New trends in physics teaching, v.4; The ... - unesdoc - Unesco

If a system receives the amount of heat AQ in a reversible process
at temperature T the entropj, of the system increases by AS =AQf T.
Entropy and Information
Thus heat is the form of energy transfer connected with entropy increase. This was to be expected
since heat increases thermal motions and thereby decreases the information available about the
system.
The choice of the special factor 0.7 k in the definition of entropy is now motivated by the
simple connection AS= AQ/T between entropy and heat which results thereby. We shall not
attempt here to give a general proof for this relation [ 121 .
THE FREE ENERGY
A satisfactory description of all reversible processes taking place in isolated thermodynamic
systems can now be given with the help of the entropy concept introduced above. In many
practical problems - especially in chemistry and biology - thermodynamic systems are not
isolated from their surroundings. In these cases it is necessary to consider the total thermodynamic
system, composed of the original system and the laboratory. The total entropy ST
increases during irreversible processes, while the total energy ET remains constant:
AS, = AS+ AS, > 0, AE, = AE + AE, = 0. (Eq. 8)
It is desirable to eliminate the variables concerning the laboratory from these equations. Since
the laboratory can be considered a heat reservoir with approximately constant temperature we
have TAS, = AEL = - AE. Inserting this into the first of the equations given above we obtain
upon multiplication by T
Because of the constancy of the temperature we can write this also in the form
The quantity F defined by (Eq. 10) is the free energy of the thermodynamic system. Our result
shows that the free energy decreases during thermodynamic processes taking place in a system in
thermal contact with its surroundings. In thermal equilibrium the free energy becomes a
minimum.
These considerations can then be generalized to thermodynamic systems for which not only
the temperature but also the pressure is fixed by their surroundings. In this case the free enthalpy
G = E + p I' - TS decreases during irreversible processes
AG = A (E + pV - TS)= AH - TAS < 0. (Eq. 11)
The quantity AH, i.e. the change in enthalpy, is the heat given off to the surroundings during a
chemical reaction or during other irreversible processes. This heat can also be negative (endothermic
processes).
These abstract considerations will be illustrated with some concrete examples in the following
sections.
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