Chapter 12

• Gas Pressure
  • Matter occurs in 3 states or phases
    • solid -- definited volume and definite shape
  • liquid -- definite volume and indefinite shape
  • gas -- indefinite volume and indefinited shape

• A gas exerts a pressure in all directions at any point within a gas
  • pressure ---> a force (push or pull) per unit area

• Measure gas pressure by using a barometer (see page 229)
  • 2 forces act on a barometer
    • 1. force of gravity pulling down
  • 2. force due to air pressure pushing up
  • When these 2 forces are in balance the mercury stops falling

• The greater the air pressure, the higher the mercury level

• The height of the mercury in the tube is a measure of pressure

• mm of Hg ---> unit of gas pressure

• 1 mm of Hg - torr ---> unit to maintain the mercury

• at sea level at zero degrees celsius on a normal day - 760 mm (height of the mercury) - 760 torr

• gas pressure may also be measured in atmospheres

• 1 atm = 760 torr = 760 mm Hg

• atmospheric pressure is dependent on the place and time at which it is taken

• another unit for measuring pressure is the pascal (Pa) and kilopascal (kPa)

• 1.0 atm = 1.013 kPa = 1.013 x 10⁵ Pa

• Boyle's Law

• Robert Boyle --> discovered that the volume of a gas at a constant temperature is inversely proportional to its pressure (one variable gets larger by the same factor as another gets smaller)
  • Example: the faster you go the shorter the time it takes you to get somewhere

• if the pressure of a gas at a given temperature is doubled, the volume is cut in half

• for a given sample of a gas we have a pressure and volume --if we change the pressure you should have a new volume

• so,       P1V1  =  P2V2

  • the units of pressure and volume must be the same

• Charles' Law
  • Discovered a law relating the volume of a gas to its absolute temperature the volume varies with the temperature

                    so,       V1   =  V2
                              T1   =  T2

• absolute temperature is the lowest possible temperature for particles to sustain movement -- Kelvin scale

• O K = -273^o C (absolute zero)

• T = Kelvin and t = celsius scale value so, T = t + 273

• Kelvin is not a degree

• Kelvin scale must be used in all gas laws (must convert celsius to Kelvin)

• Combined Gas Laws
  • shows how the volume of a sample of gas is affected by changes in both temperature and pressure

• STP = standard temperature and pressure - a temperature of 0^o C and a pressure of 1 atm (don't forget to change 0^o C to Kelvin)

                     so,       P1V1   =  P2V2
                                T1        T2

• Ideal Gas Law

• The ideal gas law works approximaely for all samples of any gase. The law works exactly only for a hypothetical "ideal" gas, from which it gets its name.

Ideal gases are those that abide by the Kinetic Theory
1. Gases are composed of tiny particles called molecules
2. Gas molecules are in constant motion.
3. The impacts of the molecules are elastic.
4. The molecules of the gas have no attraction or repulsion to one another.
5. At any given moment, the molecules have different velocities.

where,


T = temperature in Kelvin
P = pressure

V = volume

n = moles

R = gas constant = 0.0821 L^.atm/mol^.K