Change of Phase and Latent Heat

CHANGE OF PHASE AND LATENT HEAT

Prepared by: Merie Lyn DJ. Agustin MAE - Physics

CHANGE OF PHASE

THERMODYNAMICS

The term “change of phase” means the same thing as the term “change of state”.

There are four states, or phases, of matter:•Solid•Liquid•Gas•Plasma

CHANGE OF PHASE

THERMODYNAMICS

When a substance changes from one state or phase of matter to another we say that it has undergone a change of state, or we say that it has undergone a change of phase. For example, ice melts and becomes water; water evaporates and becomes water vapor.

These changes of phase always occur with a change of heat. Heat, which is energy, either comes into the material during a change of phase or heat comes out of the material during this change. However, although the heat content of the material changes, the temperature does not.

Why Do Phase Changes Occur?

Phase changes typically occur when the temperature or pressure of a system is altered.Example: At normal atmospheric pressure, ice melts as temperature increases. If you held the temperature steady, but lowered the pressure, eventually you would reach a point where the ice would undergo sublimation directly to water vapor.

When you supply heat to a substance or you remove heat from it, then you can cause the substance to change its state.

Here are the five changes of phase:

Description of Phase Change

Term for Phase Change

Heat Movement During Phase

Change

Temperature Change

During Phase Change

Solid to Liquid MeltingHeat goes into the solid as it melts.

None

Liquid to Solid FreezingHeat leaves the liquid

as it freezes.None

Liquid to Gas EvaporationHeat goes into the

liquid as it vaporizes.None

Gas to Liquid CondensationHeat leaves the gas

as it condenses.None

Solid to Gas SublimationHeat goes into the

solid as it sublimatesNone

So, how could there be a change in heat during a state change without a

change in temperature?

During a change in state, the heat energy is used to change the bonding between the molecules. In

the case of melting, added energy is used to break the bonds between the molecules. In the case of freezing, energy is subtracted as the

molecules bond to one another. These energy exchanges are NOT changes in kinetic energy.

They are changes in bonding energy between the molecules.

If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of

the substance. The example we will use here is ice melting into water. Immediately

after the molecular bonds in the ice are broken the molecules are moving

(vibrating) at the same average speed as before, so their average kinetic energy

remains the same, and, thus, their Kelvin temperature remains the same.

Breaking the bond of the molecules as heat is added to the material.

PHASE CHANGE DIAGRAM

Heat Energy

Melts

Freezes

Condenses

Boils

Coo

ls

In a similar way heat enters a liquid to change the molecular bonding

when the liquid boils or evaporates into a gas, and heat enters a solid to change the molecular bonding when

it sublimates into a gas.

PHASE CHANGE DIAGRAM

Heat Energy

Melts

In the ice, the molecules are strongly bonded to one another, thus forming a rigid solid.When heat is added to the ice these bonds are broken and the ice melts. The molecules afterward bond to one another with less strength and a different geometry, and water is formed.Now, before the melting, the molecules were actually moving when in the solid state. They were vibrating back and forth. They had an average kinetic energy. So they had a Kelvin temperature proportional to this average kinetic energy.

After the melting, the water molecules are still vibrating. And they have the same average kinetic energy as they had before the melting. So, the water is at the same temperature at the moment after the melting that the ice was at the moment before the melting.

In none of these changes of state is the heat (energy) that is input or output

used to change the speed of the molecules. The average speed of the

molecules is the same before and after a phase change, and so is the average

kinetic energy.

Heat (energy)

is transferred into the

ice.

The heat is used to break

the bonds between

molecules, not to increase the average

kinetic energy of the

molecules.

Since the bonds among the ice

molecules have been broken, water

is formed. The water molecules, at this moment, have the

same average kinetic energy as

they did when they were ice.

Since the ice and water molecules

both have the same average kinetic energy, they are at the same Kelvin temperature.

So, how could there be a change in heat during a state change without a change in temperature?

All phase changes involve either an increase or decrease of intermolecular forces. For each of the phase changes below, choose whether intermolecular forces increase or

decrease.

There are three primary phases of matter: solid, liquid and gas. A solid becoming liquid is called melting or

fusion. A solid becoming gaseous is called sublimation. A liquid becoming solid is called freezing.

A liquid changing to gas is called boiling or evaporation. A gas changing into a solid is called

deposition, and a gas changing into a liquid is called condensation. Half of these are endothermic, meaning they absorb heat from their surroundings. The others

are exothermic, meaning they release heat.


decrease.

