Phase Changes

Our interaction with the world of the small

What’s the Point?

• What happens to all our energy?

• How does energy convert in freezing, melting, evaporation, and condensation?

• What is “heat”?

Temperature and Energy

• Average translational molecular kinetic energy is proportional to a substance’s temperature.

• Individual molecules can have higher or lower kinetic energies than average.

Demo:

• Speed of Diffusion in hot vs cold liquid.

• Which has faster moving particles?

What is heat?• Heat is molecular energy transferred from high

to low temperature.

Heat transfers from a hotter substance to a colder substance

until they are both the same temperature.

What is heat?• System vs. Surroundings:

– You sit on a cold bench (around 600F). Your body temperature is 98.70F.

– You, the system, lose heat.• Temperature decreases.• Exothermic

– The bench, the surrounding, gains that heat.• Temperature increases.• Endothermic

– Heat lost by system = Heat gained by surroundings.– Heat exchange will continue until temps equal.

When a red-hot piece of iron is dropped into a bucket of water,

Poll Question

A. the water becomes hotter.

B. the water’s temperature increases .

C. the water’s internal energy increases .

D. the water receives heat from the iron.

E. all of the above.

What is heat?

– You sit on a cold bench (around 600F). Your body temperature is 98.70F.

– Heat lost by system = Heat gained by surroundings.– Heat exchange will continue until temps equal.

– Final temperature will NOT be half-way in-between the 2 temperatures. Why not?

Specific Heat (Capacity)

• Heat needed to change the temperature of a unit amount of a substance.

– q = heat input– m = mass of sample– T = temperature change

– Different chemicals have different specific heats!

c =q

mT

Specific Heat (Capacity)

• Heat needed to change the temperature of a unit amount of a substance.

– Metal has a very low specific heat– Needs little energy to change temp.– Greater change in temperature.

– Water (most of human body) has a very high specific heat– Needs a lot of energy to change temp.– Smaller change in temperature.

You sit on a cold bench (around 600F). Your body temperature begins at 98.70F. Assuming no other heat exchange occurs, when the 2 temperatures become equal, it will be

Poll Question

A. Below 600F

B. Above but close to 600F

C. Exactly in-between 600F and 98.70F.

D. Below but close to 98.70F

E. Above 98.70F

When a red-hot piece of iron is dropped into a bucket of water, the final temperature will be

Poll Question

A. Closer to water’s initial temperature

B. Closer to iron’s initial temperature

C. Half-way between water and iron’s initial temperature

D. Different for each: water and iron

Phase Changes

Endothermic (heat enters)

melting boiling

Solid Liquid Gas

freezing condensing

Exothermic (heat exits)

When an ice cube melts in your hand, your hand is __________ heat, going through an _____________ process. At the same time, the ice cube is ______________ the same amount of heat, going through an ___________ process.

Exothermic

Endothermic

Absorbing

releasing

Poll Question

Phase Changes

• Melting, boiling, freezing, condensing…

• Added or removed heat changes the substance’s potential rather than kinetic energy

• Water freezes at 0 °C, boils at 100 °C (well, about 92 °C in Laramie)

• Not all heat transfer is expressed as a temperature change.

ice

Liquid water

steam

Heating Curve for Water

Water boils

Ice melts

Phase Changes

• Potential energies:

Solid < Liquid < Gas

• During a phase change, potential energy, not kinetic energy (temperature) changes.

• Heating or cooling a changing phase does not change its temperature!

Latent heat

• Potential energy of phase change (energy required to change the phase of 1 kg of substance)

• Water’s latent heat of fusion (melting):

335,000 J/kg

• Water’s latent heat of vaporization:

2,255,000 J/kg

Evaporation of a Liquid

• More energetic jostling = higher temperature

• An especially fast molecule at the surface may detach!

Evaporation of a Liquid

• More energetic jostling = higher temperature

• An especially fast molecule at the surface may detach!

Evaporation

• Evaporating molecules carry away energy

• KE PE

• Remaining liquid cools (KE decreases)

Why would this occur?