GASES
Chemistry
2011
Characteristics of Gases
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Fill their containers completely
Have mass
Can be compressed
Exert pressure
Mix rapidly
Kinetic Molecular Theory
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A simple model to explain the properties of
gases
Assumptions:
1. The volume of the individual gas particles is
insignificant (zero).
2. The particles are in constant random motion.
Perfectly elastic collisions. (no loss of energy)
3. The particles exert no forces (repulsion or
attraction) on each other
4. The average kinetic energy is directly
proportional to it’s temperature in Kelvin.
Kinetic Energy
• Kinetic energy depends on
temperature KE = 3/2 RT
• Kinetic energy is also dependent on
mass and velocity of particles,
KE = mv2/2
Measuring Gases
Four Variables to describe a gas
• Pressure
• Volume
• Temperature
• Number of particles
Pressure
• The amount of force exerted by gas
molecules hitting the walls of their container
• Force per unit area
• Typical units atm, mm Hg, torr, kPa, psi
• Conversions
1 atm = 760 mm Hg = 760 torr = 101.3 kPa = 14.7
psi
Manometer and Barometers
• Manometers measure gas pressure.
• Barometers measure atmospheric
pressure.
• 1. If atmospheric pressure is 753 mm
Hg, what is the pressure of the gas in
this manometer? Express your
answer in atmospheres.
If atmospheric pressure is 0.955 atm,
what is the pressure of the gas in this
manometer? Express your answer in
atmospheres.
Homework – Copy and Complete
Express your answers in atm.
Do Now:
• Turn in your homework
• Complete pg 390 #25-28
Manometer answers
1.
2.
3.
4.
5.
6.
0.957 atm
0.901 atm
1.146 atm
0.993 atm
0.984 atm
0.0657 atm
Temperature and Kinetic
Theory
• Temperature is the measure of the
average kinetic energy of the
particles in a substance.
• KE = 1 mv2
or KE = 3 RT
2
2
Heat ≠ Temperature
• Heat is the amount of energy
transferred from a hotter object to a
cooler object.
• Heat is measured in Joules
Boyle’s Law
Boyle’s Law
• At constant temperature, pressure and
volume of a gas are inversely related.
• Mathematically,
P1V1 = P2V2
Where, P1 = Pressure at one state (initial),
V1 = Volume at one state (initial),
P2 = Pressure at another state (final),
V2 = Volume at another state (final)
Charles’ Law
Charles’ Law
• If the amount and pressure of a gas
are constant, the volume of the gas
is directly proportional to temperature
in Kelvin.
• Mathematically,
V1 = V2
T1
T2
Guy – Lussac’s Law
• If the amount and volume of gas are
constant, pressure is directly
proportional to temperature.
• Mathematically,
P1 = P2
T1
T2
P1T1 P2T2

V1
V2
Combined Gas Law
P1V1 P2 V2

T1
T2
Avogadro’s Law
• If pressure and temperature are
constant, volume is directly
proportional to the amount (moles) of
gas.
• Mathematically,
V1 = V2
n1
n2
New combined gas law
• Combines all four gas laws.
• Anything that is held constant when
changing conditions gets crossed
out.
P1V1 P2V2

n1T1 n2T2
Ideal Gas Law
• Combines all four variables of the gas,
pressure, temperature, amount, and
volume.
PV = nRT
• Where, P = pressure in kPa or atm
V = volume in Liters
n = moles of gas
R = Universal Gas constant
(0.08206 L·atm/K·mol or 8.31
L·kPa/K·mol)
T = Temperature in Kelvin
Using the ideal gas law
• When a particular gas is at one state.
No changes in any of the conditions
• When you know three of the four
variables, you can solve for the 4th.
• Example,
Calculate the volume of 3.00 mol H2 at
24oC and 100.5 kPa.
73.7 L H2
Density and ideal gas
What is the fault in the
logic?
• I noticed my tires were a bit low and
went to the gas station. As I was
filling the tires, I thought about the
kinetic molecular theory (KMT). I
realized that I was increasing both
the pressure and volume of the tires.
“Hmmm,” I thought, “that goes
against what I learned in chemistry,
where I was told pressure and
volume are inversely proportional.”
Dalton’s Law of Partial
Pressure
Dalton’s Law of Partial
Pressure
• The total pressure of a system is the
sum of the partial pressures of all the
gases that make up the system.
• Ptot = P1 + P2 + P3 + …
– Where Ptot is the total pressure
– P1 is the pressure of gas 1
– P2 is the pressure of gas 2
– P3 is the pressure of gas 3
Example
• 0.750 L of a gas is collected over
water at 23.0°C with a total pressure
of 99.75 kPa. What is the pressure of
the dry gas? The vapor pressure of
water at 23.0oC is 2.81 kPa