11 Latent Heat & Specific Latent Heat

LATENT HEAT&

SPECIFIC LATENT HEATTOPIC 11: THERMAL PROPERTIES OF MATTER

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WHAT HAPPENS TO MATTER WHEN THERMAL ENERGY IS

APPLIED TO IT?

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OF THE MATTER

THERMAL ENERGY

CAN

Change the temperature

Change the state

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FOCUS ONCHANGING THE

STATE OF THE SUBSTANCE

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WHAT DO WE KNOW?When heat is applied to a substance.

The substance may undergo changes of state.

Heat Energy Applied

Change in State of substance

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WHAT DO WE KNOW?When heat is removed from a substance.

The substance may undergo changes of state.

Heat Energy Removed

Change in State of substance

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LATENT HEATThe thermal energy absorbed or released

during a change of state.

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Energy absorbed Energy released

The unit of latent heat is Joule.

WHY IS ENERGY NEEDED

DURING CHANGES OF STATE?

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LATENT HEATIt is used to break the substance’s

intermolecular bonds during changes of state.

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Intermolecular bonds are forces of attraction that holds the particles together.

SOLID LIQUID GAS

BreakingBonds

BreakingBonds

LATENT HEATIt is used to create the substance’s

intermolecular bonds during changes of state.

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Intermolecular bonds are forces of attraction that holds the particles together.

SOLIDLIQUIDGAS

CreatingBonds

CreatingBonds

LATENT HEATAll the thermal energy is used to break / create the

substance’s intermolecular bonds during changes of state.

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SOLIDLIQUIDGAS

CreatingBonds

CreatingBonds

BreakingBonds

BreakingBonds

None of the thermal energy is used to change the kinetic energy of the particles during changes of state.

There will be no change in the temperature of the substance.

LATENT HEATThe thermal energy absorbed or released

during a change of state.

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Latent Heat of

Fusion

Latent Heat of

Vaporisation

The two types of latent heat:

LATENT HEAT OF FUSIONThe thermal energy required to change a substance

from solid to liquid state, or vice versa, without a change in temperature.

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SOLID LIQUID

L f

L f

The SI unit of latent heat is Joule.

LATENT HEAT OF VAPORISATIONThe thermal energy required to change a substance

from liquid to gaseous state, or vice-versa, without any change in temperature.

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LIQUID GAS

Lv

Lv

The SI unit of latent heat is Joule.

“The latent heat of fusion of a block of ice is 5000 J”

What does this statement means?

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The heavier the mass of the substance, more thermal energy needed to change state,

the bigger the latent heat.

m

L

mL

LATENT HEAT

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For any states of matter,

EXAMINE THE LATENT HEAT PER

UNIT MASS OF THE SUBSTANCE

OR THE SPECIFIC LATENT

HEAT

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“The specific latent heat of vaporisation of steam is

2.26 MJ/kg”

What does this statement means?

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SPECIFIC LATENT HEATThe thermal energy required to change the

state of 1 kg substance, without any change in temperature.

SpecificLatent Heat

Latent HeatLl = m

Mass

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The SI unit of specific latent heat is J/kg.

SPECIFIC LATENT HEAT OF FUSION

The thermal energy required to change 1 kg of the solid to liquid, or vice versa, without a change in temperature.

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SOLID LIQUID

l f

l f1 kg 1 kg

The SI unit of specific latent heat is J/kg.

SPECIFIC LATENT HEAT OF FUSION

The thermal energy required to change 1 kg of the solid to liquid, or vice versa, without a change in temperature.

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The SI unit of specific latent heat is J/kg.

SpecificLatent Heat of Fusion

Latent Heat of Fusion

Ll = mMass

ff

SPECIFIC LATENT HEAT OF VAPORISATION

The thermal energy required to change 1 kg of the liquid to gas, or vice-versa, without any change in temperature.

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LIQUID GAS

l v

l v1 kg 1 kg

The SI unit of specific latent heat is J/kg.

SPECIFIC LATENT HEAT OF VAPORISATION

The thermal energy required to change 1 kg of the liquid to gas, or vice-versa, without any change in temperature.

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The SI unit of specific latent heat is J/kg.

SpecificLatent Heat of

Vaporisation

Latent Heat of

VaporisationLl = m

Mass

vv

Which process is easier to achieve?Melting the ice or Boiling water.

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Ice

= 336 000 J/kgl f

Steam

= 2 260 000 J/kgl v

Thermal Energy = Power X Time Taken

WHAT ARE THE SOURCES OF THERMAL ENERGY?

Bunsen Burner Electric Burner

= P x tQ Heater

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An ice-cream has a mass of 150g. If the specific latent of fusion of ice

cream is 340000 Jkg-1, find the thermal energy required to

melt the ice cream.

