Date post: | 23-Dec-2015 |
Category: |
Documents |
Upload: | isabella-lawson |
View: | 217 times |
Download: | 0 times |
2
Characteristics of Gases
Gases are highly compressible and occupy the full volume of their containers.
Gases exert pressure, P = F/A (force/area).
Gases always form homogeneous mixtures with other gases.
Actual gas atoms and molecules only occupy about 0.1 % of the volume of their containers.
The Composition of the Atmosphere
Dry Atmosphere at Sea Level (% by Volume)Nitrogen 78.084Oxygen 20.948Argon 0.934Carbon dioxide 0.033 Neon 0.00182Hydrogen 0.0010Helium0.0052Methane 0.0002*Krypton 0.0001Xenon 0.000008Carbon monoxide, ozone, ammonia, nitrogen dioxide, sulfur dioxide <0.00001* * = variable depending upon pollution
Water content in the atmosphere isvariable in actualcontexts, rangingto over 5% in hotsteamy climates to <<1% in dryarid environments.
A Mercury Barometer
The pressure exerted by the column of mercury …
… must be the same as that exerted by the atmosphere.
5
PressureAtmospheric Pressure and the Barometer
. Standard atmospheric pressure is the pressure required to support 760 mm of Hg in a column.
Units: 1 atm = 760 mmHg 760 torr 1.013 105 Pa 101.3 kPa
1000 Pa = 1kPa
Boyle’s Law:Pressure-Volume RelationshipFor a fixed amount of a gas at constant
temperature, the volume of the gas varies inversely with its pressure.
For a fixed amount of a gas at constant temperature, the product of pressure and volume is a constant.
If you squeeze (increase pressure) a balloon gets smaller (volume decreases)
P V or V P
P1V1=P2V2
PinitialVinitial = PfinalVfinal
Example
A helium-filled party balloon has a volume of 4.50 L at sea level, where the atmospheric pressure is 782 Torr. Assuming that the temperature remains constant, what will be the volume of the balloon when it is taken to a mountain resort at an altitude of 2500 m, where the atmospheric pressure is .89 Atm?
Charles’s Law:Temperature-Volume RelationshipThe volume of a fixed amount of a gas at
constant pressure is directly proportional to its Kelvin (absolute) temperature.
Absolute zero is the temperature obtained by extrapolation to zero volume.
Absolute zero on the Kelvin scale = –273°C
Absolute ZeroAbsolute temperature is
measured in Kelvin Never be a negative KelvinAbsolute zero is zero (0) Kelvin or
-273 degrees CMolecular motion stopsDegree C + 273 = Degree
K
Charles’s Law:Temperature-Volume Relationship
If the temperature goes up a balloon expands
If the temperature goes down a balloon contracts
T must be in Kelvin2
2
1
1
T
V
T
V
Example
A weather balloon has a volume of 900 liters at 25 degrees C. What would the volume be if the same balloon was -45 degrees C?
Gay Lussac’s Law:Temperature-Pressure Relationship
If the temperature goes up the pressure goes up
If the temperature goes down the pressure goes down
T must be in Kelvin
2
2
1
1
T
P
T
P
Example
A coke can has a pressure of 1.1 atm at 25 degrees C what would the pressure be if you left the coke can in your car and it warmed to 125 degree C?
Standard Temperature and Pressure:STP
We define STP (Standard Temperature and Pressure)
= 0C (273 K) = 1 atm
STP is used often and represents a standard condition which gases can be compared to each other
The Combined Gas Law:Temperature-Pressure Relationship
Puts the first three gas laws together in one law
Allows for all three variables to be changed at once
T must be in Kelvin2
22
1
11
T
PV
T
PV
Example
A pump is attempting to compress propane gas. The propane has an initial volume of 2500 liters, temperature of 30 degree C, and a pressure of one atm. If the pump manages to compress the propane to 1000 liters at 2.5 atm what is the temperature of the propane gas?
Example (opener)
If a balloon contains 50 liters of gas at 25 degrees C and 1200 mmHg what would the volume of the gas be at STP?
Mixtures of Gases: Dalton’s Law of Partial Pressures
Dalton’s law of partial pressures is used in dealing with mixtures of gases.
The total pressure exerted by a mixture of gases is equal to the sum of the partial pressures exerted by the separate gases:
Ptotal = P1 + P2 + P3 + …
Partial pressure: the pressure a gas would exert if it were alone in the container.
Example
The gases in a light bulb are at STP. If a light bulb contains three gases, xenon at a pressure of 450 mmHg, argon at a pressure of 50 mmHg, and neon gas. What is the pressure of the neon gas in atm?
Water Vapor-Water vapor is a
problem-The pressure of the
water vapor changes with temperature
-Must look up vapor pressure of water on a chart (refer to handout)
Temperature (˚C)
Vapor Pressure (mmHg)
19 16.520 17.5421 18.722 19.823 21.124 22.425 23.826 25.2
ExampleSuppose that 120 ml of argon is collected over water at 22 degree C and the total pressure is 780 torr. Calculate the pressure of the dry argon gas?
The Ideal Gas Law:Adding Moles to the Gas Laws
V must be in LitersP must be in AtmT must be in Kelvin
Ideal gas equation: PV = nRTR = gas constant = 0.08206 L•atm/mol-
K
ExampleIf Emily had 1.4 moles of propane in a one liter bottle at 30 degrees C what is the pressure?
ExampleA helium balloon has a volume of 12 liters at a temperature of 20 degrees C and 749 mmHg. How many grams of helium are in the balloon?