Chapter 6
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Chapter 6: Thermochemistry
Understanding Heats of Reaction
The first part of this chapter lays the groundwork for understanding
what we mean by heats of reaction.
Thermodynamics is the science of the relationship between heat and
other forms of energy. Thermochemistry is one area of
thermodynamics. It involves the study of heat absorbed or evolved
during chemical reactions.
1.
Energy and Its Units
Energy is the ability to move matter.
a.
Kinetic Energy, E
K
; Units of energy
The energy an object has due to its motion is kinetic energy.
2
K
2
1
mv
E
=
In this formula,
m
is in kilograms, kg, and
v
is in m/s.
E
K
is then in kg
m
2
/s
2
. This unit is defined as a joule (pronounced “jewl”), J.
Another unit of energy is the calorie, cal. It is defined at 4.184 J.
(Originally it was defined as the amount of energy required to raise the
temperature of one gram of water by one degree C.)
To calculate kinetic energy first convert to the required units, then
substitute into the equation and solve.
b.
Potential Energy, E
P
The energy an object has due to its position in a field of force is potential
energy.
The most common example of this is gravitational potential energy, the energy an object
has by virtue of its altitude.
E
P
=
mgh
In this formula
m
is in kilograms, kg,
g
is the acceleration due to gravity, a constant equal
to 9.8 m/s
2
, and
h
is the altitude in meters, m.
E
P
is then in kg m
2
/s
2
which is equal to a
joule, J.
c.
Internal Energy, U
Internal energy is the energy of the molecules and atoms that make up the object.
The total energy of an object is therefore,
E
total
=
E
K
+
E
P
+
U
d.
Law of Conservation of Energy
Energy may be converted from one form to another, but the total quantity of energy
remains constant.
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For example, imagine an object that is held stationery at
25 m above ground. At this point
E
K
= 0. Once dropped,
the potential energy is converted to kinetic energy so that
by the time that it reaches the ground all the potential
energy has become kinetic energy.
We see this illustrated in Figure 6.3 where water crosses
a dam converting its potential energy into kinetic energy.
2.
Heat of reaction
We first need to learn to analyze the situation being studied. We
divide it into the system (the substance or mixture being
studied) and the surroundings (everything else in the vicinity of
the system).
a.
Definition of Heat
Heat is the energy that flows into or out of a system because
of a difference in temperature between the thermodynamic
system and its surroundings.
Heat flows until thermal equilibrium is established, that is,
until they are the same temperature.
The flow of heat can be explained by the kinetic
molecular theory. In Figure 6.5 we see two containers that
share a wall. The molecules in the vessel on the left are at
a higher temperature than those on the right. As the
molecules at a higher temperature collide with the shared
wall, they lose energy, slowing down. Conversely, as the
molecules at lower temperature collide with the wall, they
gain energy, speeding up. This continues until both have
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 Fall '07
 barber
 Thermodynamics, Enthalpy, Thermochemistry, Reaction, Energy, Law of Conservation of Energy, a. Kinetic Energy

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