Chapter 11

Behavior of Gases

Warm-up #1

How much force do you think it would take to crush this railroad tank car?

Stay tuned

Behavior of Gases

The properties of gases that are most easily observed are the relationships among pressure, volume, temperature, and mass. It is now possible to predict the

physical behavior of gases by the application of certain laws.

Particles and Pressure

How are number of particles and gas pressure related? The pressure of a gas is the force per unit area The pressure of a gas is directly proportional to

the number of contained particles. The more often gas particles collide with the

walls of their container, the greater the pressure.

More gas particles mean a greater number of collisions

http://demonstrations.wolfram.com/SimulationOfASimpleGasPressureModel/

Temperature and Pressure

How are temperature and gas pressure related? At higher temperatures, the particles in a gas have greater

kinetic energy. The particles move faster and collide with the walls of the

container more often and with greater force, so the pressure increases.

If the volume of the container and the number of particles of gas are not changed, the pressure of a gas increases in direct proportion to the Kelvin temperature.

http://demonstrations.wolfram.com/SimulationOfASimpleGasPressureModel/

Check This Out!

http://www.youtube.com/watch?v=Bi6sDTXE9TE http://www.youtube.com/watch?v=Zz95_VvTxZM

Warm-up #2

Yesterday we saw what happens when the pressure on the outside of a system is greater than the pressure on the inside of a system.

What happens when the pressure inside is greater than the pressure outside?

Devices to Measure Pressure 1 The barometer is an instrument that

measures the pressure exerted by the atmosphere. The height of the mercury column

measures the pressure exerted by the atmosphere.

The standard atmosphere (atm) is the unit used to measure pressure; the pressure that supports a 760-mm column of mercury.

1.00 atm = 760 mmHg

760 mmHg

Devices to Measure Pressure 2

The barometer can only measure atmospheric pressure and absolute pressure (pressure of all gases in the atmosphere).

To measure the pressure inside a bicycle tire or in an oxygen tank a pressure gauge is needed. This measures pressure above atmospheric pressure.

Pressure Units and Conversions

The SI unit for measuring pressure is the pascal (Pa). It is easier to use kilopascals (kPa) because the unit is bigger.

1 kPa = 1000 Pa Equivalent Values/Units of Atmospheric Pressure 1.00 atm = 760 mmHg = 14.7 psi = 101.3 kPa

Use the Factor Label Method to convert from one unit of pressure to another

A tire gauge reads 22.7psi, what is the same measurement in atmospheres (atm)?

More Pressure Calculations

Absolute pressure is the sum of the atmospheric pressure and the measured pressure

What is the absolute pressure when the gauge pressure is 44 psi?

What is the gauge pressure in kPa when the absolute pressure is 890 mm Hg?

44 psi + 14.7 psi = 59 psi

Warm-up #3

Assuming the walls of a container are weak and that atmospheric pressure is 1atm, would a container with a gas at a pressure of 300kPa explode or implode? Explain.

Volume and Pressure

How are volume and gas pressure related? In containers with fixed volumes

If the number of gas particles and temperature remain constant, then gas pressure will be inversely proportional to the volume of the container.

In containers with adjustable volumes As the pressure of the gas increase either due to an increase in

the number of particles or an increase in temperature, the volume will increase.

As the volume increases, the pressure decreaseshttp://demonstrations.wolfram.com/SimulationOfASimpleGasPressureModel/

Boyle’s Law

Boyle’s law states that the pressure and volume of a gas at constant temperature are inversely proportional.

Boyle’s law formula: P1V1=P2V2

Example: A sample of nitrogen collected in the laboratory occupies a volume of 725 mL at a pressure of 0.971 atm. What volume will the gas occupy at a pressure of 1.40 atm, assuming the temperature stays constant?

Warm-up #4

Using Boyle’s Law, calculate the initial pressure of a gas occupying 200mL if the final volume is 15mL and the final pressure is 18.8 psi

Volume and Temperature

How are volume and the temperature of a gas related? When a gas is heated, the kinetic energy of the particles

increase and particles move a greater distance away from one another creating a larger volume.

When a gas is cooled the kinetic energy of the particles decrease and particles mover closer together creating a smaller volume.

http://demonstrations.wolfram.com/SimulationOfASimpleGasPressureModel/

Charles’ Law

Charles’ law states that at constant pressure, the volume of a gas is directly proportional to its Kelvin temperature.

Charles’ law formula:

Why does the air in balloons expand when heated and contract when cooled?

Charles’ Law Practice Problem

A container of oxygen has a volume of 349 mL at a temperature of 22°C. What volume will the gas occupy at 50°C?

Temperatures in any gas law problem must be in Kelvin. Remember to always convert Celsius to Kelvin before attempting to solve the problem.

382 mL

Warm-up #5

What would have been the initial temperature of a system that had an initial volume of 550mL, a final volume of 250mL and a final temperature of 297K?

Combined Gas Law

The combination of Boyle’s law and Charles’ law is called the combined gas law.

The set of conditions 0°C and 1 atm are used so often that they are called standard temperature and pressure or STP.

Combined gas law formula:

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°C. What volume will the hydrogen occupy at 0.984 atm and 25°C?

The Law of Combining Gas Volumes

The observation that at the same temperature and pressure, volumes of gases combine or decompose in ratios of small whole numbers is called the law of combining gas volumes.

Avogadro’s principle states that equal volumes of gases at the same temperature and pressure contain the same number of particles