Thermochemistry
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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
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Kinetic Energy
• Kinetic Energy, the energy of motion.
• When atoms or molecules move, their mass and
speed give them energy:
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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).
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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.
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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.
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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.
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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 Esys
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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!
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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)(m2)/s2
• 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?
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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.
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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 qp
• If the system is closed and has a constant
volume, then q is called qv
• We will typically do problems which are at
constant pressure, so qp is used.
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Internal Energy
• Work in a chemical system is:
w = -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.
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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%.
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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 H = qp
H is also just called q
• You will use both terms!
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Enthalpy and H
• Enthalpy, H, and H are state functions as are E
and E.
• 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?
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Enthalpy and H
• We can find the change in enthalpy, H, for a
chemical reaction or for a chemical process:
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Enthalpy and H
• If H is positive, then heat energy was absorbed
by the system. This is an endothermic process or
rxn.
• If H 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 H values for all
the products and reactants. (more on this later)
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Meaning of H
• Let’s look at a rxn:
• Look at the ways we can show H.
• What does this H 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?
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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?
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Calorimetry and Enthalpy Changes
• One common experimental method to find the
q or H 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.
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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 T = Tf - Ti
• But how can we get from T to H?
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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.
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Calorimetry and Enthalpy Changes
• We have tables of heat capacities, given in 2
forms:
– Specific Heat Capacity, s or cp, 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 Cm, 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.
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Calorimetry and Enthalpy Changes
• So the heat capacity, the amount of a substance and the
T for a rxn can let us calculate the H for a rxn, Hrxn
• 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.
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Calorimetry and Enthalpy Changes
•Here’s the equations and some problems!
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Hess’s Law: Adding Rxns Together
• If we want to find the H for a rxn, and we have
H values for other rxns, sometimes we can use
Hess’s Law to calculate the desired H from the
given H 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 Hrxn
for the following rxn?
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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 Hrxn for Rxn 1)? It’s the sum of
the Hrxn for Rxn 2) and 2 x 3).
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Standard Heats of Formation and Hrxn
•Earlier, you learned the following:
H°rxn = H°products - H°reactants
•This means that we need to know the H
values for all of the reactants and
products in order to calculate Hrxn
•Where can we find these values?
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Standard Heats of Formation and Hrxn
• There are Tables and Books of Tables which list
the thermodynamic values for H for thousands
of compounds.
• To avoid confusion, these values are listed as
Hf°, 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.
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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.
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H° Tables
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Standard Heats of Formation, H°f
• How would you make propane, C3H8, 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.
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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!
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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:
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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!)
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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.
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Heat of Combustion, H°comb
• Write the combustion rxn equation for benzene, C6H6,
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.
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Enthalpy Changes for Phase Changes
• There are 6 changes of state that a chemical may
undergo:
–
–
–
–
–
–
Vaporization
Condensation
Sublimation
Melting or Fusing
Freezing
Deposition
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Enthalpy Changes for Phase Changes
• There is a H associated with all of these phase
changes.
Hvap is the enthalpy change associated with the
vaporization process or the heat of vaporization;
Hf is the heat of fusion; and Hsub 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 –Hvap
• 3 of the phase changes are exothermic: which?
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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 H is independent of path (it’s a State
Function), we could first melt the solid and then
vaporize it to the gas. The H value for the 2
paths would be the same, so long as the
temperature was held constant.
• So for constant T, Hsub = Hvap + Hf
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