4.3 specific latent heat
TRANSCRIPT
4.3 UNDERSTANDING SPECIFIC LATENT HEAT
Learning Outcome
State the transfer of heat during a change of phase does not cause a change in temperature.
Define specific latent heat , (l ) State that l = Q/m Determine the specific latent heat of fusion Determine the specific latent of vaporisation Solve problems involving specific latent heat.
Process of changing phase
Solid Liquid
gaseous
Melting
Boiling
Heat is absorbed during melting and boiling
Solid Liquid
gaseous
Solidification
Condensation
Heat is released during condensation and solidification
Process of changing phase
Heating curve
Melting point
temperature
time
A
B C
0
Boiling point
temperature
time
A
B C
0
solid
Solid +
liquid
liquid
Liquid + gaseous
AB : heat absorbed raise the temperature
BC : heat absorbed change the phase without changing the temperature
Melting Boiling
Solid to liquid Liquid to gaseous
Cooling Curve
Freezingpoint
Boiling point
time
liquid
gaseous
PQ : heat released cause a drop in temperature
QR : heat released change the phase without changing the temperature
Solidification Condensation
Liquid+ solid
temperature
time0
P
Q R
S
temperature
0
P
Q R
SLiquid +
gaseousLiquid to solid Gaseous to liquid
Change of phase ………
Heat energy is transferred
temperature remains constant even though there is transfer of heat.
occurs at a particular temperature.
steam
Melting point
Boiling
point
Freezing point
absorbedreleased
Until the whole
process is completed
Latent Heat
The heat absorbed or the heat released at constant temperature during a change of phase is known as latent heat.
1
Latent heat of fusion
Solid liquid Liquid gaseous
Latent heat of vaporisation
Latent heat of fusion
Melting
Used to loosen up bonds
between the particles
The particles are able to vibrate and
move among each other
Latent heat of fusion
Latent heat of fusion has to be removed for a liquid to solidify at its freezing point.
Vice versa ….
Latent heat of vaporisation
Boiling
Used to completely break the bonds
between the particles
The particles are free to move .
Latent heat of vaporisation
Latent heat of vaporisation has to be released for vapour to condensed at its boiling point
Vice versa ….
Specific Latent Heat
The amount of heat required to change the phase of 1 kg of the substance at a constant temperature.
The specific latent heat ,
Q = latent heat m = mass of the substance
and
Qm
Q = mlQm
J kg-1
Specific Latent Heat
The specific latent of fusion is the amount of heat required to change 1 kg of the substance from solid to liquid phase without a change in temperature.
The specific latent of vaporisation is the amount of heat required to change 1 kg of the substance from liquid to gaseous phase without a change in temperature.
Worked example
How much heat must be removed from 0.75 kg of water at 0o C to make ice cubes at 0oC ?
Pg 120
Solution
Change phase
No change in temperature
Use Q = m l
Specific latent heat of fusion
Heat , Q = ml
= 0.75 x 3.36 x 105
= 2.52 x 105J
Worked example
How much heat must be supplied to 0.60 kg of water at 25o C to change it to steam at 100oC ?
Pg 120
Solution Change phase
Change in temperature
Use Q = mc and Q = m l
Heat supplied= Heat to raise temp + Heat to boil = mc + ml
= 0.60(4200)(75) + 0.60(2.26x106)
= 1.545 x 106 J
Specific latent heat of vaporisation of water = 2.26x106 J kg-1
Specific heat capacity of water , c = 4200 J kg-1 oC-1
Evaluation
1.
T / oC
t /min0
P
QR
ST
U
Diagram shows the heating curve of a substance. The substance is a mixture of liquid and gas at
A PQB QRC RSD ST
solid
Solid + liquid
liquid
Liquid + gas
Evaluation
2. Latent heat of fusion is absorbed to
A change liquid into gas
B loosen the bonds between particles
C completely break up bonds between
particles.
D increase the kinetic energy of the particles
Heat capacity
Latent heat of vaporisation
Latent heat of
vaporisation
Evaluation
3.
Which of the above process absorbed
heat ? A P and R C Q and S B P and Q D R and S
P = melting R = condensation
Q = boiling S = solidification
Exercise
Mastery Practice 4.3 pg 121
Q 1, 2, 3