1
Thermochemistry
2
Energy
•
Thermochemistry is the study of energy
changes and exchanges in chemical systems.
•
Energy is basically the ability of a system to
supply heat or to perform work.
•
You already know the 2 principal classes of
energy.
–
Kinetic
–
Potential
3
Kinetic Energy
•
Kinetic Energy
, the energy of motion.
•
When atoms or molecules move, their mass and
speed give them energy:
4
Potential Energy
•
Potential Energy,
internal or stored energy
.
•
It may be stored because of position, or it may
be stored internally in chemical substances.
•
A ball or boulder on a hill has potential energy
because of its position in space: if it gets
pushed, it will roll down the hill, converting
potential energy into kinetic energy (and other
energies).
5
Chemical Energy
•
Chemical Energy
is actually both potential and
kinetic energy
.
•
It is the energy a chemical substance has based
on the positions and motions of its atoms and
electrons.
•
When a chemical rxn takes place, new chemical
substances are produced which have different
energies as they have different positions and
motions of electrons, etc.
6
Conservation of Energy & the First Law
•
You learned the
Law of Conservation of
Energy,
which is
also called the
First Law
of Thermodynamics
:
•
Energy can’t be created nor destroyed; it
can only be converted from one form of
energy to another and transferred from
one object to another.
7
Conservation of Energy & the First Law
•
To follow energy changes, we have defined a
system
and the
surroundings
.
•
The contents of a rxn vessel constitute the
system.
•
Everything else is the surroundings.
•
Note: if a rxn takes place in a solvent, like
water, then the solvent is usually classified as
part of the surroundings.
8
Conservation of Energy & the First Law
•
The surroundings either supply energy to the
system or absorb energy released by the
system. (and of course we can state the same
for the system)
•
This means that the energy change for the
system equals the negative of the energy change
of the surroundings:
•
As we are following the rxn (system), when we
say energy or E, we typically mean the E
sys
9
The First Law of Thermodynamics
•
The first law is stated in 2 ways:
–
The energy of the Universe is constant.
–
The energy of an isolated system (there is
NO energy transfer with any surroundings)
is constant.
•
So there is no free lunch in the Universe!
•
If someone tries to sell you a product that
makes energy out of nothing, don’t buy it!
10
Units of Energy
•
Scientists use the energy unit joule, J.
•
The J is an abbreviated unit, here’s the units that you
would get from the above equation:
KE = (kg)(m
2
)/s
2
•
We still use the old unit calorie, cal, where:
1 cal = 4.184J (exact)
•
If you look at your food label and it says 140 Cal, this is
a food calorie, Cal, where 1 Cal = 1000 cal.
•
The typical male is supposed to eat 2000 Cal/day. How
many J is this?
11
Internal Energy
•
The internal energy of a chemical system is the
sum of all of the kinetic and potential energies
of all of the particles in the system.
E = KE + PE
•
As stated earlier, it is the ability to produce
heat and work.
12
Internal Energy
•
Unfortunately, it is basically impossible to
measure the actual internal energy of a system.
•
What we can measure is the change in energy
of a system as it undergoes a chemical
rxn
.
E = Heat + Work =
q
+
w
•
If the system is open to atmospheric pressure
(as many
rxns
are), then
q
is called
q
p
•
If the system is closed and has a constant
volume, then
q
is called
q
v
•
We will typically do problems which are at
constant pressure, so
q
p
is used.
13
Internal Energy
•
Work in a chemical system is:
w
㴠

