Heats of Fusion and
Solidification
Ck12 Science
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Printed: May 14, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Heats of Fusion and Solidification
1
Heats of Fusion and
Solidification
• Define heat of fusion.
• Define heat of solidification.
• Perform calculations of energy changes during the transition from solid to liquid or liquid to solid.
What makes the ice melt?
Suppose you hold an ice cube in your hand. It feels cold because heat energy leaves your hand and enters the ice
cube. What happens to the ice cube? It melts. However, the temperature during a phase change remains constant. So
the heat that is being lost by your hand does not raise the temperature of the ice above its melting temperature of
0°C. Rather, all the heat goes into the change of state. Energy is absorbed during the process of changing ice into
water. The water that is produced also remains at 0°C until all of the ice is melted.
Heats of Fusion and Solidification
All solids absorb heat as they melt to become liquids. The gain of heat in this endothermic process goes into
changing the state rather than changing the temperature. The molar heat of fusion (∆Hfus ) of a substance is the heat
absorbed by one mole of that substance as it is converted from a solid to a liquid. Since the melting of any substance
absorbs heat, it follows that the freezing of any substance releases heat. The molar heat of solidification (∆Hsolid )
of a substance is the heat released by one mole of that substance as it is converted from a liquid to a solid. Since
fusion and solidification of a given substance are the exact opposite processes, the numerical value of the molar heat
of fusion is the same as the numerical value of the molar heat of solidification, but opposite in sign. In other words,
∆Hfus = −∆Hsolid . The Figure 1.1 shows all of the possible changes of state along with the direction of heat flow
during each process.
Every substance has a unique value for its molar heat of fusion, depending on the amount of energy required to
disrupt the intermolecular forces present in the solid. When 1 mol of ice at 0°C is converted to 1 mol of liquid water
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FIGURE 1.1
From left to right, heat is absorbed from
the surroundings during melting, evaporation, and sublimation.
Form right to
left, heat is released to the surroundings
during freezing, condensation, and deposition.
at 0°C, 6.01 kJ of heat are absorbed from the surroundings. When 1 mol of water at 0°C freezes to ice at 0°C, 6.01
kJ of heat are released into the surroundings.
H2 O(s) → H2 O(l)
∆Hfus = 6.01 kJ/mol
H2 O(l) → H2 O(s)
∆Hfus = −6.01 kJ/mol
The molar heats of fusion and solidification of a given substance can be used to calculate the heat absorbed or
released when various amounts are melted or frozen.
Enjoy the video about the heat of fusion at the link below:
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/64470
http://www.youtube.com/watch?v=jaaGqui9NVY
Sample Problem Heat of Fusion
Calculate the heat absorbed when 31.6 g of ice at 0°C is completely melted.
Step 1: List the known quantities and plan the problem.
Known
• mass = 31.6 g ice
• molar mass H2 O(s) = 18.02 g/mol
2
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Chapter 1. Heats of Fusion and Solidification
• molar heat of fusion = 6.01 kJ/mol
Unknown
• ∆H =? J
The mass of ice is first converted to moles. This is then multiplied by the conversion factor of
find the kJ of heat absorbed.
6.01 kJ
1 mol
in order to
Step 2: Solve.
31.6 g ice ×
6.01 kJ
1 mol ice
×
= 10.5 kJ
18.02 g ice 1 mol ice
Step 3: Think about your result.
The given quantity is a bit less than 2 moles of ice, and so just less than 12 kJ of heat is absorbed by the melting
process.
Summary
• Molar heats of fusion and solidification are defined.
• Calculations of heat changes during fusion and solidification are described.
Practice
Work on problems 4-5 at the link below:
http://ths.sps.lane.edu/chemweb/unit4/problems/heatcalc/index.htm
Review
Questions
1. In the transition from liquid to solid, is energy absorbed or released?
2. In the transition from solid to liquid, is energy absorbed or released?
3. How much energy is released when one mole of water at 0°C changes from liquid to solid?
• molar heat of fusion (∆Hfus ) : The heat absorbed by one mole of that substance as it is converted from a solid
to a liquid.
• molar heat of solidification (∆Hsolid ) : The heat released by one mole of that substance as it is converted
from a liquid to a solid.
References
1. Image copyright Cher_Nika, 2014. http://www.shutterstock.com .
2. CK-12 Foundation - Christopher Auyeung. .
3