CHAPTER 8
Latent Heat
Capacity
The use of an equation
Definition
• When a solid is heated, it may melt and
change its state from solid to liquid. If ice
is heated it becomes water. The opposite
process of freezing occurs when a liquid
solidifies.
• A pure substance melts at a definite
temperature, called the melting point; it
solidifies at the same temperature - the
freezing point.
Experiment: cooling curve of
ethanamide
• Half-fill a test-tube with ethanamide (acetamide) and
place it in a beaker of water. Heat the water until all
the ethanamide has melted.
• Remove the test-tube and arrange it as in figure with a
thermometer in the liquid ethanamide.
• Record the temperature every minute until it has fallen
to 70oC.
• Plot a cooling curve of temperature against time.
• What is the freezing (melting) point of ethanamide?
Types of latent heat
•
There exists two types of latent heat.
1 This occurs when solids becomes liquids
or vice-versa and is called specific latent
heat of fusion.
2 This occurs when liquids becomes gases
or vice-versa and is called specific latent
heat of vaporization.
Latent heat of fusion
• The previous experiment shows that the
temperature of liquid ethanamide falls until it
starts to solidify (at 82oC) and remains
constant till it has all solidified.
• The cooling curve in the following figure is for
a pure substance;
• The flat part AB occurs at the melting point
when the substance is solidifying.
• During solidification a substance loses heat to
its surroundings but its temperature does not
fall.
• Conversely when a solid is melting, the heat
supplied does not cause a temperature rise.
• For example, the temperature of a well-stirred
ice-water mixture remains at 0oC until all the ice
is melted.
• Heat which is absorbed by a solid during
melting or given out by a liquid during
solidification is called latent heat of fusion.
• Latent means hidden and fusion means
melting.
• Latent heat does not cause a temperature
change; it seems to disappear.
The specific latent heat of fusion of a
substance is the quantity of heat needed to
change unit mass from solid to liquid
without temperature change.
• It is measured in J/kg or J/g.
The latent heat equation
•
In general, we can write the ‘latent heat
equation’ as
Heat
Latent
received or = mass ×
Heat
given out
Capacity
i.e.
E = mL
•
Note that this equation does not depend on
temperature since there is no temperature
change during the latent heat.
Latent heat graph of water
Experiment: specific latent heat of
fusion of ice
• Place a 12V electric
immersion heater of
known power in a
filter funnel and pack
small pieces of ice
round it.
• Switch on the heater
for 3 minutes and find
the mass of water
which collects in a
beaker.
Experiment: specific latent heat of
fusion of ice
• Arrange the results as shown.
Power of immersion heater
Time heat supplied
 Heat supplied to ice
Mass of beaker empty
Mass of beaker + melted ice
 Mass of melted ice
= W(J/s)
=s
=J
=g
=g
=g
• Calculate the heat needed to melt 1g of ice. What are
causes of error in this experiment?
Latent heat of vaporization
• Latent heat is also needed to change a liquid
into a vapour.
• The reading of a thermometer in boiling water
remains constant at 100oC even though heat,
called latent heat of vaporization, is still being
absorbed by the water from whatever is
heating it.
• When steam condenses to form water, latent
heat is given out.
• This is why a scald from steam may be more
serious than one from boiling water.
The specific latent heat of vaporization of a
substance is the quantity of heat needed to
change unit mass from liquid to vapour
without change of temperature.
Table 1
• The following table gives the Latent heat capacity of
some common substances.
• Note carefully the units of the latent heat. They are
J/g or J g-1
• If you want to change them to kilograms, just multiply
by 1000.
• Ex. 334 J g-1 = 334 × 1000 = 334 000 J kg-1
Substance
Water
Ethanol
Ethanoic
acid
Chloroform
Mercury
Sulphur
Hydrogen
Oxygen
Nitrogen
oC
Specific
Specific latent
latent heat
heat of
of fusion
vaporization
J.g-1
J.g-1
334
0
2258
109
-114
838
192
17
395
74
11
54
60
14
25
-64
-39
115
-259
-219
-210
254
294
1406
449
213
199
oC
100
78
118
62
357
445
-253
-183
-196
Example 1
• How much heat is needed to change 20g of ice
at 0oC to water at 0oC?
• Answer: Since here we have only a change of
state then we are going to use the formula of
latent heat of fusion (because it is ice).
i.e.
E = mL
Latent heat of fusion of water = 334 Jg-1
So
E = 20 × 334 = 6680 J
Example 2
• How much heat is needed to change 50 g of
mercury from liquid at 357oC to gas at 357oC ?
• Answer:
Again we are going to use the
Latent heat equation since this involves only a
change of state.
Latent heat of vaporization of mercury = 294 J/g
E = 50 × 294 = 14700 J
Example 3
• How much heat is needed to change 50 g of ice
at 0oC to water at 10oC?
• Answer:
• This time we have to find the heat needed to
change 50g of ice at 0oC to water at 0oC by
using the latent heat equation.
• Then we have to find the heat needed to
change 50g of water at 0oC to water at 10oC by
using the specific heat capacity.
1)
Latent heat of fusion of water = 334 J/g
E = mL = 50 × 334 = 16700 J
2)
Specific heat capacity of water = 4200J/kgoC
Change to grams = 4200  1000 = 4.2J/goC
To change from 0oC to 10oC
E = mcT = 50 × 4.2 × 10 = 2100 J
Total Energy = 16700 J + 2100 J = 18800 J
Exercise 1
1) How much heat is needed to change 250 g of
Ethanol from solid at -114oC to liquid at
-114oC?
Ans: 27250 J
2) How much heat is needed to change 335 g of
water at 50oC to steam at 100oC?
Ans: ES = 70350
EL = 756430
ET = 826780 J