Latent Heat

PhysicsMontwood High School

R. Casao

Change of StateMatter exists in 3 states:•Solid•Liquid•Gas

water steam

fusion (at melting point)

vaporization (takes place at boiling point)

solidification condensation

Consider water:

at freezing point at boiling point

ice

Heats of Transformation

When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise.

The thermal energy may cause the mass to change from one phase, or state, to another.

The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.

Heat of Fusion To melt a solid means to change its

from the solid state to the liquid state. This process requires energy because the molecules of the solid must be freed from their rigid structure.

To freeze a liquid to form a solid is the reverse of melting and requires that energy be removed from the liquid so that the molecules can settle into a rigid structure.

Heat of Fusion When the phase change is from

solid to liquid, the sample must absorb heat; when the phase change is from a liquid to solid, the sample must release heat.

The heat of transformation for these phase changes is called the heat of fusion, Lf.

Water: Lf = 334 J/g = 79.5 cal/gfLmQ

Heat of Vaporization To vaporize a liquid means to change it from the

liquid state to the vapor or gas state. This process requires energy because the molecules must be freed from the liquid state.

Condensing a gas to a liquid is the reverse of vaporizing; it requires that energy be removed from the gas so that the molecules can cluster together instead of flying away from each other.

The heat of transformation for these phase changes is called the heat of vaporization, Lv.

Water: Lv = 2256 J/g = 539 cal/gvLmQ

Phase Changes

Copyright 1999, PRENTICE HALL

Chapter 118

Gas-Liquid Equilibration

Copyright 1999, PRENTICE HALL

Chapter 1110

Phase ChangesPhase ChangesEnergy Changes Accompanying Phase Energy Changes Accompanying Phase

ChangesChanges

Copyright 1999, PRENTICE HALL

Chapter 1112

Phase ChangesPhase ChangesHeating CurvesHeating Curves

Copyright 1999, PRENTICE HALL

Chapter 1113

Heating Curve Illustrated

Phase Change Diagram - Water

vaporization

energy in 2260

kJ

fusion

energy in 334 kJ

condensation

energy out

2260 kJ

solidification

energy out 334

kJ

steam(1 kg)

water(1 kg)

ice(1 kg)

Summary: Change of State

Energy involved in heating 1 kg of water

icesteam(ice and water) melting

(334) (420)water

boiling

(water and stream)(2260) energy /

kJ

Summary: from ice to steam

0

100

Heat of Combustion (HC) The amount of heat released per unit

mass or unit volume of a substance when the substance is completely burned.

Heats of combustion are used as a basis for comparing the heating value of fuels, since the fuel that produces the greater amount of heat for a given cost is the more economic.

Equation: CHmQ

Phase Change Example Heat is added to 0.5 kg of water at 20° C.

How many joules of heat energy are required to change the water to steam at 110° C?

Heat has to be added to the water to raise its temperature from 20° C to the boiling point 100° C.

Heat has to be added to vaporize all of the water that is at 100° C.

JQ

CCCkg

JkgTcmQ ooo

167440

2010041865.0

Phase Change Example

Heat has to be added to raise the temperature of all of the steam from 100° C to 110° C.

Total heat energy: Q = 167440 J + 1130000 J + 10050 J = 1307490 J

J1130000QkgJ2260000kg5.0LvmQ

J10050Q

C100C110Ckg

J2010kg5.0TcmQ ooo

Final Temperature Example A 0.3 kg piece of ice at 0° C is placed in 1 kg of

water at 40° C in an insulated container. If no heat is lost to the container, what is the final temperature of the water?

Q lost water = Q gained ice + Q gained ice water (m·c·T)water = m·Lf + (m·c·T)ice water

FFo

oFo

Fo

o

TkgJ1255.8J100200TC

J4186J167440

C0TCkgJ4186kg0.3

kgJ334000kg0.3TC40Ckg

J4186kg1

Final Temperature Example

C.CJ5441.8

JT

TCJ5441.8J

TCJ4186TC

J1255.8J100200J167440

o

o

F

Fo

FoFo

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