222
Thermochemistry
This equation makes it simple to calculate enthalpies of reaction from values of
AE obtained at constant volume, and vice versa.
PROBLEM:
The combustion of C 2 H 5 OH a) in a bomb calorimeter at constant volume gives a
value of A£° = -326.1 kcal/mole. Calculate the value of A//°.
SOLUTION:
The balanced chemical equation
C 2 H 5 OH U) + 30 2(fl) -» 2C0 2(9) + 3H 2 0 (()
shows that A;; = 2 - 3 = -1. Therefore, at standard conditions (25.0°C),
A//° = AE° + (An)KJ
(
cal \ /
kcal\
1.987
;
10! —- (298 K)
mole K/ \
cal /
= -326.7 kcal (per mole of C 2 H 5 OH)
THE FIRST LAW OF THERMODYNAMICS
A concise summary of the principles of the last section is given by the first law
of thermodynamics, which states that any change in internal energy (AE) of a
system is just equal to the difference between the heat (Q) it absorbs and the
work (W) it performs:
AE = Q - W
In other words, energy cannot be created or destroyed; all energy must be
accounted for.
A "system" is any carefully defined object or collection of materials that is
under discussion or study. For example, it may be the substances in a chemical
reaction mixture, the contents of a calorimeter, a solid of prescribed dimensions
or amount, or a gas at a given temperature, pressure, and volume. Everything
in the lab or the universe that exchanges heat or work with the system is called
"the surroundings."
For a chemical reaction at constant volume, W = 0, so AE = Q. If the
reaction absorbs heat from the surroundings, Q is positive, and the reaction is
endothermic (AE is positive).
For a chemical reaction at constant pressure, Q = A//, and W = (&n)RT, so
A// = AE + (&n)RT. If the reaction absorbs heat from the surroundings, A// is