Intermolecular Forces of
Liquids
and Solids; Phase
Changes
GENERAL CHEMISTRY-Q3W2
Learning Competency
 Describe
the nature of the following
phase changes in terms of energy
change and the increase or
decrease in molecular order: solidliquid, liquid vapor, and solid-vapor
(STEM_GC11IMF-IIIa-c-106)
Specific Learning Outcomes
At the end of the lesson, the learners will be able to:
 • describe the transitions among gas, liquids, and solids
in terms of increase or decrease in molecular order;
 • explain what is happening as a system is heated and
relate phase changes to heat and temperature
changes;

• explain solid-liquid, liquid vapor, and solid-vapor
transitions in terms of amount of energy change; and
 • calculate heat changes in phase and temperature
changes.

Relevant vocabulary that will be
used in the lesson:

Fluid - A gas or a liquid; a substance that can flow.

Phase - A homogeneous part of a system in contact with
other parts of the system, but separated by well-defined
boundaries.

Solid - A phase of matter with definite shape and volume.

Liquid - A phase of matter with definite volume but no
definite shape.

Gas- A phase of matter with no definite shape or volume
of its own.

Intermolecular forces - are attractive forces
between molecules.

Phase changes - Transformations of matter from
one phase to another.

1. Melting - A phase change from solid to liquid.

2. Vaporization - A phase change from liquid to gas.

3. Sublimation - A phase change from solid to gas.
 4.
Condensation - A phase change from gas
to liquid.
 5.
Freezing - A phase change from liquid to
solid.
 6.
Deposition - A phase change from gas to
solid.
 Exothermic
process - Process that gives off
or release heat to the surroundings.
 Endothermic
process - Process that
absorbs heat from the surroundings.
 Specific
heat of a substance - The amount
of heat needed to raise the temperature
of 1 gram of a substance by 1 *C.
What phase(s) of matter exist in the
following images?
 Phase
changes are transformations of matter
from one physical state to another. They occur
when energy(usually in the form of heat) is
added or removed from a substance. They are
characterized by changes in molecular order;
molecules in the solid phase have the greatest
order, while those in the gas phase have the
greatest randomness or disorder.
What
changes in
molecular order occur
during phase changes?
The figure below illustrates the difference in
molecular order of a substance in the solid, liquid
and gaseous states.
The next figure shown below summarizes
the types of phase changes.

The change from solid to liquid is melting, liquid to
gas is vaporization, and solid to gas is sublimation.
These changes take place when heat is absorbed
(heat gained). They are endothermic processes.

The reverse change from gas to liquid is
condensation, gas to solid is deposition, and liquid to
solid is freezing. These changes give off heat (heat
lost) and are exothermic processes.
How does a change in energy
affect phase changes?

Phase changes occur when heat is added or removed from a
substance.

When a substance is heated, the added energy is used by the
substance in either of two ways:

a. The added heat increases the kinetic energy of the particles
and the particles move faster. The increase in kinetic energy is
accompanied by an increase in temperature.

b. The added heat is used to break attractive forces between
particles. There is no observed increase in temperature when
this happens. Often a change in the physical appearance of
the substance is observed, such as a phase change.
Conversely, the removal or release
of heat results in two ways:
 a. A decrease
in kinetic energy of the
particles. The motion of the particles slow
down. A decrease in temperature is
observed.
 b. Forces of attraction are formed, and a
phase change may occur. No change in
temperature is observed.
 The
change in temperature of a substance as
it is being heated can be shown in a graph
called the heating curve. The heating curve
is a plot of temperature and heat added to the
substance. Often, time is used instead of heat
added in the abscissa, because it is assumed
that heat is uniformly added per unit time.

In both the heating and cooling curves, there are
certain portions where the temperature changes as
heat is being added or removed, and portions where
the temperature remains constant even if heat is
being added or removed. What is happening at these
portions?

1. When heat change is accompanied by a change in
temperature, a change in kinetic energies of the
particles in the substance is occurring. The particles
are either moving faster or slowing down.

2. When temperature remains constant during heat change,
the particles move at the same speed. The heat added or
removed is involved in breaking or forming attractive forces. A
phase change occurs at this temperature: solid melts or liquid
freezes at the melting point, which is also the freezing point;
liquid boils, or gas condenses at the boiling point, which is
also the condensation point.

During phase changes, two physical states of the substance
exist at the same time. When addition or removal of heat is
stopped at this temperature, the two physical states will
interconvert from one state to the other, and will be at
equilibrium.
MELTING AND FREEZING: SOLIDLIQUID EQUILIBRIUM

When a solid is heated, its temperature increases
until it reaches its melting point. At this temperature,
the average kinetic energy of the molecules has
become sufficiently large to begin overcoming the
intermolecular forces that hold the molecules of a
solid state together. The heat absorbed is used to
break apart more and more of the molecules in the
solid. The transformation of solid to liquid is called
melting, and the reverse process is called freezing.

During the transition, the average kinetic energy of the molecules does not
change, so the temperature stays constant. The melting point of a solid or
the freezing point of a liquid is the temperature at which solid and liquid
phases coexist in equilibrium.

• Melting points are distinct for each substance. It is dependent on the
strength of attractive forces that hold the particles in the solid. The stronger
the attractive forces that hold the particles in the solid, the higher is the
melting point of the substance.

