A Method of Teaching Thermodynamic Functions1 HAROLD BECHER Polytechnic Institute of Brooklyn, Brooklyn, New York
I
N TEACHING beginners in physical chemistry or chem~calthermodynamics, difficulty is often encountered in making the student visualize the relationships between thermodynamic functions such as A H = A F f TAS
or F = A f P V
Particularly, it has been found that in the Lewis and Randall notation, trouble is found in defining A H or
- AH.
However, from several years' experience, it has been found that the subject matter is readily visualized by means of the accompanying simple diagrams.
I=F-~
1
I
Total Heat Content
TS
= Unavailable energy = Faraday E L = Voltage
fi
From an inspection of the diagrams it becomes obvious that if heat is evolved, H, the total heat content, is diminished by a quantity - A H , and if heat is absorbed H is increased by A H . This increase or decrease is indicated by the shaded portion of the rectangle (Figure 3) and the notations above it. The same idea follows for the other functions. The relationship between A and F is at once clear from Figure 3, The Gibbs-Helmholtz equation is derived by differentiating
Free or Available Energy
Y H . Total Heat Content
-
PV
External Energy
A H = AF+ T A S
--
E
>
or AE = AA
Internal Energy
FIGURE2
Each rectangle represents a thermodynamically isolated isothermal system in which H = Heat content or total energy = Pressure-volume content or available external energy F = Free energy content or available energy A = Work content E = Internal energy content S = Entropy content n = Valence T = Absolute temperature
PV
+ TAS
(the latter at constant volume), obtaining the relationship TAS
=
-TbAA/bT
TAS
=
-TbAF/bT
The connection between the last two relations and the electrochemical function nE& becomes clearer when referred to Figure 3. When the diagram is explained to the class at the very beginning of the course the further study of thermodynamics is facilitated. The iirst law
-
Presented before the Division of Chemical Education at the 102nd meeting of the A. C. S., Atlantic City, New Jersey, September 11, 1941.
is grasped at once. Particularly is this so if heat is a t first represented (although incorrectly) as a fluid. This
is advisable as an aid in visualizing the functions. The heat transfer into or from the system becomes analogous to transferring a quantity of liquid from a pitcher into a glass. The liquid fills the glass to the limit of its capacity and overflows into the space surrounding it. Like-
wise, heat from an external source can enter the system, increasing its internal energy to the limit of its capacity, and the excess will increase the pressure-volume content, or, in other words, do work. This last is analogous to the water which overflows the glass.
