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Chapter Six
Gas Laws– Properties of Gases– Gas Pressure– Empirical Gas Laws
• Boyle’s, Charles’ and Gay-Lussac’s
– Combined Gas Law– Avogadro’s Law– Dalton’s Law
Kinetic Theory of Gases
1. A gas is composed of very small particles.
2. The particles of a gas are very far apart.
We assume that the volume of the particles is negligible when compared to the size of the container.
Kinetic Theory of Gases
3. Gas particles move very fast in random direction and collide with each other and the walls of the container.
An O2 molecule at 25oC has an average speed of 1040 mph!
pressure is always uniform on any surface of the container.
Kinetic Theory of Gases
4. Gas particles do not attract or repel each other.
5. Gas particles move faster, on average, as the temperature increases.
Variables
Gases are measured in terms of four variables.
1. Pressure (P) – the force exerted by the gas particles
– measured in mmHg, atm, torr, and many more.
2. Volume (V) – the space occupied by the gas particles
– measured in milliliters or liters
Variables
3. Temperature (T) – proportional to the average kinetic energy (speed) of the gas molecules
- Celsius or Kelvin (must be used for calculations)
4. Quantity (n) – the amount of gas particles
- grams or moles (must be used for calculations)
Gas Pressure
Gas particles move in random directions and at various speeds.
Area
Force Pressure
Atmospheric Pressure
Atmospheric pressure arises because air particles have mass and are acted upon by the force of gravity.
Water exerts a similar, larger force as every 33 feet under water adds an additional force equal to the air.
Atmospheric Pressure
As altitude increases the pressure ___________.
Thus, on top of a mountain, we would see a ______ pressure.
And, in Death Valley, which is below sea level, we would see a ______ pressure.
Measuring Air Pressure
Air pressure is measured with a barometer.
At sea level, on an “average” day, the mercury column measures 760 mmHg.
This is defined as 1 atmosphere of pressure.
Units of Pressure
Pressure can be measured in a variety of units.
Vapor Pressure
If a liquid like water or ethanol is placed in a closed container, some of the liquid particles will evaporate and become gas particles.
As temperature increases, so does the vapor pressure.
Boiling Point
Does water always boil at 100oC?
What would we expect to see in Denver, Colorado?
What does a “Pressure Cooker” do?
Empirical Gas Laws
Allow to examine the relationship between two of the four variables.– Boyle’s Law– Charles’ Law– Gay-Lussac’s Law
Boyle’s Law
P, V vary while n, T are constant As P , V and as P , V When two variables behave this way,
we say that they are inversely related. The product of any P and V equals a
constant. P * V = constant
Boyle’s LawBoyle's Law
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Volume (mL)
Pre
ssu
re (
atm
)
Boyle’s Law
Formula:
A 25mL sample of air has a pressure of 750mmHg. If the sample is compressed to 16mL, then what is the pressure of the gas?
ffii V P V P
Charles’ Law
V, T vary while n, P are constant As V , T and as V , T When two variables behave this way,
we say they have a direct or linear relationship
Under certain conditions, the ratio of V / T equals a constant.
Charles’ Law
Charles' Law
330.00
340.00
350.00
360.00
370.00
380.00
390.00
400.00
410.00
270 280 290 300 310 320 330
Temperature (K)
Vol
um
e (m
L)
Charles’ Law
Formula:
A 55mL sample of air in a balloon at 21oC is warmed to 64oC. What is the new volume of the balloon?
f
f
i
i
T
V
T
V
Gay-Lussac’s Law
P, T vary while n, V are constant Result is a linear relationship Formula is: A sample of air has a pressure of
0.750atm at 24oC. If the pressure decreases to 0.552atm, then what is the new temperature (in Celsius)?
Combined Gas Law
All three gas laws can be combined into one overall formula
Once again, Temperatures MUST be in Kelvin
f
ff
i
ii
T
V P
T
V P
Combined Gas Law
A 752mL sample of Neon gas has a pressure of 327mmHg at 15oC. If the pressure is increased to 749mmHg and the temperature is increased to 42oC, then what is the new volume of the Neon gas?
Avogadro’s Law
Looks at the relationship between the quantity of gas and its volume
The moles of gas are proportional to the volume of that gas
i
f
i
i
n
V
n
V
STP
STP = standard temperature and pressure– Temperature = 0oC or 273K– Pressure = 760mmHg or 1atm
One mole of any gas at STP will occupy 22.4L
STP
Using STP
A reaction produces 0.160g of O2 at STP. What
volume would this occupy? The decomposition of H2O2 generated 152mL of O2 at
STP. What mass of H2O2 reacted?
2 H2O2(aq) 2 H2O(l) + O2(g)
What volume of N2 gas at STP would be generated from the decomposition of 2.50g of NaN3? What volume would it occupy at 22oC?
2 NaN3(s) 2 Na(s) + 3 N2(g)
Dalton’s Law
For any gas mixture, each gas exerts a pressure proportional to its quantity
Ptotal = PA + PB + PC + …
PA = Ptotal cA
Where
moles total
A of moles χ A
Dalton’s Law
A gas mixture contains 0.50mol Xe, 0.25mol Kr, and 0.75mol Ar at a total pressure of 3800mmHg. What are the partial pressures of each of the three gases?