• The melting (or freezing) point of a substance when the external pressure
is 1 atm pressure is called its normal melting (or freezing) point. For water,
this is 0oC.

A practical illustration of this dynamic equilibrium is
provided by a glass of ice water. As the ice cube
melts to form water, some of the water between ice
cubes may freeze, thus joining the cubes together.

• When heat is added to this system at equilibrium,
ice will continue to melt until all have been
transformed to the liquid state. The amount of heat
needed to convert the solid to the liquid state at the
melting point is called the heat of fusion of the
substance.
MOLAR HEAT OF FUSION AND
MELTING POINT
 Heat
of fusion is an extensive property. The
actual amount of energy involved in the
transformation of a substance from solid to
liquid is dependent on the amount of sample
used. Thus, this property is often expressed in
terms of molar quantities of sample.
 Molar
heat of fusion (ΔHfus) is the energy
required to melt 1 mole of a solid.

Like melting points, heats of fusion are influenced by
the strength of attractive forces that exist between
particles in the solid. The stronger the attractive
forces that hold the particles of the solid together, the
larger is the heat of fusion.

Cooling a substance has the opposite effect of heating it, as
can be seen from the cooling curve.

• If heat is removed from a liquid at a steady rate, its
temperature should decrease until the freezing point is
reached. As the solid is being formed, heat is given off by the
system, as attractive forces form and become stronger
between particles. Even if heat is being removed, the
temperature of the system remains constant over the freezing
period.

• After all the liquid has frozen, the temperature of the solid
drops.
BOILING AND CONDENSING:
LIQUID-VAPOR EQUILIBRIUM

In the liquid phase, there are still attractions among its particles. The
particles are still in contact with each other but are not locked into fixed
positions and are free to move past each other. Although they lack the total
freedom of gaseous molecules, these molecules are in constant motion.

• When a liquid is heated, its temperature increases as the kinetic energy
of the molecules increases.

When the molecules have sufficient energy to escape from the surface, a
phase change occurs.

Evaporation or vaporization is the process in which a liquid is
transformed into a gas. The temperature at which this occurs is the boiling
point of the substance. While the liquid vaporizes, the temperature
remains constant.

The boiling point is a characteristic of each substance, and is dependent on
the strength of attractive
forces that hold the particles or molecules in the liquid state. It is also dependent
on the external or atmospheric pressure. The boiling point of a liquid at 1 atm
pressure is called its normal boiling point. For water, this is at 100 C.
 • The reverse of vaporization or boiling is called condensation, the change
from the gas phase to the liquid phase. Condensation occurs because a
molecule strikes the liquid surface and becomes trapped by intermolecular
forces in the liquid. This process occurs at the same temperature when the
liquid vaporizes into the gaseous state. The boiling point can thus be also
called condensation point (dew point), and occur at the same temperature.
 • At the boiling point, both liquid and gaseous states of the substance are
present, and the transformations of liquid to gas and gas to liquid happen at
the same time.
 • At 100 C and 1 atm, the dynamic equilibrium for water and steam
o
O
 As
heat is absorbed, some water will boil off
but the temperature remains at 100 OC
(373.15 K) until all the liquid has vaporized.
The amount of heat absorbed by the sample
as the liquid transforms into gas is called heat
of vaporization.When all of the sample has
turned into gas, further heating will cause the
temperature of the gas to increase again.
MOLAR HEAT OF VAPORIZATION
(ΔH ) AND BOILING POINT
vap

The heat of vaporization is an extensive property and is thus
dependent on the amount of sample undergoing phase
change. Hence, published quantities of heats of vaporization
specify the amount of substance, and is often expressed as
molar heat of vaporization.

Molar heat of vaporization (ΔHvap) is defined as the energy
(usually in kilojoules) required to vaporize 1 mole of a liquid at
a given temperature, usually, at the boiling point. The molar
heat of vaporization of water at 100oC is 40.8 kJ/mol.
SOLID-VAPOR EQUILIBRIUM

In a solid, the particles may be in fixed positions, but
they are able to vibrate in place and with increasing
intensity as temperature increases. When particles
are able to acquire enough energy to break attractive
forces with adjacent particles, the energetic particles
may move into the gaseous state. This phase
change is called sublimation. One of the most
familiar examples of sublimation is that of dry ice.
The figure below shows iodine subliming into a
purple gas.

Sublimation is the process in which molecules go
directly from solid into vapor phase. The reverse
process is called deposition, where molecules make
a transition directly from vapor to solid.
MOLAR HEAT OF SUBLIMATION

Molar heat of sublimation (ΔHsub) of a substance is the amount
of energy that must be added to a mole of solid at constant
pressure to turn it directly into a gas, without passing through the
liquid phase. This enthalpy change associated with sublimation is
always greater than that of vaporization even if both sublimation
and evaporation involve changing a substance into its gaseous
state because in sublimation, the starting physical state of the
substance is the solid state, which is lower in energy than the liquid
state where vaporization starts.

Sublimation requires that all the forces are broken between the
molecules (or other species, such as ions) in the solid as the solid is
converted into a gas..
A comparison of the magnitudes of these
thermochemical quantities
can be seen from the heating curve shown below.