PHASE CHANGE

NAME INTERMOLECULAR FORCES

Increase or Decrease?

SOLID

LIQUID

Melting or

Fusion

Increaseor

Decrease


decrease.

PHASE CHANGE



LIQUID

GASEvaporation

Increaseor

Decrease


decrease.

PHASE CHANGE



GAS

SOLIDDeposition

Increaseor

Decrease


decrease.

PHASE CHANGE



GAS

LIQUIDCondensati

on

Increaseor

Decrease


decrease.

PHASE CHANGE



SOLID

GASSublimation

Increaseor

Decrease


decrease.

Very Good!!! For any given substance,

intermolecular forces will be greatest in the solid

state and weakest in the gas state.


decrease.

PHASE CHANGE



LIQUID

SOLID

Freezing or

Crystallization

Increaseor

Decrease

LATENT HEAT

When a substance changes its state from a solid to liquid or from a liquid to a gas heat energy is needed. This energy is used not to heat up the substance but to separate the molecules from each other.

This energy is called LATENT HEAT energy.While a solid is melting and while a liquid is boiling there is no temperature change. The temperature only changes when the change of state is complete.

It is the “hidden” heat when a substance absorbs or releases heat without producing a change in temperature of the substance, e.g. during a change of phase.

Substance(at 1 atm)

Melting PointTm (K)

Heat of FusionLf (cal/g)

Boiling PointTb (K)

Heat to VaporizeLv (cal/g)

Hydrogen 13.8 14.0 20.3 108

Oxygen 54.4 3.3 90.2 50.9

Nitrogen 63.3 6.1 77.3 48.0

Ethyl Alcohol

156 24.9 351 205

Mercury 234 2.7 630 70.0

Water 273.15 79.7 373.15 539

Lead 600 5.9 2023 208

Aluminum 932 94.5 2740 2500

Gold 1336 15.4 2933 377

Copper 1359 32.0 1460 1210

Iron 1808 69.1 3023 1520

Latent Heats Table

Specific and Latent Heat Values

Specific Heat Latent Heat of Fusion

Material (cal/g °C) (J/kg K) (cal/g) (J/kg)

Aluminum

0.215 900 94.5 3.96x105

Copper 0.092 385 49.0 2.05x105

Iron 0.107 448 63.7 2.67x105

Lead 0.031 130 5.5 0.23x105

Brass 0.092 385 Unknown Unknown

Magnesium

0.245 1030 88.0 3.7x105

Zinc 0.093 390 27.0 1.1x105

Styrofoam

0.27 1131 Unknown Unknown

Air 0.240 1006 N/A N/A

Water 1.000 4190 N/A N/A

Ice 0.500 2095 79.7 3.34x105

Latent Heat of Fusion or Liquefaction

The amount of heat required per unit mass of a substance at melting point (constant) and under 1 Atmosphere pressure to convert it from the solid to the liquid state is called Latent Heat of Fusion. L = Q/m Calorie/gm or

K.Calorie/kg

When a substance is changing its state, the temperature of the substance remains constant. For example boiling water at 100 °C on changing state becomes steam (vapour) at 100 °C

Latent heatThe heat energy that is taken in or given out by a substance when it changes state is called latent heat. When a substance changes from solid to liquid, the latent heat involved is called the latent heat of fusion. When the substance changes from a liquid to a vapour, latent heat of vaporisation is involved.

EXAMPLE PROBLEMS

LATENT HEAT 1. How much heat energy is needed to change 2.0 kg of ice at 0°C to water at 0°C?

QL = mLQL = ( 2.0 kg ) ( 3.3 x 105 J/kg)QL = 6.6 x 105 J

EXAMPLE PROBLEMS

LATENT HEAT 2. How much heat energy is needed to change 0.50 kg of water at 100°C to steam at 100°C?

QL = mLQL = ( 0.50 kg ) ( 2.3 x 106 J/kg)QL = 1.2 x 106 J

EXAMPLE PROBLEMS

3. How much heat does a refrigerator need to remove from 1.5 kg of water at 20.0 °C to make ice at 0°C?

[Hint: find heat removed for water at 20.0°C to water at 0°C, then find latent heat for water at 0°C to ice at 0°C, and add the t]

Q total = mc∆ T + ml

EXAMPLE PROBLEMS

Heat with Temperature ChangeQ = mc∆TQ = (1.5 kg)(4.2 x 103 J/kgºC)(20.0ºC)Q = 1.25 x 105 J

Latent HeatQL = mLQL = (1.5 kg)(3.3 x 105 J/kg)QL = 4.95 x 105 J

The total amount of heat needed is:Q + QL1.25 x 105 J + 4.95 x 105 J6.2 x 105 J

Latent Heat ApplicationsWater has one of the highest latent heat of fusion values of all substances and

therefore has several applications.1. Picnic CoolersThe specific latent heat of fusion for water is 3.3 x 105 J/kg. That is, 3.3 x 105 J of heat are needed or absorbed to melt 1 kg of ice. The heat required to melt the ice comes from the food or drinks in the cooler. Since heat leaves the food, it gets cold.

