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Gases

- Properties:
- Gases are fluids because their molecules/atoms can flow
- Gases have low density
- Highly compressible – their volume can be reduced
- Gases will completely fill their container

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 is greater than the pressure of the gas molecules on the outside.

http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html

Gas Pressure

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.

http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html

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

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

Measuring Pressure

Atmospheric Pressure is measured using a barometer

This picture shows a mercury barometer. As the atmospheric pressure presses down on the mercury in the pan, the mercury is forced into the tube. The height of the mercury is measured to obtain the pressure.

Measuring Pressure

- Manometers

A manometer is a U-shaped tube full of a liquid (usually mercury). As the pressure is changed on one side of the tube, the mercury is forced up or down accordingly.

The change in height of the mercury (h) indicates the change in pressure

Manometers

- Animation
- http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/gaslaw/manometer4-1.html

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.

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

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?

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

First Gas Law: Boyle’s Law

- Pressure-Volume Relationships

Boyle’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

Boyle’s

As volume decreases, the concentration, and therefore the pressure, increases

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

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

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?

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

For a fixed amount of gas at a constant pressure, the volume of a gas increases as the temperature of the gas increases

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

Charles’ Law Formula:

Remember! ALWAYS USE KELVIN when dealing with temperature in gas laws! All °C temperatures MUST be converted to Kelvin!!!

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?

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)

Gay-Lussac’s Animation

- http://www.chm.davidson.edu/ChemistryApplets/KineticMolecularTheory/PT.html

Avogadro’s Law

- In 1811, Avogadro proposed that equal volumes of all gases, under the same conditions, have the same number of particles.

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

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

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?

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…………

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?

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.

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

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.

Gas Simulator

- http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm

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