Gases

Introduction

  • At ordinary temperature & pressure: He, Ne, Ar, Kr, and Xe are monatomic; H2 , N2 , O2 , F2 , Cl2 are diatomic.
  • Gases tend to be nonmetallic, low molecular weight substances
  • Expand spontaneously and are highly compressible (unlike solids & liquids)
  • Form homogeneous mixtures regardless of identities
  • This behavior is due to the very small volume take up by gas molecules
      * They behave independently of one another.

Pressure

  • Pressure: the amount of force applied to an area:
      * P = F/A
  • Atmospheric pressure is the weight of air per unit of area.
  • Manometer: used to measure the difference in pressure between atmospheric pressure and that of a gas in a vessel.

The Gas Laws

  • Boyle’s Law: the volume of a fixed quantity of gas, at constant temperature, is inversely proportional to the pressure.
  • Charles’ Law: the volume of a fixed amount of gas maintained at a constant pressure is directly proportional to its absolute temperature
  • Avogadro’s Law: the volume of a gas maintained at constant temperature and pressure is directly proportional to the number of moles of gas.

Ideal Gas Law

  • PV = nRT
  • Assumptions necessary to arrive at this form of the gas law:

  
  1. There are no interactions between molecules
  2. Molecules take up no space in the container (actually take up about 0.1% by volume)
  3. Corrections for non-ideal gas made with the Van der Waals equation

Dalton’s Law of Partial Pressures

  • The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone.
      * Ptotal = P1 + P2 + P3 + …
  • When one collects a gas over water, there is water vapor mixed in with the gas.
      * Ptotal = Pgas + PH2O

Kinetic Molecular Theory Gases

  • Gases consist of large numbers of molecules in continuous, random motion.
  • Volume of the gas molecules is negligible compared To the total volume of the “container”.
  • Attractive and repulsive forces between gas molecules is negligible.
  • Average kinetic energy (KE) is constant at constant temperature
  • Average kinetic energy is proportional to the absolute temperature (Kelvin).