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# Gases - PowerPoint PPT Presentation

Gases. Chapter 13 Page 298. 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.

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### Gases

Chapter 13

Page 298

• 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

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 (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

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

• 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

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.

• 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

• Animation

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

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.

 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

• 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?

• 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

• 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

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

• 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

A graph of Boyle’s Law shows the relationship between pressure and volume is inversely proportional: as one variable increases, the other decreases

http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html

Animation of 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?

• 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

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

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

• http://wright.nasa.gov/airplane/aglussac.html

Animation of 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?

• 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)

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

At constant volume:

P1 = P2

T1 T2

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

• 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

• 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

• 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?

• 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…………

• 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?

• 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.

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

• 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 Fraction

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