Chapter 12:

The Behavior of Gases

Think of Chem 1A….

• Mrs. Richards, Mr. Mazurek, Ms. Knick, Ms. Olin…..

• What do you remember about gases????

The Behavior of Gases

• The behavior of gases rely heavily on the properties of pressure, temperature and volume.

Pressure & Force

• Pressure (P) is defined as the force per unit of area on a surface.

• Atmospheric pressure (atm) is the pressure exerted on an object due to the weight of the column of the air above it in the atmosphere.

• A barometer is a device used to measure atmospheric pressure.

Units of Pressure• Pascal (Pa)

• millimeters of mercury (mm Hg)

• torr (torr)

• pounds per square inch (psi or lbs/in2)

• atmosphere (atm) - This is the unit of pressure that we will use in chemistry. All pressure units must be converted to atmospheres

Relationships between units of pressure

• 1.0 atm = 760 mmHg

• 1.0 atm = 760 torr

• 1.0 atm = 1.0135 x 105 Pa

• 1.0 atm = 14.700 psi

PRACTICE: Convert 78.9 psi to units of atm.

Temperature

• A temperature is a numerical measure of hot and cold.

• We will measure or calculate temperature in units of Kelvin, not Celsius.

• K = °C + 273.15

Example

• Convert 30 degrees C to Kelvin

• Pretttyyy easy!!!

Volume

• A gas’ volume can change; gases can be compressed or expanded.

• Units of liters (L).

• Remember, there are 1000 mL in 1L.

Standard Temperature and Pressure

• Abbreviated as STP

• Standard temperature is equal to 0 °C, which is 273.15 K.

• Standard Pressure is 1 Atm

Direct Relationship vs. Inverse Relationship

• Direct Relationship: both variables increase together or both variables decrease together.

• Inverse Relationship: one variable increases while the other variable decreases or vice versa.

The Gas LawsBoyle’s Law: the pressure-volume relationship of gases

• Boyle’s Law states that the volume of a fixed mass of a gas at a constant temperature varies inversely with its pressure.

P1V1 = P2V2

Problem #1A balloon filled with helium gas has a volume of

500 mL at a pressure of 1 atm. The balloon is released into the air where the pressure is 0.5 atm. If the temperature has remained the same, what volume does the gas occupy at this height?

Change of website…

http://www.grc.nasa.gov/WWW/k-12/airplane/boyle.html

The Gas LawsCharles’ Law: the Volume-Temperature relationship

• Charles’ Law states that the volume of a fixed mass of gas held at a constant pressure varies directly with its Kelvin temperature.

V1 = V2

T1 T2

Problem #2

A gas has an initial volume of 752 mL at a temperature of 25 degrees C. The gas is heated to 100 degrees C. What is the new volume?

The Gas LawsGay-Lussac’s Law: the pressure-temperature relationship

in gases

• Gay-Lussac’s Law states that the pressure of a fixed mass of a gas held at a constant volume will vary directly with its Kelvin temperature.

P1 = P2

T1 T2

Problem #3

A gas has an initial temperature of 120 degrees C with a pressure of 1.07 atm. The gas is heated to a temperature of 205 degrees Celsius.

The Gas LawsThe Combined Gas Law

• The combined gas law expresses the relationship between pressure, volume, and the Kelvin temperature of a fixed amount of gas. (This means that the number of moles is held constant)

P1V1 = P2V2

T1 T2

Problem #4

V1 = 27.5 mL P1 = 0.974 atm T1 = 22˚C

V2 = ? P2 = 0.993 atm T2 = 15˚C

The Ideal Gas Law

• The ideal gas law is the mathematical relationship among pressure, temperature, volume, and the number of moles of a gas.

• PV = nRT P = pressureV = volumen = # molesR = ideal gas constant (pg

384)T = Kelvin temperature

.

Problem #5

• Calculate the grams of CO2 that would occupy a volume of 22.4 liters at STP.

Diffusion & Effusion• diffusion is the gradual mixing of two gases

due to their spontaneous, random motion

• effusion is the process by which molecules of a gas confined in a container randomly pass through a small opening in the container