Page 116 - Essentials of physical chemistry

78 Essentials of Physical Chemistry

OTHER TYPES OF THERMOCHEMISTRY

Here we face the problem of showing the mainstream applications of thermodynamics in one
semester, so we neglect other uses of the algebraic additivity of enthalpy values. We only mention
here that carefully weighed amounts of minerals can be dissolved in HCl=HF solvents in special
solution calorimeters to make accurate estimates of heats of formation of minerals that occur
over a period of thousands of years with use of Hess’s rule. Other special calorimeters can be
used to measure the heat of the process of denaturing (unfolding) proteins in a strong base. Still
other calorimeters can be used to measure the heats of sublimation (solid-to-gas), heats of fusion of
solids (solid-to-liquid), and heats of vaporization (liquid-to-gas) transitions phase changes but the
key idea is the algebraic summation of enthalpy values, which we have treated above mainly for gas
phase reactions.

PERSPECTIVE

The student needs to understand that instead of skimming the surface of thermodynamics in a short
one-semester treatment we have plowed more deeply into just a few examples of the first law in this
chapter. The idea is that we have selected what we think are important illustrations with sufficient
detail to prepare an interested student to elect a second semester and yet provide a good foundation
for students who stop at just one semester of study and need to apply principles of thermodynamics
to other disciplines. We have shown details for partial derivatives and ways to find heats of
reactions, but as chemical engineers and chemistry graduate students will tell you, there is a lot
more to thermodynamics, except now you have a good preparation for further study.

KEY FORMULAS AND EQUATIONS

For reversible ideal gas processes:

V ð 2 V ð 2
nRT V 2 P 1
dV ¼ nRT ln ¼ nRT ln :
w ¼ PdV ¼
V V 1 P 2
V 1 V 1
For adiabatic (Q ¼ 0) temperature changes:

C V C V
V 1 T R ðÞ ¼ V 2 T R ðÞ :
1 2
For adiabatic (Q ¼ 0) pressure changes:

C P C P
P 1 V C V ¼ P 2 V C V :
1 2
For constant volume heats of combustion:

nDH comb ¼ C V DT þ (Dn gas RT ave )n:

For heats of reaction at standard conditions:

X X
0 0 0
rxn
f,i
f, j
DH (298:15 K) H H :
product i reactant j