Phase Transitions

Inter Molecular Forces in Play

Van der Waals (Atraction)

Electron Overlap (Repulsion)

Inter Molecular Forces in Play

Temperature and pressure make molecules be either too far, at the optimal distance for attraction, or too close

Phase transitions depend on the molecular properties of the material, temperature and pressure

Evaporation

Evaporation is the change of phase from liquid to gas

Evaporation

Evaporation is the change of phase from liquid to gas

Evaporation can happen below the surface when boiling

Evaporative Cooling

Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, thus evaporation cools

Condesation

Condensation is the reverse of evaporation, a change of phase from gas to liquid

Condesation

Condensation is the reverse of evaporation, a change of phase from gas to liquid

a)  At high speeds, molecules of water vapor bounce apart and remain a gas.

b) At lower speeds, molecules of water vapor are more likely to stick together and form a liquid

Checkpoint 

Why is it that a 90 degree day in a dry climate feels more comfortable than a 90 degree day in a humid place?

Melting

Melting is the change of phase from solid to liquid.

While melting, the solid absorbs heat from the environment 

Freezing

Freezing is the change of phase from liquid to solid.

While freezing, the solid releases heat into the environment 

Sublimation

Sublimation is the change of phase from solid to gas without passing through the liquid phase.

Solid carbon dioxide (dry ice) sublimates at -109 °F.

Energy and Changes of Phase

Energy and Change of Phase

1 Calorie = 4.2 Joules

Phase transitions require energy

Heating 1 gram of water

Water Heat Capacity in calories = 1 cal/g °C

Checkpoint 

It takes 80 Calories for 1 gram of water to change phase from solid to liquid, and the specific heat capacity of  water is 1 Calorie/(g °C). How much energy do you need to melt 1 gram of ice and end up with water at 20 °C ?

Thermodynamics

Connecting heat to mechanical energy

Remember

Heat (Q) : Energy transfer due to temperature differences

Work (W): Energy transfer due to acting forces

First Law of Thermodynamics

Whenever heat is added to a system, it transforms to an equal amount of some other form of energy

Energy is conserved!

First Law of Thermodynamics

Whenever heat is added to a system, it transforms to an equal amount of some other form of energy

Energy is conserved!

Adiabatic Process

Compressing or expanding a gas while no heat enters or leaves the system is said to be an adiabatic process

Adiabatic Process

When a gas adiabatically expands, it does work on its surroundings and gives up internal energy, and thus becomes cooler.

Second Law of Thermodynamics

Heat of itself never flows from a cold object to a hot object

The second law of thermodynamics describes the direction of heat flow in natural processes

Heat Engine and Second Law

A heat engine is any device that changes internal energy into mechanical work

How a Refrigerator Works?

The End

Adiabatic Process