This document provides an overview of phase changes and energy changes that accompany phase changes. It discusses the molecular order in different phases of matter (solid, liquid, gas) and how phase changes result in an increase or decrease of molecular order. Phase changes that decrease molecular order are endothermic and require energy input, while those that increase order are exothermic and release energy. Heating and cooling curves are introduced to track temperature and energy changes during complex phase change processes. Formulas are provided to calculate energy for temperature changes and phase changes. Examples are given to practice these calculations.

1. Power
competenc
y
Apply the properties of liquids
and solids to the nature of
forces in designing a simple
investigation to determine the
effect on boiling point or
freezing point when a solid is
dissolved in water

2. Sub-
competency
● interpret the phase diagram of water and
carbon dioxide STEM_GC11IMF-IIIa-c-107
● Determine and explain the heating and
cooling curve of a substance
STEM_GC11IMF-IIIa-c-10

3. Learning
targets
● I can interpret the phase
diagram of water and carbon
dioxide.
● I can determine and explain
the heating and cooling curve
of a substance

4. 4
Observe an ice cube melting. Have you ever wondered
what happens in the molecular level when ice melts?

5. 5
Intermolecular forces
and the energy from
the surroundings
interact in such a way
that will define what
phase of matter the
substance will take.

6. 6
In this lesson, you are going to learn about phase
changes and how they occur on a molecular level.
You will also learn to calculate the change of energy
that occurs whenever there is a transition between
phases of matter.

7. General Chemistry 2
Science, Technology, Engineering, and Mathematics
Lesson 2.1
Energy Changes Accom
panying
Phase Changes

8. 8
How can phase change
affect the molecular order
in matter?

9. 9
● Phase is defined as a homogeneous state in which the
substance has a uniform composition and governed by
the same intermolecular forces throughout the
material.
● There are three fundamental phases of matter:
solid, liquid, and gas.
Phases and Phase Changes

10. 10
In solids, molecules are
tightly packed and can be
compared to people inside a
train during rush hour.
Phases and Phase Changes

11. 11
In liquids, there is more
space. This can be analogous
to people walking in a busy
street.
Phases and Phase Changes

12. 12
The gaseous state has very
large spaces between them.
This figure is an attempted
representation of this phase,
but in reality gas molecules
are much farther part.
Phases and Phase Changes

13. 13
Phase Changes

14. 14
Phase Changes
Phase changes are
accompanied by a change
that occurs as a result of
energy interactions and the
intermolecular forces in the
substance itself.
Molecular Order in Phase Changes

15. 15
In solids, molecules are well
ordered. The particles are
not free to move around.
When a solid transitions to a
liquid or gas, there is a
decrease in the order in the
material.
Molecular Order in Phase Changes

16. 16
The same is true for a liquid
that would turn to a gaseous
state. There is a decrease in
the order in the material
because gas molecules are
free to move around.
Molecular Order in Phase Changes

17. 17
The gaseous state has the
most disorder because of the
large spaces these molecules
can move around. Gas
transitioning to liquid or gas
would result in decrease in
order.
Molecular Order in Phase Changes

18. 18
In summary, there is a decrease in molecular order if
the transition is
Molecular Order in Phase Changes
more condensed
state
less condensed
state

19. 19
How about if you reverse the process? What is the
expected change in molecular order?
Molecular Order in Phase Changes
more condensed
state
less condensed
state

20. 20
● When molecular order decreases, this results in
absorption of energy. It is an endothermic process.
● When molecular order increases, this results in
release of energy. This is an exothermic process
Energy in Phase Changes

21. 21
The amount of heat energy
transferred from the
surroundings to the
substance is a change in
enthalpy, represented by
the symbol ΔH.
Energy in Phase Changes

22. 22
● For an endothermic process, the change in enthalpy is
always positive.
● When ice melts, heat energy from the environment
causes the water molecules to break free from the
intermolecular forces that hold it in the solid state.
Energy in Phase changes

23. 23
● For an exothermic process. The change in enthalpy will
always be negative.
● When water condenses on a leaf on a cold morning,
Intermolecular forces become stronger, and the spaces
between molecules become closer. Heat is released to
the environment.
Energy in Phase changes

24. 24
Energy in phase changes can be likened to breaking sticks.
Energy in Phase Changes

25. Tips
25
To remember if a process is
endothermic or exothermic:
○ When heat is absorbed, it is
endothermic; heat enters the
system.
○ When heat is released, it is
exothermic; heat exits the
system.

