Thermodynamics
Energy
Energy in general is the ability to cause a change.
In chemistry, energy can do work or produce heat.
Energy is typically divided into two types: potential
energy and kinetic energy.
Energy in general is the ability to cause a change.
In chemistry, energy can do work or produce heat.
Energy is typically divided into two types: potential
energy and kinetic energy.
Potential energy is stored energy in the form of
position (gravity-related, like a skateboarder at the
top of a half-pipe) or chemical composition (like the
chemicals in food or a battery).
Kinetic energy is energy of motion, and is the usual
use for potential energy (food energy allows you to
move).
A chemical reaction could have both kinetic energy,
as the atoms or molecules move around, and
potential energy, as there could be chemicals still
waiting to react.
Energy can be converted between kinetic energy
and potential energy, even several times, but no
matter how many times energy is converted the
Law of Conservation of Energy states that no
energy can be lost or created.
The energy stored in a substance because of the
atoms that make it up is chemical potential energy.
For example, when octane (C8H18) is placed into an
automobile’s gas tank, it can be burned later to
move the automobile. Unfortunately a lot of the
chemical potential energy in gasoline is “lost” in
the engine as heat, so it is not available for motion.
Heat
Heat Energy
Heat Energy
Heat (symbolized q) is the energy that naturally
moves from something hot to something cold. The
amount of heat is often measured as temperature,
but temperature does not directly measure heat.
For example, water at 20 °C does not have twice as
much heat as water at 10 °C, but it does have more
heat!
To measure heat directly, the calorie (abbreviated
cal) was defined as the amount of heat needed to
increase the temperature of 1 gram (1 milliliter) of
water by 1 °C.
Note that the Calories reported on food wrappers
are the measurement for heat to increase the
temperature of 1000 grams (one liter) of water by 1
°C, in other words a kilocalorie.
Drinking a can of
Pepsi actually
means drinking
150,000 calories!
Keep in mind that a
normal person’s
diet is probably
around 2,000,000
calories per day.
The measurement
of how much heat
energy can come
from food is called
calorimetry.
Calories are still used in the nutrition field, but in
chemistry we will use the Joule (abbreviated J).
There are 4.184 Joules in 1 calorie, so Joules can
measure energy a little more accurately.
Both the calorie and Joule are defined using pure
water, but only pure water gains and loses heat like
pure water. Everything else in the world gains and
loses heat at a different rate – a factor called the
specific heat (abbreviated c).
When a cake has been baked and is removed from
the oven it would cause severe burns to touch the
metal pan, but often the top of the cake is touched
to test if the cake is ready.
The metal of the pan has a low specific heat so it
can lose heat quickly (which would be absorbed by
your finger) and the cake contains a lot of trapped
air and other material with a higher specific heat
which lose heat slowly, allowing more time before
your finger has absorbed enough heat to cause a
burn.
Additionally, the density of the cake is usually
much less than the density of the metal pan, so for
the same surface area (the surface area touched by
your finger) there are a lot more metal atoms that
could transfer heat energy than cake molecules.
Thus not only does the amount of heat that could
be transferred relate to the specific heat, the mass
also has an affect.
(Also, metals tend to be good conductors of heat
while air is a poor conductor of heat.)
In total, to calculate the amount of heat that can be
gained or lost by an object the formula is
q = m × c × ∆T
q = heat, measured in J
m = mass, measured in g
J
c = specific heat, measured in
g °C
∆T = Temperature final – Temperature initial, measure in °C
If the object is heating up, then the final
temperature is greater than the initial temperature,
so ∆T is positive and the number for heat is
positive.
If the object is cooling down, then the final
temperature is less than the initial temperature, so
∆T is negative and the number for heat is negative.
-q
+q
What is the heat absorbed by 100 g of water to
raise it from 25 °C to 75 °C? The specific heat of
water is 4.184 J/g°C.
q = m × c × ∆T
q = ________ x ________ x (________ - _______)
q = _______________
What is the heat given off by 100 g of iron to cool
from 50 °C to 20 °C? The specific heat of iron is
0.449 J/g°C.
q = m × c × ∆T
q = ________ x ________ x (________ - _______)
q = _______________
Heat Movement
Heat Movement
The Law of Conservation of Energy reminds us
that heat energy cannot just appear out of nowhere
and disappear to nowhere, so if an object warms
up, then that heat had to be absorbed from
somewhere and if an object cools down, then that
heat had to be given off to something.
