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Gas Behavior

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Chapter 16 Section 3

Gas Behavior

- Definition of pressure is:
- Amount of force exerted on an area
- P = F/A
- Unit in SI: Pascal (Pa)
- One Pascal is one Newton force on an area 1 square meter

- Gases exert pressure due to KE of their particles

- Robert Boyle (1627-1691)
- Studied relationship between volume and pressure of gases
- Found: as volume goes up, pressure goes down
- Product of pressure and volume of a gas is a constant if temperature does not change

- As an equation: PV=constant
- http://www.grc.nasa.gov/WWW/K-12/airplane/Animation/frglab.html

- More commonly, we compare pressure and volume of the same gas
- Since PV is constant, we can say:
- P1V1 = P2V2 for any gas

- To see this in action, let’s do Math Skills Activity p 505!

- A balloon has a volume of 10.0 L at a pressure of 101 kPa. What will the balloon’s new volume be if the pressure changes to 43 kPa?
- What we know:
- P1 = 101 kPa
- V1 = 10.0 L
- P2 = 43 kPa
- V2 = ?

- What equation?
- P1V1 = P2V2
- So V2 = P1V1
P2

- Put in the values!
- V2 = (101 kPa)(10.0L)
(43 kPa)

- V2 = (101 kPa)(10.0L)
- Do the math!
- V2 = 23.488 L

- Check units and math!

- A volume of helium occupies 11.0 L at 98.0 kPa. What is the new volume if the pressure increases to 101.0 kPa?
- What do we know?
- P1 = 98.0 kPa
- V1 = 11.0 L
- P2 = 101.0 kPa
- V2 = ?

- What equation?
- P1V1 = P2V2
- So V2 = P1V1
P2

- Put in the numbers!
- V2 = (98.0 kPa)(11.0 L)
(101.0 kPa)

- V2 = (98.0 kPa)(11.0 L)
- Do the math!
- V2 = 10.67 L

- Check your math and units.

- Jaques Charles (1746-1823)
- Studied relationship of temperature and volume with constant pressure
- As temperature goes up, volume increases
- Volume/Temperature is a constant
- As an equation: K = V/T
- This relationship is the basis for the Kelvin temperature scale

- More commonly, we compare one situation to another for a gas
- V1 = V2 With constant pressure
T1T2

Temperatures must be used in Kelvins!

- If a balloon has a volume of 2.0 L at 25oC, what is the volume of the balloon at 3oC?
- What do we know?
- T1 = 25oC = 298 K
- V1 = 2.0 L
- T2 = 3oC = 276 K
- V2 = ?

- What equation?
- V1 = V2
T1 T2

So V2 = V1T2

T1

- V1 = V2
- Put in the numbers! Remember, temperature must be in Kelvins!
- V2 = (2.0L)(276 K)
(298 K)

- V2 = (2.0L)(276 K)
- Solve!
- V2 = 1.85 L

- Check for math and units.

- Earth’s atmosphere divided into 5 layers

- Exosphere: outer layer
- Thermosphere
- Mesosphere
- Stratosphere
- Troposphere: layer we live in!

- Very thin outer portion of atmosphere
- Negligible pressure

- Extends from about 90 km to about 500 km above Earth
- Very low pressure
- Very high temperatures (1000oC) in upper regions
- Gases here broken into atoms by energy from sun
- Much of X-ray and UV radiation absorbed here
- Aurora occurs in this layer

- About 50 to 85 km above Earth’s surface
- Temperatures get very cold (-25oC to -90oC)
- Air is mixed by currents
- Most meteors burn up in this layer

- Altitude of the bottom of this layer varies with latitude and season
- 16 km at equator
- 8 km at poles

- Ozone heats this layer as it absorbs UV from sun
- Very few clouds, very dry air

- Layer we live in!
- From surface up to 7 to 20 km altitude
- Almost all weather occurs in this layer
- Almost all dust particles and water vapor in this layer
- This layer heated from below, so warmer at lower altitudes than higher