Gas and Pressure

Review Kinetic Vs. Potential Energy

Kinetic Molecular Theory

Explains the behavior of gases

Real gases– do not follow all the assumptions of this theory.

Theoretical, applies to an ideal gasGas following all the assumptions of the theory.

Kinetic Molecular Theory

1) Gases are made up of TONS of particles, constantly moving, and spread out.

2) Gas particles drive straight until they hit/collide with something (ex. Wall, particles).

3) Small particles, HUGE space for them to roam! Gas volume mainly empty space

Kinetic Molecular Theory (cont.)

4) No force of attraction! –gas particles randomly move around

5) When particles collide with each other or container wall, elastic collisions are the result.

-elastic collisions—no kinetic energy is lost when gas particles collide

6) Temperature determines average kinetic energy of gas particles.

-not all gas particles are moving at the same kinetic energy

Properties of Gases

1) ExpandableNo shape

No defined volume

Fills whatever container is available to it.

Move quickly and no attraction

2) FluidGases move similar to liquids

No attraction to worry about

3) Low Density Least dense state of matter for most substances

Due to distance between particles

Properties of Gases (cont.)

4) DiffusionGas particles mix with each other and disperse

“Spontaneous mixing of gas particles from 2 substances due to random motion” (ex. Perfume)

5) EffusionDescribes gas movement through a small opening. (ex. Tire puncture)

Related to how fast gas particles can move.

6) CompressibilityGas particles are able to be packed close together.

Decreased volume

Pressure

“force per unit area on a surface”

Gas particles colliding against container and creating force

Force a gas exerts on its surroundings

Unit = Newton (N)

Amount of pressure dependent on Volume, Temperature, and particle/molecule number.

Ex. Atmospheric pressure

How do we measure pressure?

Barometer– atmospheric pressure

Evangelista Torricelli (1600s) Mercury falls to 760 mm

Air pressure holds 760 mm mercury column.

BarometerThe pressure of the atmosphere at sea level will hold a column of mercury 760 mm Hg.

1 atm = 760 mm Hg1 atm Pressure

760 mm Hg

Vacuum

Pressure Units

Common: mmHg (millimeters of Hg)

Measurements done in “atmosphere units”

1 atmosphere of pressure (atm) = (sea level, 0° C)

760 mmHg

760 torr

1.01325 x 105 Pa

101.325 kPa

Example 1:

1.75 atm of pressure to mmHg

Example 2:

570 torr of pressure to atmospheres.

kPa?

Gas Laws

Describes the relationship between variables associated with gases

Volume (V)

Temperature (T)

Pressure (P)

Concentration/amount of gas (n)

**Two variables change in relation to each other while the remaining variables are held constant.

Gas Laws

1) Boyle’s Law

2) Charle’s Law

3) Gay-Lussac Law

4) Avagadro’s Law

5) Combined Gas Law

6) Ideal Gas Law

Boyle’s Law

Pressure is INVERSELY proportional to the volume of the gas.

Temperature and particle amount constant

P , V

P , V

Pressure and volume relationship

P1V1 = P2V2

Remember to keep units the same.

Examples

1) What would happen to the volume of a balloon filled with 0.357 L of H2 gas collected at 741.3 mmHg if the atmospheric pressure increased to 758.1 mmHg? (temperature is constant)

2) Calculate the volume of a balloon that could be filled at 1.00 atm with helium in a 2.50L compressed gas cylinder in which the pressure is 200 atm at 25°C.

Charle’s Law

Temperature and volume relationship

The volume of a gas is DIRECTLY proportional to the temperature

T , V

T , V

V1 = V2 SOOO T1 = T2

T1V2 = T2V1

Remember to keep units the same. Temperature MUST be in Kelvin

Examples

1) A sample of O2 gas with a volume of 0.357L was collected at 21°C. Calculate the volume of the gas when it is cooled to 0°C if the pressure remains constant.

2) How hot will a 2.3L balloon have to get to expand to a volume of 400L? Assume the initial balloon temperature is 25°C.

Gay-Lussac’s Law

Pressure and temperature relationship Pressure results from molecular collisions

Pressure of gas is DIRECTLY proportional to temperature.

