CBE2124, Levicky
1
Chapter 7 – Energy and Energy Balances
The concept of energy conservation as expressed by an
energy
balance
equation is
central to chemical engineering calculations. Similar to mass balances studied
previously, a balance on energy is crucial to solving many problems.
________________________________________________________________
System
A “system” is an object or a collection of objects that an analysis is done on.
The
system has a definite boundary, called the system boundary, that is chosen and
specified at the BEGINNING of the analysis. Once a system is defined, through
the
choice
of
a
system
boundary,
everything
external
to
it
is
called
the
surroundings
.
All energy and material that are transferred out of the system enter
the surroundings, and vice versa.
In the general case there are very few restrictions
on what a system is; a system can have a nonzero velocity, a nonzero acceleration,
and a system can even change in size with time.
An
isolated system
is a system that does not exchange heat, work, or material with
the surroundings.
If heat and work are exchanged across a system’s boundary, but material is not, it
is a
closed system
.
An
open system
can exchange heat, work, and material with the surroundings.
Examples.
Discuss each situation below as approximating an isolated, a closed, or
an open system.
(i) A river.
(ii) The interior of a closed can of soda.
(iii) The interior of a closed refrigerator that is turned on.
(iv) The interior of a closed refrigerator that is turned off.
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State of a System
Once a system is defined, a certain number of variables will specify its state fully.
For example, one may need to provide the temperature, pressure, composition,
total amount of material, velocity, and position in order to specify a system’s
“state.” The exact information that is needed to specify the state of a system
depends on the type of system and the analysis to be performed.
State Functions and State Properties
The state of a system can be changed, for example by increasing its temperature or
changing its composition. Properties of the system whose change depends only on
the initial (before) and final states of the system, but not on the manner used to
realize the change from the initial to the final state, are referred to as
state
properties
or
state
functions
. In other words, the change in a state function or
state property
X
, between some final (state 2) and initial (state 1) situations, can be
expressed as
1
state
2
state
X
in
change
initial
final
X
X
X
X
X

≡

=
Δ
4
43
4
42
1
(1)
In equation 1,
X
final
only depends on the final state of the system, and
X
initial
only on
the initial state of the system. Equation 1 does not require any information
whatsoever as to how the system got from the initial to the final state, since
X
does
not depend on the details of the path followed.
Example
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 Fall '11
 Levicky
 Thermodynamics, Energy, Potential Energy, Levicky

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