phase changes - antelope valley college...phase changes of a substance vary with both temperature...

© 2017 Pearson Education, Inc. Slide 19-1

Phase Changes

Suppose you were to remove an ice cube from the freezer, initially at –20ºC, and then warm it by transferring heat at a constant rate.

Figure (b) shows the temperature as a function of time.

During the phase changes of melting then boiling, energy is being added to break molecular bonds, but the temperature remains constant.


Phase Changes

A phase diagram is used to show how the phases and phase changes of a substance vary with both temperature and pressure.

At the normal 1 atm of pressure, water crosses the solid-liquid boundary at 0ºC and the liquid-gas boundary at 100ºC.

At high altitudes, where p < 1 atm, water freezes at slightly above 0ºC and boils at a temperature below 100ºC.

In a pressure cooker, p > 1 atm and the temperature of boiling water is higher, allowing the food to cook faster.


If the pressure of liquid water is suddenly

decreased, it is possible that the water will

A. Freeze.

B. Condense.

C. Boil.

D. Either A or C.

QuickCheck


A phase change is characterized by a change in thermal energy without a change in temperature.

The amount of heat energy that causes 1 kg

of substance to undergo a phase change is the heat of transformation (or latent heat)L of that substance.

The heat required for a system of mass M to undergo a phase change is

Phase Change and Heat of Transformation

Lava—molten rock—undergoes a

phase change when it contacts the

much colder water. This is one way

in which new islands are formed.


If you heat a substance in an insulated container,

is it possible that the temperature of the substance

remains unchanged?

A. Yes

B. No

QuickCheck


The specific heat c can be

measured from the slope of

the linear increases.

The heat needed for the

phase changes is:

Suppose you start with a system in its solid phase and heat

it at a steady rate.

where Lf is the latent heat of

fusion and Lv is the latent

heat of vaporization.

Phase Change and Heat of Transformation


Melting/Boiling Temperatures and Heats of

Transformation

Example 1 - Latent Heat

A bag containing 0ºC ice is much more effective in absorbing energy than

one containing the same amount of 0ºC water. How much heat transfer is

necessary to raise the temperature of 0.800 kg of water from 0ºC to 30.0ºC?

How much heat transfer is required to first melt 0.800 kg of 0ºC ice and then

raise its temperature?

Example 2 – Latent Heat

How much heat transfer is required to raise the temperature of a 0.750-

kg aluminum pot containing 2.50 kg of water from 30.0ºC to the boiling

point and then boil away 0.750 kg of water? (b) How long does this take

if the rate of heat transfer is 500 W?


Consider two systems with different temperatures T1 and T2 that can interact thermally with each other but are isolated from everything else.

Heat will naturally flow from the hotter to the colder system until they reach a common final temperature Tf.

If Qi is the heat transferred to system i, then

Calorimetry


50 g of ice at 0ºC is added to 50 g of liquid water at

0ºC in a well-insulated container, also at 0ºC. After a

while, the container will hold

A. All ice.

B. > 50 g of ice, < 50 g of liquid water.

C. 50 g of ice, 50 g of liquid water.

D. < 50 g of ice, > 50 g of liquid water.

E. All liquid water.

QuickCheck


Problem-Solving Strategy: Calorimetry Problems


Example 3

A 200 g piece of iron at 120ºC and a 150 g piece of copper

at -50ºC are dropped into an insulated beaker containing

300 g of ethyl alcohol at 20ºC. What is the final temperature?

In-class Activity #1

In a calorimeter of negligible heat capacity, 200 g of steam at 150ºC and

100 g of ice at −40ºC are mixed. The pressure is maintained at 1 atm.

What is the final temperature, and how much steam, ice, and water are

present?


Conduction Convection

Radiation Evaporation

Heat-Transfer Mechanisms


For a material of cross-section area A and length L, spanning a temperature difference ΔT = TH – TC,the rate of heat transfer is

where k is the thermal conductivity, which characterizes whether the material is a good conductor of heat or a poor conductor.

Conduction


Conduction


Example 4

A 1.8-m-wide by 1.0-m-tall by 0.65-m-deep home freezer is

insulated with 5.0-cm-thick Styrofoam insulation. At what

rate must the compressor remove heat from the freeze to

keep the inside at -20ºC in a room where the air

temperature is 25ºC?


Thermal energy is easily transferred through air, water, and other fluids because the air and water can flow.

When the fluid is heated it generally expands and becomes less dense.

Buoyancy then causes the warmer fluid to flow upward, while the cooler fluid sinks to take its place.

This transfer of thermal energy by the motion of a fluid is called convection.

Convection

Warm water (colored) moves

by convection.


All objects emit energy in the form of radiation, electromagnetic waves generated by oscillating electric charges in the atoms that form the object.

This satellite image shows radiation emitted by the ocean waters off the east coast of the United States.

You can clearly see the warm waters of the Gulf Stream, a large-scale convection that transfers heat to northern latitudes.

Radiation


If heat energy Q is radiated in a time interval Δt by an object with surface area A and absolute temperature T,the rate of heat transfer is

The parameter e is the emissivity of the surface, a measure of how effectively it radiates.

The value of e ranges from 0 to 1.

σ = 5.67 ×10–8 W/m2K4 is the Stefan-Boltzmann constant.

Radiation


Suppose the temperature of the sun suddenly dropped

to half its present value. The sun’s power output would

decrease by a factor of

A. 1/2

B. 1/4

C. 1/8

D. 1/16

QuickCheck


Example 5

The intensity of solar radiation is 1370 W/m2. What is the

temperature of the sun’s surface? Note: The distance to

the sun is on average 1.50 x 1011 m and the radius of the

sun is 6.96 x 108 m.

In-class Activity #2

A firewalker runs across a bed of hot coals without

sustaining burns. Calculate the heat transferred by

conduction into the sole of one foot of a firewalker given

that the bottom of the foot is a 3.00-mm-thick callus with a

conductivity at the low end of the range for wood (0.08 W/m

ºC) and its density is 300 kg/m3. The area of contact is 25.0

cm2, the temperature of the coals is 700ºC, and the time in

contact is 1.00 s.

phase changes - antelope valley college...phase changes of a substance vary with both temperature...

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