2. Preventing Frost DamageWhen 1 kg of water freezes it releases 3.3 x 105 J of heat energy. Farmers use this principle to prevent frost damage to their orchards and other crops. When a frost is predicted, farmers will turn on the water sprinklers. As the water falls on the plants and starts to freeze, heat is released to the surroundings and plants. The heat helps the plants stay warm enough to prevent damage. This only works when the temperature does not drop much below freezing.

3. Cooling Off When WetYou can cool off your body by just coming out of a shower or swimming pool if you do not dry off with a towel. It takes 2.3 x 106 J of heat to vaporize 1 kg of water. When you are wet, heat from your body is used to vaporize the water. Because heat leaves your body, you feel cool. If you don't want to cool off, just dry yourself well with a towel.

4. Avoid Steam BurnsWhen steam condenses it releases heat to the surroundings. When just 1 mL of steam water condenses it releases 2.3 x 103J of heat. Then the condensed water cools from 100°C to 37°C (body temperature), so even more heat is released and absorbed by the skin. That's why a steam burn is much worse than a burn from boiling water.

Q = mL

Heat absorbed or released as the result of a phase change is called latent heat. There is no temperature change during a phase change, thus there is no change in the kinetic energy of the particles in the material. The energy released comes from the potential energy stored in the bonds between the particles.

EXOTHERMIC (warming processes)condensation

warmer in the showersteam radiators

freezingorange growers use ice to stop oranges from freezing

depositionsnowy days are warmer than clear days in the winter

ENDOTHERMIC (cooling processes)evaporation/boiling

sweatalcohol is "cool"

meltingmelting ice in drinks

sublimationcooling with dry ice

Summary

All phase changes …

take place at a specific temperature.take place without a change in temperature. (There is no temperature change during a phase change.)involve changes in internal potential energy.release or absorb latent heat.

Endothermic phase changes absorb heat from the environment. (They are cooling processes.)Exothermic phase changes release heat to the environment. (They are warming processes.)

Scattered thoughts …

Under extreme conditions of heat and exercise, an individual may sweat more than a liter of liquid per hour.

The interior of roasted meat can never reach temperatures higher than the boiling point of water until all the water is cooked out of it, at which point it would resemble shoe leather. The outside is quickly dried out, however, and can reach the temperature of the surrounding cooking medium.

Cocoa butter is unique among the fats in that it is very regular in composition; whereas most other fats are actually mixtures. This gives it a very definite point; unlike butter, which softens gradually. As it melts in your mouth, it absorbs latent heat. This makes chocolate bars taste "cool". Cocoa butter is remarkably uniform in composition and structure: only three fatty acids in the majority of its triglycerides, with the same one occupying the middle position. Pure cocoa butter is quite brittle up to about 34 (93 ), at which point it melts quite quickly.℃ ℉

The specific latent heat (L) of a material …

is a measure of the heat energy (Q) per mass (m) released or

absorbed during a phase change.

is defined through the formula Q = mL.

is often just called the "latent heat" of the material.

uses the SI unit joule per kilogram [J/kg].

There are three basic types of latent heat each associated with a

different pair of phases.

The following picture shows what happens to ONE gram (one ml) of water. The orange arrow on the top represents adding heat, from left to right, and the blue arrows represent releasing heat from right to left on the bottom.

So, let’s look at the whole process together in the form of a graph. The graph describes how the temperature of 1 gram of water in its three forms (ice, water, steam) changes as heat is added or taken away.

A to B: pure iceB to C: a mixture of water and ice all at 0 ºCC to D: pure waterD to E: a mixture of water and steam all at 100 ºC

E to F: pure steam

EXAMPLE PROBLEMS

EXAMPLE PROBLEMS

Change of Phase and Latent Heat

Science

change of heat

change of phaseor heat

term change of phase

liquid melting heat

changes of phase

astate change

bea change

change intemperature