Behavior of Gases

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# Behavior of Gases - PowerPoint PPT Presentation

Behavior of Gases. What is suction?. What is suction?. What is suction?. What is suction?. What is suction?. What is suction?. There is no such thing as suction, it is simply a difference in air pressure. What Do You Already Know?. True or False: Kinetic energy is energy in motion.\_\_\_\_\_\_

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### Behavior of Gases

What is suction?
• There is no such thing as suction, it is simply a difference in air pressure.

True or False:

• Kinetic energy is energy in motion.______
• Potential energy is stored energy. ______
• Gas particles are tightly packed. ______
• Gas molecules expand uniformly to occupy a container. ______
• Gas molecules have strong intermolecular forces. ______
• Gas molecules have very little kinetic energy. ______
Properties of Gases
• Shape? Indefinite Shape (take the shape of a container)
• Volume? Indefinite volume
• Energy? Molecules move at high speeds = high kinetic energy
• Particle Diagram:
Properties of Gases
• Compressibility:
• The ability of the volume of a substance to be decreased by adding pressure
• Why can gases be easily compressed?
• Because there is mostly empty space between the molecules.
Importance of Gases

During an accident, a sudden stop in motion triggers this decomposition reaction:

2NaN3 2Na + 2N2 (g)

N2 gas fills the airbag and saves your life!!!!

Gas Pressure
• Gas pressure:
• The result of billions of collisions of gas molecules with an object
• Atmospheric Pressure:
• The result of billions of collisions of AIR molecules with an object
• How is atmospheric pressure measured?
• By using a device called a barometer
Barometer
• Why do we use mercury in a barometer?
• Mercury has a high density. It does not respond to pressure as much as liquid water would.
• What does the height of the column depend on?
• The amount of pressure pushing down on the mercury determines how high the mercury will rise in the column.
Atmospheric Pressure
• Does elevation have an effect on atmospheric pressure? YES!!!
• As elevation increases, atmospheric pressure decreases (baseballs fly further because there are less collisions with air molecules to slow them down)
• As elevation decreases, atmospheric pressure increases, there are more collisions with air molecules.
STP
• Standard Temperature and Pressure: TABLE A!!!
• Temperature = 273 K or 0° C
• Pressure = 101.3 kPa (kilopascals) OR 1 atm (atmosphere)
Converting from atm to kPa
• 1 atm = 101.3 kPa
• How many atmospheres would be present in 253.25 kPa of pressure?

101.3 kPa

253.25 kPa

=

X = 2.5 atm

1 atm

X atm

Conversion Practice
• Convert the following to atmospheres:
• 658.45 kPa = ___________ atm
• 253.25 kPa = ___________ atm
• 1013 kPa = ___________ atm
• 25.325 kPa= ___________ atm

Convert the following to kilopascals:

• .75 atm = ______________ kPa
• 4 atm = ______________ kPa
• 2 atm = ______________ kPa
Evaporation
• During evaporation:
• Particles with the highest KE can overcome IMFs and escape the liquid into the vapor phase
• During condensation:
• Particles that lose enough KE will fall back into the liquid phase

http://www.chm.davidson.edu/vce/kineticmoleculartheory/Maxwell.html

What is Vapor Pressure?
• Vapor pressure is the pressure exerted by a vapor in a closed container.
• How is pressure exerted by the vapor?
• By billions of particles colliding with the walls of the container

Vapor Pressure

Vapor Pressure Equilibrium

Liquid begins to boil, particles are evaporating

An equilibrium is established when the rate of evaporation = the rate of condensation

Question
• What factors effect the rate of evaporation?
• Temperature
• IMFs (Intermolecular forces)
• Surface area
Rate of Evaporation
• Which substance will evaporate faster?
• Hand sanitizer (alcohol)
• What IMFs hold H2O molecules together?
• Hydrogen bonding
• What IMFs hold alcohol molecules together?
• Dipole-dipole
• Which IMF is stronger?
• Hydrogen bonding
IMFs and Vapor Pressure

What is the relationship between IMFs and vapor pressure?

• As IMFs get stronger, there are less particles evaporating, therefore there are less collisions with the container walls, vapor pressure decreases.
• As IMFs get weaker, there are more collisions with the container walls, vapor pressure increases.
• http://www.chm.davidson.edu/ronutt/che115/Phase/Phase.htm
Vapor Pressure

100 mL H2O

@ 50°C

100 mL Ethanol

@ 50°C

• Will they exert the same pressure? Why or why not?
• The flask with ethanol will exert a higher vapor pressure because it has weaker
• IMFs, more particles will evaporate, causing more collisions with the container walls.
Vapor Pressure and Temperature
• As the temperature of a liquid increases, what happens to kinetic energy?
• Kinetic Energy increases
• How does this affect the number of molecules that can evaporate?
• More molecules will evaporate as temperature and KE increase.
• What does this do to the vapor pressure?
• Since more molecules are in the vapor phase, there are more collisions with the container walls, vapor pressure increases.

http://www.chm.davidson.edu/ronutt/che115/Phase/Phase.htm

Vapor Pressure

100 mL H2O

@ 50°C

100 mL H2O

@ 95°C

Will they exert the same pressure? Why or why not?

The flask that is at 95°C will exert a higher vapor pressure because at a higher temperature, more particles are evaporating and colliding with the walls of the container.

Questions
• How does a gas exert pressure on an object?
• What is “suction”?
• Describe the relationship between elevation and atmospheric pressure.
• What units do we use to measure pressure?
• Explain, in terms of KE, the process of evaporation
• Describe the meaning of vapor pressure equilibrium.
• Explain the relationship between vapor pressure and temperature?
• Explain the relationship between vapor pressure and IMFs.