures. Van Der Waals was a physicist that modified the ideal gas law to account for the volume of the particles themselves and the collision of the particles. The corrected equation is Pobs=(nRT/V-nb)-a(n/V)2.

VIII. Chemistry in the Atmosphere
A. The principal components of the atmosphere are N2 and O2. Due to gravitational effects pressure decrease as altitude increases. Sever air pollution is found in many large cities. Photochemical smog is pollution that is often initiated by sunlight.