EXAMPLE EXERCISE 1

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EXAMPLE EXERCISE 1

=Qice mice X

= 0.150 kg 340 000 J/kgX

=

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Mass Specific Latent Heat of Fusion of Ice

= X

Thermal Energy to melt Ice = Latent Heat of Fusion of Ice

icelf

Qice

A heater which supplies thermal energy at a constant rate of 1000 W is completely

immersed in a 3.00 kg block of ice at 00C.

The block of ice takes 1020s to melt completely.

Calculate a value for the specific latent heat of fusion of ice.

EXAMPLE EXERCISE 2

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EXAMPLE EXERCISE 2

=

Thermal Energy supplied to ice by heater

=Thermal Energy use to

melt the ice

mice XP X t =heater

Q =heater

= 3kgX X1000W 1020s

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icelf

iceLf

icelficelf

A jet of steam at 1000C is directed for a short time at a large block of ice at 00C.

Some of the steam condenses to form water and some ice is melted. The condensed steam

forms 0.40 kg of water at 00C.

EXAMPLE EXERCISE 3

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EXAMPLE EXERCISE 3

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Calculate the thermal energy given out by this mass of steam in changing to water at 1000C.

Calculate the thermal energy given out by this water as it cools to the temperature of ice.

specific latent heat of vaporisation of water = 2200 kJkg-1

specific heat capacity of water = 4.2 kJkg-1K-1

EXAMPLE EXERCISE 3

=Qsteam m steam X

= 0.40 kg 2200 kJ/kgX

=

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Mass Specific Latent Heat of Vaporisation

= X

Thermal Energy to condense Steam

= Latent Heat of Vaporisation of Steam

steaml v

Qsteam

EXAMPLE EXERCISE 3

=Q water

=

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Mass Specific Heat Capacity

=

XThermal Energy Lost by water

=

mwater cwater water

X Change in Temperature

0.4kg 4.2kJ/kg0C 0C(100- 0)XX

Q water

EXAMPLE EXERCISE 4

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What is the amount of energy required to change 10g of ice at 0ºC

to water at 20ºC?

specific latent heat of fusion of ice = 336 J/g

specific heat capacity of water = 4.2 J/gºC

EXAMPLE EXERCISE 4

Total Thermal Energy Needed

= Thermal Energy to melt Ice

=Q Total

= mwater cwater water

=

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Thermal Energy to increase temperature

of water+

Q WatericeLf +

m ice icel f + (( ) )

10g 4.2J/g0C 0C(20 - 0)XX ][+10g 336J/g X( )

=Q Total

EXAMPLE EXERCISE 5

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A glass contains 250g of hot tea at 90ºC.What is the minimum amount of ice at 0ºC

needed to cool the drink to 0ºC?

specific latent heat of fusion of ice = 336 J/g

specific heat capacity of tea = 4.2 J/gºC

EXAMPLE EXERCISE 5

Initial Final

Hot Tea

= 4.2 J/g0C c tea

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Ice00C 900C

Iced Tea00C

250g

mice = ?

= 336 J/gl icef

EXAMPLE EXERCISE 5

=

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=mtea c tea tea m ice icel f

250g 4.2J/g0C 0C(90 - 0)XX ][ 336J/g X( )

Q Tea iceLf=

Thermal Energy lost by hot tea

Thermal Energy gained by melting ice=

m ice

m ice

SUMMARIZATION

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QL =

The amount of thermal energy required to change the state

of a substance without a change in temperature.

“Latent Heat, L”

Ll =m

The amount of thermal energy required to

change the state of 1 kg of a substance without a change in temperature

“Specific Latent Heat, l”

J/kg Joule (J)

SUMMARIZATION

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The thermal energy required to change a

substance from solid to liquid state, or vice versa, without a change in temperature.

“Latent Heat of Fusion”

The thermal energy required to change a

substance from liquid to gaseous state, or vice-

versa, without any change in temperature.

“Latent Heat of Vaporisation”

SI unit = Joule

“L ”f “L ”v

SI unit = Joule

The heavier the mass of the substance, more thermal energy needed to change state,

the bigger the latent heat.

m

L

mL

LATENT HEAT

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For any states of matter,

Ll =mf

f Ll =mv

v

SUMMARIZATION

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The thermal energy required to change 1 kg of the solid

to liquid, or vice versa, without a change in

temperature.

“Specific Latent Heat of Fusion”

The thermal energy required to change 1 kg of the liquid to gas, or vice-

versa, without any change in temperature.

“Specific Latent Heat of Vaporisation”

SI unit = J/kg SI unit = J/kg

SUMMARIZATION

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Thermal Energy supplied to substance by heater

=Thermal Energy to change the state of the substance

=

=

SUMMARIZATION

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Thermal Energy lost by Substance A

=Thermal Energy gained by

Substance B to change state

=

=

SUMMARIZATION

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Total Thermal Energy Needed

=Thermal Energy

Needed to Change State

Thermal Energy Needed to Change

Temperature+

=Q Total Q substancesubstanceL +

=Q Total

=Q Total

=Q Total