P
V
•
The negative sign reflects the standard
terminology that work produced is a negative
quantity.
•
For work produced, the volume increases so
V
is positive.
14
Internal Energy
•
Work produced by a chemical system is typically small
so the following assumption is made:
•
Although this is an approximation, it is generally (but
not always) within 1%.
15
Energy and Enthalpy
•
Chemists also have defined another energy term,
enthalpy, H, or the enthalpy change,
H
.
H
is the heat energy change, or the enthalpy
change of a constant pressure system.
•
So
䠠
㴠
q
p
H
is also just called
q
•
You will use both terms!
16
Enthalpy and
H
•
Enthalpy, H, and
䠠
are state functions as are E
and
䔮
•
What does this mean?
–
State functions are independent of path, or
how the system arrives at its final state.
–
What’s another state function that you
know?
17
Enthalpy and
H
•
We can find the change in enthalpy,
H
, for a
chemical reaction or for a chemical process:
18
Enthalpy and
H
•
If
䠠
is positive
, then heat energy was absorbed
by the system. This is an
endothermic
process or
rxn
.
•
If
䠠
is negative
, then heat energy was released
by the system. This is an
exothermic
process or
rxn
.
•
Since we want to find
H
for a
rxn
or process,
we need to know the individual
䠠
values for all
the products and reactants. (more on this later)
19
Meaning of
H
•
Let’s look at a
rxn
:
•
Look at the ways we can show
H
.
•
What does this
䠠
value mean?
•
As the units are kJ/mol
rxn
, it is a conversion
factor, which can take us between mol of a
reactant or product and heat energy required or
released!
•
What if we write the reverse reaction?
20
Stoichiometry
and
H
•
For the above rxn, 2043 kJ of heat energy is
released for every mol of propane which is
burned.
•
So how much heat energy would be released if
3.5 moles of propane were burned?
•
How much heat energy would be released if 25.0
g of carbon dioxide was produced?
21
Calorimetry and Enthalpy Changes
•
One common experimental method to find the
q
or
䠠
for a
rxn
is to conduct the
rxn
inside a
calorimeter.
•
Calorimeters may be constant pressure or
constant volume.
•
In the lab, you will use a “coffee cup”
calorimeter, which is constant pressure.
•
Another common type is a “bomb” calorimeter,
which is constant volume.
22
23
24
Calorimetry and Enthalpy Changes
•
Whatever type of calorimeter is used, the
temperature change of the system, or the
surrounding water reservoir, is measured.
•
This gives us
T
, where
吠
㴠
T
f

T
i
•
But how can we get from
吠
瑯
H
?
25
Calorimetry and Enthalpy Changes
•
A property called the heat capacity of a chemical
(or a mixture) lets us make this conversion.
•
Heat capacity is a measure of a substance’s (or a
mixture’s) ability to store heat.
•
The higher the heat capacity of an object, the
more heat energy it can store without its
temperature changing.
26
Calorimetry and Enthalpy Changes
•
We have tables of heat capacities, given in 2
forms:
–
Specific Heat Capacity, s or c
p
, which is defined as
the amount of heat necessary to raise exactly 1 g of a
substance by exactly 1
°
C. The units are J/g•
°
C.
–
Molar Heat Capacity, C or C
m
, which is the amount
of heat necessary to raise exactly 1 mol of a
substance by exactly 1
°
C. The units are
J/mol•
°
C.
27
28
Calorimetry and Enthalpy Changes
•
So the heat capacity, the amount of a substance and the
T
for a
rxn
can let us calculate the
䠠
f潲 愠
r确
,
H
rxn
•
What kind of substances have high heat capacities?
•
Water has one of the highest heat capacities, much
higher than most common substances. It’s specific heat
capacity is 4.184 J/
g
•
°
C
.
•
Water can store a lot of heat energy, and this is crucial
for life on our planet.
•
As our planet is a liquid water

based planet, water is a
heat sink or heat reservoir.
•
So our oceans and large lakes moderate temperature
fluctuations on our planet, keeping it from getting too
hot during the day and too cold at night.
29
Calorimetry and Enthalpy Changes
•
Here’s the equations and some problems!
30
Hess’s Law: Adding Rxns Together
•
If we want to find the
䠠
for a
rxn
, and we have
䠠
values for other
rxns
, sometimes we can use
Hess’s Law to calculate the desired
䠠
from the
given
䠠
values.
•
Hess’s Law: the overall enthalpy change is equal
to the sum of the individual
rxns
which make up
the
rxn
.
•
For example, what if we want to find the
H
rxn
for the following
rxn
?
31
Hess’s Law: Adding Rxns Together
•
Can you see how to manipulate
Rxn
2) and 3) in
order to get
Rxn
1)?
•
Do you add them, add the reverse of one,
multiply or divide them by some whole number,
etc?
•
In this case, addition of
Rxn
2) and 2 times
Rxn
3) gives you
Rxn
1)
•
So what’s the
H
rxn
for
Rxn
1)? It’s the sum of
the
H
rxn
for
Rxn
2) and 2
x
3).
32
Standard Heats of Formation and
H
rxn
•
Earlier, you learned the following:
H
°
rxn
=
H
°
products