P , T

P , T

P1 = P2 SOOO T1

= T2

P1T2 = P2T1

Remember to keep units the same. Temperature MUST be in Kelvin

Examples

1) A coke can has 5.00atm of gas at 21°C. Calculate the pressure inside the can when it is found in a warehouse during the summer at 38°C.

2) The pressure of my tires before a road trip to Wyoming was 1.5atm at 25°C. After returning to North Carolina, my tire pressure is 1.7atm. What is the temperature (in °C) outside?

Gas Worksheet

Avogadro’s Law

Volume of a gas is DIRECTLY proportional to # of gas particles (moles of gas)

Temperature and Pressure are held constant

V1 = V2

n1 = n2

# gas particles, volume

# gas particles, volume

Ex. Blowing up a balloon

Ideal Gas Law

Describes the general relationship among the variables:

Temperature

Pressure

Volume

Number of moles of gas

Enables us to determine the value of a variable if the other three variables are known

Ideal Gas Law (cont.)

PV = nRT

P = pressure (atmospheres)

V = volume (liters)

T = temperature (Kelvin)

n = moles of the gas

R = 0.08206 Latm/molK (ideal gas constant)

Examples

1) Many gases are available for use in the laboratory among compressed gas cylinders stored at high pressures. Calculate the mass of O2 (in grams) that could be stored at 21°C and 170atm in a cylinder with a volume of 60.0L.

2) Calculate the molecular weight of butane if 0.5813g of the gas fills a 250.0ml flask at a temperature of 24.4°C and a pressure of 742.6 mmHg

Examples (cont.)

3) Calculate the density in grams per liter of O2 gas at 0°C and 1.00 atm

Combined Gas Law

Do variables remain constant for gases???

Temperature, pressure, and volume are CONSTANTLY changing for a gas based on the conditions Gas amount (n) is constant

Combined Gas Law (cont.)

Combination of all three laws into one equation (Boyle’s, Charle’s, and Gay-Lussac’s Laws)

Describes the relationship between pressure, volume, and temperature

Focus on initial and final conditions

Rearrange ideal gas law with the gas constant (R) remaining the same

Combined Gas Law

P1V1 = P2V2

T1 T2

Temperature—Kelvin

Examples

1) A gas has a volume of 80.0ml at 27°C and 0.200 atm. What volume will the gas have at standard conditions?

2) A gas has a volume of 60.0ml at standard conditions. This volume is reduced to 10.0ml at 25.0°C. What is the necessary pressure for this volume reduction?

Dalton’s Law of Partial Pressures

Pressure of each gas DIRECTLY proportional to amount of moles of a gas Increase gas particles, increase pressure Decrease gas particles, decrease pressure

Partial pressure— Pressure of one gas that contributes to the total

pressure in a mixture of gases

Total mixture pressure---- The sum of the individual gas pressures in a

mixture

Dalton’s Law of Partial Pressures

Total Pressure (PT) of gas mixture =

Sum of partial pressures of each gas in the mixture

PT = P1 + P2 + P3

Example

Calculate the partial pressure (in mmHg) exerted by the 4 main gases in air at 760 mmHg: nitrogen, oxygen, argon, and carbon dioxide. Their abundance by volume is 78.08%, 20.95%, 0.934%, and 0.035% respectively.

Water Displacement with Dalton’s Law

How do we collect and measure gases? Water displacement

Gas displaces water but the gas is mixed with water vapor

Application of Dalton’s Law allows the adjustment for the amount of water vapor to be made so just the amount of gas collected can be measured.

Water Displacement with Dalton’s Law

(cont.)Water vapor is mixed in with gas of interest so need to separate.

PT = Pgas + Pwater

look up vapor pressure of water at different temperatures (p. 196, Table 15. 4)

Examples

1) A sample of nitrogen gas is collected over water at a temperature of 23.0°C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mmHg?

2) A student has stored 100.0 ml of neon gas over water on a day when the temperature was 27.0°C. If the barometer in the room reads 743.3 mmHg, what is the pressure of the neon gas in the container?

Homework

Gas Worksheet #2