In our world ecosystem heat is gained by the sun’s
radiation and given off to space at night. This is
why a cloudy night is typically warmer than a clear
night – the heat gets trapped in by the clouds.
Heat can move from one object to another by three
ways: convection, conduction, and radiation.
It was already mentioned how the energy from the
sun reaches earth by radiation, but radiant heat
energy is the movement of heat by electromagnetic
waves.
The infrared portion of the electromagnetic
spectrum is the “heat” form of radiation. Other
examples of radiation are the heat from a fire, nonfluorescent lightbulbs, and the coil in an electric
oven that heats up to cook the food.
Convection is the movement of heat by circulating
materials, so anything that is a fluid (gases and
liquids) can move heat by convection, often called
convection currents.
Examples of convection include most airconditioning systems, the ocean currents, and even
the warm and cold “fronts” the weather-person
talks about on the news.
Conduction is the movement of heat between
objects that are touching. This is the most common
form of heat movement when cooking. The food
touches a pan that is touching a burner, and as the
pan absorbs heat from burner the food absorbs
heat from the pan. When you touch a hot pan heat
moves from the pan into your hand and gives you a
burn. Often the lasting effects of a burn can be
minimized if you can quickly get the heat to move
out of your hand by running cold water over your
hand for a long time.
Regardless of the method of heat transfer, heat
energy will always naturally move from an object
with higher heat to an object with lower heat. Also,
the Law of Conservation of Energy tells us that the
amount of heat lost by the hot object must be equal
to the amount of heat gained by the cold object. As
heat lost is a negative number, we will need to add a
negative sign to the lost heat number so that it
matches the gained heat number.
Hot becomes cold, and cold becomes hot.
- qlost = qgained
so
- (mlost × clost × ∆Tlost ) = mgained × cgained × ∆Tgained
Phases and Changes
Sometimes when an object gains or loses heat, it is
not just the temperature that changes but it could
be that the phase of the object changes. The phases
we are concerned with are solid, liquid, and gas
(vapor) - which are sometimes called the states of
matter. The phase that an object currently exists in
depends on how much heat energy is in the object,
often measured by the temperature. Thus the solid
form of any object is going to have lower
temperature than the liquid form, which will have
lower temperature than the gas (vapor) form.
Phase changes occur when sufficient heat energy is
added or removed to change the object’s phase.
Melting is the solid phase changing to the liquid
phase, and freezing is the liquid phase turning to a
solid phase. Melting and freezing both occur at the
melting point – the temperature at which a solid
becomes a liquid. If heat is being absorbed (+q),
then melting is occurring. If heat is given off (-q),
then freezing is happening.
Boiling (vaporizing) is the liquid phase changing to
the gas phase, and condensation is the gas phase
changing to the liquid phase. Boiling and
condensing occur at the boiling point – the
temperature at which a liquid becomes a gas. If
heat is being absorbed (+q), then boiling is
occurring. If heat is given off (-q), then condensing
is occurring. Sometimes liquids evaporate instead
of boiling.
Evaporation is boiling, but occurs below the boiling
point. Evaporation can only happen at the surface
of the liquid (boiling happens everywhere inside)
and is a cooling process.
There are two other phase change possibilities, but
they are rare in everyday life. Sublimation is the
solid phase changing directly to the gas phase (no
liquid in-between),
and deposition is the
gas phase changing
directly to the solid
phase. Once again, a
+q would indicate
sublimation and a –q
would indicate
deposition.
Pressure also has a small effect on the phases. For
a liquid to boil, the pressure inside the liquid needs
enough energy to equal the pressure outside the
liquid.
In Denver where there is less atmospheric pressure
than here in Haltom City, the water does not need
as much heat energy to boil, so water will boil
around 95 °C. Most of the time, the solid phase is
more dense than a liquid, so high pressure can
force a liquid to become a solid, even if the
temperature is above the normal melting point.