H
°
reactants
•
This means that we need to know the
䠠
va汵敳 fo爠a汬lof t桥h牥a捴a湴s a湤
products in order to calculate
H
rxn
•
Where can we find these values?
33
Standard Heats of Formation and
H
rxn
•
There are Tables and Books of Tables which list
the thermodynamic values for
䠠
for thousands
of compounds.
•
To avoid confusion, these values are listed as
H
f
°
, or the
Standard State Enthalpy Changes
.
•
What’s Standard State?
–
Standard State is defined as 1
atm
pressure (now it’s
1 bar); 1 M for all solutions; and at a specified
temperature, which is usually 25
°
C.
34
Standard Heats of Formation,
H
°
f
•
But what do these
H
°
f
values mean?
•
They are the standard heats of formation for a
substance.
•
They are the enthalpy change when exactly 1
mol of the substance is made from its elements
under standard state conditions.
•
We can write equations which show exactly
what we mean.
35
H
°
Tables
36
Standard Heats of Formation,
H
°
f
•
How would you make propane, C
3
H
8
, from the
elements?
•
You make it from the most stable elemental
form of the element.
•
Here’s the heat of formation equation for
propane:
•
Note that graphite is the stable elemental form
of carbon.
37
Standard Heats of Formation,
H
°
f
•
What’s the heat of formation equation for liquid
water?
•
Note in the above that we are ALLOWED
(actually we are MANDATED) to have fractions
in the
rxn
equation!
•
Why? Because of the definition of a
H
°
f
:
exactly 1 mol of the substance is made!
38
Standard Heats of Formation,
H
°
f
•
Now we can use
H
°
rxn
=
H
°
products

H
°
reactants
•
There really is a more mathematically correct
way to write this equation:
39
Standard Heats of Formation,
H
°
f
•
So we just look up the
H
°
f
for all of the products and
reactants and add and subtract them together.
•
Example: Using Tables of
H
°
f
values, find the
H
°
rxn
for the combustion of propane:
•
What is interesting is that the
H
°
f
for the stable
elemental state of an element is 0.
•
In problems, you will not usually be given the
H
°
f
value
for stable elemental forms, you are expected to
know
that
it is zero! (Remember this!)
40
Heat of Combustion,
H
°
comb
•
You noticed that in Heat of Formation equations, you
could legally have fractions in the
rxn
equation.
•
There is another common case where it is legal to have
fractions in the
rxn
equation: Heat of Combustion
Rxn
Equations.
•
The Heat of Combustion,
H
°
comb
(or just
H
°
c
) is
defined is the heat energy change when exactly 1 mol of
a substance is combusted with oxygen gas.
•
We are allowed to have fractions in combustion
rxn
equations if we are calculating heats of combustion.
41
Heat of Combustion,
H
°
comb
•
Write the combustion
rxn
equation for benzene, C
6
H
6
,
and calculate its
H
°
comb
.
•
Of course, combustions
rxns
are very important to us as
they heat our homes, power our cars, and light our
homes (as most electricity is produced by burning fossil
fuels).
•
We can use
H
°
comb
values for the reactants and
products in a chemical reaction to find
H
°
rxn
•
However, as combustion is for
breaking apart
a
compound, while heats of formation were for
making
a
compound, we have to change the signs of the
H
°
comb
values before we use them in
products

reactants.
42
43
Enthalpy Changes for Phase Changes
•
There are 6 changes of state that a chemical may
undergo:
–
Vaporization
–
Condensation
–
Sublimation
–
Melting or Fusing
–
Freezing
–
Deposition
44
Enthalpy Changes for Phase Changes
•
There is a
䠠
associated with all of these phase
changes.
H
vap
is the enthalpy change associated with the
vaporization process or the heat of vaporization;
H
f
is the heat of fusion; and
H
sub
is the heat of
sublimation.
•
As condensation if the reverse of vaporization,
we don’t have a special term for the heat of
condensation, it’s just
–
H
vap
•
3 of the phase changes are exothermic: which?
45
Enthalpy Changes for Phase Changes
•
We can also combine 2 phase changes!
•
When we sublime something, it goes from the
solid state to the gas state directly.
•
But as
䠠
is independent of path (it’s a State
Function), we could first melt the solid and then
vaporize it to the gas. The
䠠
value for the 2
paths would be the same, so long as the
temperature was held constant.
•
So for constant T,
H
sub
=
H
vap
+
H
f
46
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