26. Tips
26
● In endothermic processes, heat is
added (+). Therefore, ΔH is positive
(+).
● In exothermic processes, heat is
removed (-). Therefore, ΔH is
negative (-).

27. 27
In endothermic processes like melting, evaporation and
sublimation, energy is supplied to overcome the
intermolecular attractive forces that hold the particles in
their present state.
Enthalpy and Phase Changes

28. 28
● The temperature at which a
substance melts is called the
melting point.
● The amount of heat needed
to melt a substance is called
the heat of fusion or the
enthalpy of fusion
symbolized by ΔHfus.
Enthalpy and Phase changes

29. 29
Energetics of Phase Changes: A Recall
For phase changes at constant P,
where q = heat absorbed or released (J);
n = amount of substance
(moles), and
ΔH = molar heat associated with phase change
(J/mol).

30. 30
Energetics of Phase Changes: A Recall
For temperature changes,
where q = heat absorbed or released, J;
m = mass, g;
c = specific heat, J/(g x OC-1); and
ΔT = change in temperature.

31. Let’s Practice!
31
How much energy is required to raise the
temperature of 70.0 g of water in the liquid state by
12 ºC, if the specific heat of water is 4.186 J/(g∙°C) ?

32. Let’s Practice!
32

33. Let’s Practice!
33
At a temperature of 0 ºC, how many grams of ice can
be melted by 500 joules of heat energy if the molar
enthalpy of fusion of water is 6.01 kJ/mol?

34. Let’s Practice!
34

35. 35
Heating and Cooling Curves
● Transformations of substances in real life involve
multiple phase changes and temperature changes.
● There are instances when a substance in its solid state
is converted into its gaseous state.
● In between phase changes, additional heat is needed
to satisfy temperature changes.

36. 36
Heating and Cooling Curves
The amount of heat in
complex phase
transformations can be
tracked in heating and
cooling curves.

37. 37
Heating and Cooling Curves
In these curves, the
temperature (y-axis) is
plotted against total heat
changes (x-axis).

38. 38
Heating and Cooling Curves
This heating curve shows
the heat associated with
the transformation of 1
mol of ice initially set at –
25 ºC and 1 atm to steam
at 125 ºC and 1 atm.

39. 39
Heating and Cooling Curves
How do we calculate the
amount of heat associated
with the entire process?

40. 40
How can one calculate the
heat associated with
consecutive phase
transformations?

41. 41
Heating and Cooling Curves
The amount of heat can be
calculated by adding all
heat absorbed from each
phase and temperature
change.

42. 42
Heating and Cooling Curves
The process can be divided
as represented by the
segments.

43. 43
Heating and Cooling Curves
Blue segments represent
temperature changes
without phase change,
while red segments
represent phase changes
without temperature
change.

44. 44
Heating and Cooling Curves
Segment BC
The ice cube starts to melt
from point B and ends with
point C.

45. 45
Heating and Cooling Curves
Segment DE
Water starts to boil from
point D and ends with
point E.

46. 46
Heating and Cooling Curves
Segment EF
Segment EF represents a
change in temperature of
steam from 100 ºC to 125
ºC.

47. 47
Heating and Cooling Curves
Total Heat
Total heat is additive, from
point A to F.
What is the total heat of the
phase change of ice to steam
from point A to F?

48. Bibliography
48
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General, Organic, and Biochemistry. Boston, MA: Cengage Learning, 2016. Print.
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Patrick M. Woodward, and Matthew Stoltzfus. Chemistry: The Central Science, 13th Edition. NJ:
Pearson, 2015. Print.
Chang, Raymond, and Kenneth A. Goldsby. General Chemistry: The Essential Concepts. New York:
McGraw-Hill, 2014. Print.
Petrucci, Ralph H, F G. Herring, Jeffry Madura, and Carey Bissonnette. General Chemistry: Principles
and Modern Applications. 2016. Print.
Silberberg, Martin S. Principles of General Chemistry. Boston: McGraw-Hill Higher Education, 2007.
Print.