Chapter 13 Notes – Gases

Gases are different

Properties of Gases

The volume of a gas is highly variable Gases can be compressed Gases can be very “diffuse”, or spread out

Solids can only be compressed a littleLiquids can not be compressedGases have low densities Gases have small amounts of mass per unit

of volume

Behavior of Gases

Effusion vs diffusion

Kinetic Molecular Theory

Assume: all gases are ideal Is this a correct assumption?

Kinetic Molecular Theory (KMT) developed in the 1800’s

Kinetic Molecular Theory

1. Ideal gas particles are so small that their individual volume is zero (compared to the total volume of the gas)

2. They move constantly, rapidly, and randomly, till they collide

3. There are no attractive or repulsive forces between ideal gas molecules

1. Collisions between molecules are 100% elastic

KMT (cont’d)

4. Average kinetic energy is proportional to temperature in Kelvin

1. 0 degrees K means what?

Kinetic Energy

½ mv2

Energy of motionWhat does this mean in the equation above? As mass increases, velocity must

decrease As mass decreases, velocity increases A smaller particle moves faster!

Variables that describe gases

Average kinetic energy is proportional to temperature The average number of molecules is

moving at an average speed, with an average KE

Some move higher, and some lower

Volume

Volume = length x height x width Can be measured in cm3, mL, L, m3

Gas volumes are often measured by water displacement The amount of water the gas

displaces is equal to its volume

Pressure

Pressure is force per unit area Pressure = force/unit area

For gases, it is a measure of the force when molecules collide with the container.The atmosphere exerts pressure on the earth

STP = Std. Temp. and Press.

“Standard” temperature and pressure is: 0°C and 1.0000 atm

This is the same as: 273.15 K and 101.3 kPa (kilopascals) 273.15 K and 760 mm Hg 273.15 K and 760 torr

Volatile Liquids

Liquids that are flammable!!They have high vapor pressures.They boil at low temperaturesThey have weak attractive forcesThey often have strong smells!

Vapor pressure

The pressure of a vapor over its liquidEquilibrium VP: when molecules leave the liquid at the same rate as they enter.Boiling: when vapor pressure equals atmospheric pressure, liquids boil.

Gas Laws

Dalton’s Law of Partial Pressure The total pressure of a gas is the sum of

the pressures of all its parts.

Ptot = Pa + Pb + Pc + … + Pz

Example:.

Gas Laws

Charles’ Law Volume is proportional to

temperature

Gas Laws

Boyle’s Law Pressure and volume are inversely

proportional

The combined gas law

Combines Boyle’s and Charles’ Laws: 2

22

1

11

T

VP

T

VP

What variable must we assume is held constant? Example: A helium balloon with a volume of 410 mL is cooled from 27 to –27°C. The pressure is reduced from 110 kPa to 25 kPa. What is the volume of the gas at the lower temperature and pressure?

The ideal gas law

The ideal gas law relates all 4 variables, P,V,T, and n.PV = nRTWhat is R? It is a constant: 8.314 L•kPa/mol•K Or, 0.0821 L•atm/mol•K Which one do I use? Use the one that

will cancel out units appropriately.

Example:

How many liters of hydrogen gas can be produced at 300K and 104kPa if 20g of sodium metal react with water? 2Na + 2H2O 2NaOH + H2

Gases and condensation

These two assumptions from KMT aren’t exactly true:

1. Volume of gases is zero2. No attractive forces between

particlesAt high pressure, gases don’t condense like you think because they DO have volume! And, they do have attractive forces, especially if they are close together.

Phase diagrams

A graph showing the relationships between solid, liquid, gas, and temperature and pressureNormal boiling point: the temperature at which a substance boils at 1.0000 atm.

Things you’ll see on a phase diagramSublimation: when a solid goes to a gasTriple Point: temperature and pressure where solid, liquid, and gas all coexistCritical Point: Is it a gas or liquid – who knows?

Interactive Phase Diagram

Gases: a review

1. A child receives a balloon filled with 2.30 L of helium from a vendor at a fair. The outside temperature is 311K. What will the volume be when she goes home to her air conditioned house at 295K?

2. At a deep sea station 200 m below the surface of the pacific ocean, workers live in a highly pressurized environment. How many liters of gas at STP on the surface must be compressed to fill the underwater environment with 2x107 L of gas at 20 atm? Assume constant temperature.

Real Gases

In the real world, the behavior of gases only conforms to the ideal-gas equation at relatively high temperature and low pressure.

Real Gases

Even the same gas will show wildly different behavior under high pressure at different temperatures.

Deviations from Ideal Behavior

The assumptions made in the kinetic-molecular model (negligible volume of gas molecules themselves, no attractive forces between gas molecules, etc.) break down at high pressure and/or low temperature.