To place all this information about the pressure,
temperature, and phases into a simple to read
form, chemists construct a phase diagram.
Solid
Liquid
Gas
Solid
Liquid
Sublimation
Gas
Solid
Liquid
Sublimation
Melting Point
Gas
Boiling Point
The phase diagram is helpful to compare the phase
that will exist at any particular pressure and
temperature, but if the amount of energy that is
needed to change a phases’ temperature or change
the phase is needed, it is easier to put the
information into a heat curve.
From this heat curve (for water) we can see how
the temperatures change and the phases change.
Notice that the temperature does not change during
a phase change (the two flat lines).
This means our equation for heat q = m × c × ∆T
will not work during a phase change, but it makes
sense that heat is needed to melt something
so new equations are needed for use during a phase
change:
during melting/freezing, q = m × Hf
q = heat (add “-“ sign for freezing), measured in J
m = mass, measured in g
Hf = heat of fusion, measured in
J
g
so new equations are needed for use during a phase
change:
during boiling/condensing , q = m × Hv
q = heat (add “-“ sign for condensing), measured in J
m = mass, measured in g
Hv = heat of vaporization, measured in
J
g
Example Calculations
Calculate the heat needed for the heat curve above
for 100 g of water.
Hfwater = 334 J/g
Hvwater = 2260 J/g
c ice = 2.06 J/g °C
c water = 4.18 J/g °C
c steam = 2.08 J/g °C
Step 1: Ice from -40 °C to 0 °C.
q = m × c × ∆T
q = ________ x ________ x (________ - _______)
q = _______________
Step 2: Turning ice into liquid water
q = m × Hf
q = ___________ x ___________
q = _______________
Step 3: Water from 0 °C to 100 °C.
q = m × c × ∆T
q = ________ x ________ x (________ - _______)
q = _______________
Step 4: Turning liquid water into water vapor
(steam).
q = m × Hv
q = ___________ x ___________
q = _______________
Step 5: Water vapor from 100 °C to 140 °C
q = m × c × ∆T
q = ________ x ________ x (________ - _______)
q = _______________
Total heat involved = ________ + _______ +
_______ + _______ + _______
Total heat = __________
Heat and Chemical Reactions
Heat and Chemical Reactions
Although we did not write heat as a reactant or
product back in chapter 10, every chemical
reaction involves a change in energy. If a chemical
reaction feels cold when it is happening, then the
reaction is absorbing heat. If a chemical reaction
feels warm when it is happening, then it is giving
off heat.
When energy is absorbed by a chemical reaction, it
is written as a reactant, and is called endothermic.
For example:
solid + energy (heat) → liquid
For this reaction to happen, heat needs to be
removed from the environment surrounding the
reaction for it to occur. This is why your skin gets
cold when you hold ice; the ice is pulling heat out of
your skin to melt.
When energy is given off by a chemical reaction, it
is written as a product, and is called exothermic.
For example:
Iron + oxygen gas → iron(III) oxide + energy (heat)
When this reaction happens heat is given off by the
chemical reaction, which is why it is used in instant
hot pads.
In both endothermic and exothermic chemical
reactions, the energy does not need to be heat only.
Some chemicals are stored in dark brown bottles to
block out the light.
Light can cause some chemicals to deteriorate
faster than a chemist would like, ruining the purity
of the chemical. To separate water into hydrogen
and oxygen (hydrolysis) electricity is often used.
Is hydrolysis of water endothermic or exothermic?
Some chemical reactions give off light, like the
reaction in a glow stick. When hydrogen peroxide
reacts with a phenyl oxalate ester energy is given
off, but in the form of light and only a little heat.
Sometimes the energy in a chemical reaction can be
used to speed up or even reverse the reaction. Most
chemical reactions are reversible and usually both
the forward and reverse reactions are occurring at
the same time, but at the same rate so you will not
notice a difference, called an equilibrium reaction.
Changing the energy can remove the equilibrium
and then a change is noticed, like ice melting in a
cup sitting on the counter and freezing when the
cup is moved into a freezer.
ice + heat ↔ liquid water