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receptivetrucksMechanics

Oct 27, 2013 (3 years and 10 months ago)

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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 E
sys

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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)(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?

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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
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.

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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




q
p



H
is also just called
q


You will use both terms!

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Enthalpy and


H


Enthalpy, H, and



are state functions as are E
and





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



is positive
, then heat energy was absorbed
by the system. This is an
endothermic

process or
rxn
.


If



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



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



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



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
f

-

T
i


But how can we get from






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 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.

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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



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.

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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



for a
rxn
, and we have



values for other
rxns
, sometimes we can use
Hess’s Law to calculate the desired



from the
given



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
?

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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


H
rxn

for
Rxn

1)? It’s the sum of
the


H
rxn

for
Rxn

2) and 2
x

3).

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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



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?

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Standard Heats of Formation and


H
rxn


There are Tables and Books of Tables which list
the thermodynamic values for



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.


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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, 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.

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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, 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.

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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


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Enthalpy Changes for Phase Changes


There is a



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?


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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



is independent of path (it’s a State
Function), we could first melt the solid and then
vaporize it to the gas. The



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


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