Characteristics of gases

1
Gases

• Characteristics of gases
• Often the case that gases at room temperature and
  atmospheric pressure are substances that have
  low molecular masses and are formed from
  nonmetal elements
• H2, N2, O2, F2, Cl2, CH4, HCl, H2S
• The term vapor refers the gas phase of substances
  that are liquids or solids at room temperature
  and atmospheric pressure.
• Gases take on the shape and volume of their
  container.
• Gases, as opposed to liquids or solids, are
  easily compressed.
• It is relatively easy to change the volume of a
  gas by applying pressure.
• Mixtures of gaseous substances form homogeneous
  mixtures.
• Many of the properties of gases result from the
  fact that the size of the molecules making up a
  gas are very small compared to the the total
  volume of a sample of gas.
• The molecules in a bulk sample of gas are very
  far apart.
• The physical behavior of different gaseous
  substances is very similar.

2
Gases

• Properties of gases to be examined
• Pressure
• Temperature
• Volume
• Amount of substance in bulk sample of gas
• Pressure is defined as force per unit area.
• Traditionally chemists measure the pressure of
  gases using manometers
• Closed end manometers are simpler to use because
  they requir only one measurement - the
  difference in height of the two columns of mercury

3
Gases
Measuring gas pressures with a closed end
manometer
Fg
FHg, r
A
Thus, the pressure of the gas, Pg, is directly
proportional to the net height of the Hg
column, hhr-hl
FHg, l
4
Gases
Using Open End Manometers the atmospheric
pressure must be taken into account
PggtPatm PgPhlPatmPhr PgPatmPhr-Phl PgPatm(P
hr-Phl) PgPatm Ph
PatmgtPg PgPhlPatmPhr PgPatmPhr-Phl PgPatm-(P
hl-Phr) PgPatm - Ph
5
Gases
Atmospheric pressure By definition the standard
atmoshpere will support a column of mercury in
a closed tube that is 760 mm high at 0 oC.

• The current definition of the standard atmosphere
  is
• 1 atm 1.01325 x 105 Pa
• Other useful conversions are
• 1 atm 760 mm Hg 760 torr 101.325 kPa
• Example convert 610 mm Hg to atm and kPa. This
  is the typical atmospheric pressure at UCCS.

6
Gases

• The Pressure - Volume Relationship Boyles Law
• For a fixed quantity of gas at constant
  temperature if the pressure is increases the
  volume decreases.
• Experiment shows that if the pressure is doubled
  the volume is halved

Example A gas at 255 torr and 555 mL is changed
to 325 mL. What is the new P?
Note decreasing the volume means the pressure
must increase.
7
Gases

• The Temperature - Volume Relationship Charless
  Law
• For a fixed quantity of gas at constant pressure,
  it is found that increasing the temperature
  increases the volume

-273 oC
Plots of volume vs. temperature for different
masses of the same gas at constant pressure. If
the temperature scale is converted to the K
scale, the volume (V) is directly proportional
to the temperature (T)
Example If a gas has V 255 mL at 20 oC and the
temperature is changed to 40 oC, what is the new
volume?
8
Gases Combined Charless and Boyles Laws
Example A sample of He having P621 torr
occupies 375 mL at 25 oC undergoes a temperature
change so that I ts new P is 760 torr and
occupies 300 mL. What is the new temperature?
This gives rise to a 2rd law - The Pressure -
Temperature Relationship Gay-Lussacs Law
9
Gases

• The Amount of Gas - Volume Relationship
  Avogadros Law
• At constant temperature and pressure, the volume
  of a gas is directly proportional to the amount
  of gas.
• For a given gas, doubling the mass of the gas
  doubles its volume
• Avogadros law states that equal volumes of gas
  at the same temperature and pressure contain
  the same number of moles of gas.

The Ideal-Gas Equation Combine the four gas laws
into a single equation
T must be in Kelvins Most gas law problems
involve L, atm, and mol as units for variables
10
Gases

• Ideal Gas Law
• The ideal gas law is a limiting law in that it is
  valid under a set of limiting conditions.
• Most gases follow this law at temperatures that
  are high compared to the boiling temperature of
  the substance and at pressures that are low, 1
  atm or less.
• Gases will deviate from this equation when the
  temperature is near the boiling temperature of
  the substance and the pressure is high.
• Example What mass of N2 is contained in 500 mL
  at 610 torr at 25 OC?