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First Law – Introduction to Terms:
The First Law of thermodynamics can be expressed in either of two equivalent forms, both of
which convey the essential nature of the law, to wit: conservation of energy in an isolated system
(
aka
universe).
0
=
+
=
∆
+
∆
=
∆
w
q
E
E
E
surr
sys
univ
The subscripts refer to “universe”, “system”, and “surroundings” respectively.
Let us examine
each term.
Delta E (
∆
E):
•
Denotes the change in internal energy during a chemical/physical process.
•
Units of Joules (J).
•
The capital letter denotes that the internal energy is a “state function”.
This means that
the change in internal energy (like potential energy) depends only on the initial and final
states of the process (e.g. reactants and products).
That is,
i
f
E
E
E

=
∆
.
In other
words, one simply subtracts the internal energy of the initial state from that of the final
state to obtain the change.
•
For those with interest in mathematics, this is related to exact differentials:
)
(
)
(
)
(
1
2
2
1
x
F
x
F
x
dxf
x
x

=
∫
where F(x) is the antiderivative of f(x).
That is to say, the
integral is “replaced” by a function evaluation at the endpoints (the function being the
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 Fall '07
 Warren
 Chemistry

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