SPH3U: Thermal Energy and Changes of State
Date: __________
The energy and organization of the particles in a sample of matter determine the physical state,
or phase, of the matter. The three physical states of matter are:
-
Solid (particles have some energy)
Liquid (particles have more energy)
Gas (particles have the most energy)
The particles in all matter are in constant motion and are continually transferring energy to each
other through collisions. Sometimes through these collisions a particle gets enough energy that
it can break its bonds with nearby particles. Breaking bonds takes a lot of work/energy. Once
bonds have been broken, a new type of organization is possible – and hence the substance will
change state.
When the temperature of a substance is at the substance’s melting or boiling point, a change of
state or phase transition occurs. The common changes of state caused by transfers of thermal
energy are shown in the figure below.
In the figure below, we see the effect over time of heating a substance at a constant rate. When
a solid is heated, the kinetic energy of the vibrating particles increases and the temperature of
the substance rises. When the melting point is reached, continued heating causes the bonds
between particles to begin to break, and the potential energy of the particles increases. Some of
the particles make it to the liquid state. Particles in the liquid state collide with particles in the
solid state and they exchange kinetic energy. This keeps the particles all at the same
temperature during melting. The temperature does not change during a phase transition. It is
not until all of the particles are liquid that we see an increase in temperature.
The temperature and rates and which changes of state occur depend on the substance. Water
will evaporate and a different rate than alcohol.
Latent Heat
Latent heat is the thermal energy needed for a phase change. Latent means “hidden”. The
increase in thermal energy of the object is hidden, because you can’t actually see a change in
temperature.
Latent heat of fusion, QF – quantity of thermal energy transferred in the change from solid to
liquid
The specific latent heat of fusion, Lf, is the amount of thermal energy needed to melt 1kg of the
substance at its melting point. It is also the amount of thermal energy you need to remove in
order to change it from liquid to solid without any temperature change.
Latent heat of fusion is given by the equation:
Example
Gold melts at 1063C. It takes 3250 Joules of energy to melt 50 grams of gold at this
temperature. Determine the specific latent heat of gold.
Similarly, the latent heat of vaporization, QV is – the quantity of thermal energy transferred in
the change from liquid to gas
The specific latent heat of vaporization, Lv is the quantity of thermal energy needed to vaporize
1 kg of the substance at its boiling point. It is also the quantity of thermal energy you must
remove from 1kg of the substance to make it condense.
The equation for the latent heat of vaporization is:
Example 2
The boiling point of ethanol is 78.4C. Calculate the quantity of energy needed to completely
vaporize 75 grams of ethanol if its initial temperature is 22.4C.
Homework
Read pages 288-295
Answer questions 1-9 on page 295
SPH3U: Thermal Energy and Changes of State
Date: __________
The ____________________ and ____________________________ of the particles in a sample
of matter determine the physical state, or phase, of the matter. The three physical states of
matter are:
-
_____________________ (particles have some energy)
_____________________ (particles have more energy)
_____________________ (particles have the most energy)
The particles in all matter are in __________________________ and are continually transferring
energy to each other through __________________________. Sometimes through these
collisions a particle gets enough energy that it can break its __________________ with nearby
particles. Breaking bonds takes a lot of ________________________. Once bonds have been
broken, a new type of organization is possible – and hence the substance will change
______________________.
When the temperature of a substance is at the substance’s melting or boiling point, a
________________________________ or __________________________ occurs. The common
changes of state caused by transfers of thermal energy are shown in the figure below.
In the figure below, we see the effect over time of heating a substance at a constant rate. When
a solid is heated, the kinetic energy of the vibrating particles increases and the temperature of
the substance rises. When the melting point is reached, continued heating causes the bonds
between particles to begin to break, and the potential energy of the particles increases. Some of
the particles make it to the liquid state. Particles in the liquid state collide with particles in the
solid state and they exchange kinetic energy. This keeps the particles all at the same
temperature during melting. The temperature does not change during a phase transition. It is
not until all of the particles are liquid that we see an increase in temperature.
The temperature and rates and which changes of state occur depend on the
_______________________. Water will evaporate and a different rate than alcohol.
Latent Heat
Latent heat is the thermal energy needed for a phase change. Latent means “hidden”. The
increase in thermal energy of the object is hidden, because you can’t actually see a change in
temperature.
_______________________________, QF – quantity of thermal energy transferred in the change
from solid to liquid
The ______________________________________, Lf, is the amount of thermal energy needed
to melt 1kg of the substance at its melting point. It is also the amount of thermal energy you
need to remove in order to change it from liquid to solid without any temperature change.
Latent heat of fusion is given by the equation:
Example
Gold melts at 1063C. It takes 3250 Joules of energy to melt 50 grams of gold at this
temperature. Determine the specific latent heat of gold.
Similarly, the _____________________________________________, QV is – the quantity of
thermal energy transferred in the change from liquid to gas
The _________________________________________________, Lv is the quantity of thermal
energy needed to vaporize 1 kg of the substance at its boiling point. It is also the quantity of
thermal energy you must remove from 1kg of the substance to make it condense.
The equation for the latent heat of vaporization is:
Example 2
The boiling point of ethanol is 78.4C. Calculate the quantity of energy needed to completely
vaporize 75 grams of ethanol if its initial temperature is 22.4C.
Homework
Read pages 288-295
Answer questions 1-9 on page 295