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Gases
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Gases
Properties: Gases are fluids because their molecules/atoms
can flow Gases have low density - atoms are far apart
from each other Highly compressible – their volume can be
reduced Gases will completely fill their container
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Gas Pressure
PRESSURE is a force exerted by the substance per unit area on another substance.
GAS PRESSURE is the force that the gas exerts on the walls of its container.
A balloon expands because the pressure of the gas molecules inside is greater than the pressure of the gas molecules on the outside.
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html
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Gas Pressure
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html
Gas (air) on the outside of the balloon is exerting pressure onto surface of the balloon.
The atmospheric pressure outside a balloon, PA, is the impact of moving gas molecules as they collide with the skin of the balloon.
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Atmospheric Pressure
If you could measure the weight of a column of air above the surface of the Earth, it would be14.70 lbs per square inch!
This is also known as 1 atmosphere or 1 atm
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Atmospheric Pressure
As you descend toward earth, the atmosphere is denser and the pressure is higher
When flying your ears may “Pop” due to the change in pressure
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Pressure Conversions
There are many units to express pressure:
- pounds per square inch (tire pressure)
- atmospheres
- torr
- pascals
- mm of Hg
- bars
We need to be able to convert between units.
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Converting between units
Use dimensional analysis!
Ex: Convert the pressure of 1.000 atm to mm of mercury
Conversion factor: 101325 Pa and 1 mm Hg 1 atm 133.322 Pa
Calculation:1.000 atm x 101325 Pa x 1 mm Hg = 760 mm Hg 1 atm 133.322 Pa
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Gases – Converting Between UnitsUnit Abbreviation Conversion Factor
Atmosphere atm 1 atm = 101.325 kPa
Millimeters of Hg mmHg 760 mm Hg = 1 atm
Torr torr 760 torr = 1 atm
Pascal Pa 101.325 kPa = 101325 Pa
Millibar bar 1013.2 millibar = 1 atm
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Pressure Conversions
Practice The pressure of carbon dioxide is 72.7 atm.
What is this value in units of kilopascals?
The pressure of water vapor at 50 deg C is 12.33kPa. What is this value in mm of Hg?
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Variables in Gas Laws
Whenever we discuss gas laws, we are interested in 4 variables: Number of moles of gas Volume of gas Pressure of gas Temperature of gas
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First Gas Law: Boyle’s Law
Pressure-Volume RelationshipsBoyle’s Law
Based on the following facts:
1. Gases can be compressed
2. Gases exert pressure
Boyle found that:
As volume decreases, the concentration, and therefore the pressure, increases
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Boyle’sAs volume decreases, the concentration, and
therefore the pressure, increases
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Gases
Boyle’s Law States: For a fixed amount of gas at a constant
temperature: as the volume of the gas decreases the pressure increases
We can use the following equation to calculate changes in pressure or volume
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Boyle’s Law
A graph of Boyle’s Law shows the relationship between pressure and volume is inversely proportional: as one variable increases, the other decreases
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Boyle’s Law
http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html
Animation of Boyle’s Law
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Boyle’s Law
Example:
A given sample of gas occupies 523 mL at 760 torr. The pressure is increased to 1.97 atm, while the temperature remains the same. What is the new volume of gas?
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Second Gas Law: Charles’ Law
Temperature-Volume Relationships Charles’ Law
For a fixed amount of gas at a constant pressure, the volume of a gas increases as the temperature of the gas increases
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For a fixed amount of gas at a constant pressure, the volume of a gas increases as the temperature of the gas increases
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Charles’ Law Graph
The graph of Charles’ Law is a straight line (linear).
It shows the relationship between temperature and volume is directly proportional: as one variable increases or decreases, so does the other
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Charles’ Law Formula:
Remember! ALWAYS USE KELVIN when dealing with temperature in gas laws! All °C temperatures MUST be
converted to Kelvin!!!
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Charles’ Law
http://wright.nasa.gov/airplane/aglussac.html
Animation of Charles’ Law
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Charles’ Law
Example:
A balloon is inflated to 665 mL volume at 27 deg C. It is immersed in a dry-ice bath at -78.5 deg C. What is its new volume, assuming the pressure remains constant?
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Third Gas Law: Gay-Lussac’s Law Temperature-Pressure
Relationships Gay-Lussac’s Law: The pressure of a gas at
a constant volume is directly proportional to the absolute temperature (temperature in Kelvin)
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Gay-Lussac’s Animation
http://www.chm.davidson.edu/ChemistryApplets/KineticMolecularTheory/PT.html
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Gay-Lussac’s Law Formula:
At constant volume:
P1 = P2
T1 T2
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Avogadro’s Law
In 1811, Avogadro proposed that equal volumes of all gases, under the same conditions, have the same number of particles.
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Avogadro’s Law
We know volume of a gas can change with temperature and pressure, but what about the number of molecules?
Through Avogadro’s observations, the following has been defined:
1 mole of any gas at STP (0°C and 1 atm) occupies 22.41 L The mass of 22.41L at STP is the Molecular
Mass of the gas
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Combined Gas Law
When you take Boyles, Charles’, Gay-Lussac’s and Avogadro’s Laws and combine them, you get the COMBINED GAS LAW
This law is used to solve problems where pressure, volume and temperature of a gas
vary with a constant molar quantity of the gas
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Combined Gas Law
Example:
A sample of hydrogen gas has a volume of 65.0 mL at a pressure of 0.992 atm and a temperature of 16 deg C. What volume will be hydrogen occupy at 0.984 atm and 25 deg C?
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Dalton’s Law of Partial Pressures
John Dalton showed that in a mixture of gases, each gas exerts a certain pressure as if it were alone with no other gases with it.
This is called “partial pressure” Equation:
Pt = P1 + P2 + P3…………
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Dalton’s Law of Partial Pressures
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Dalton’s Law of Partial Pressures
Example: A 2.5 L flask at 15 deg C contains a mixture of
three gases: N2, He, and Ne. The partial pressures are: N2 = 0.32 atm, He = 0.15 atm and 0.42 atm for Ne. What is the total pressure of the system?
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Gases – Dalton’s Law of Partial Pressures – Mole Fraction
As Dalton’s Law tells us, if a number of gases are mixed, each contributes to the pressure in its vessel.
To figure out how much pressure one gas is contributing, we need to find: 1. Mole Fraction 2. Its partial pressure.
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Gases – Mole Fraction/Partial Pressure
To calculate mole fraction (X):
X? = moles of gas of interest
total moles of gas in the mixture
To calculate the partial pressure of that gas:
Px = X?Ptotal
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Gases – Mole Fraction/Partial Pressure
A mixture of gases contains 4.46 moles of neon (Ne), 0.74 moles of argon (Ar) and 2.15 moles of xenon (Xe). Calculate the partial pressure of Ne.
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Gas Simulator
http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm