In a semi-ideal gas, entropy changes cannot be determined through the medium
specific heat capacity in a manner as determined by the change of internal
energy and enthalpy, i.e. the amount of heat exchanged. Taking this into
account, the authors conducted two models through which it is possible to
determine the change in the specific entropy of a semi-ideal gas for
arbitrary temperature interval using the spread sheet method, using the mean
values of the appropriate functions. The idea is to replace integration,
which occurs here in evitably, with mean values of the previous functions.
The models are derived based on the functional dependence of the actual
specific heat capacity on the temperature. The theorem used is that of the
mean value of a function as well as the mathematical properties of the
definite integral. The mean value of a fractional function is determined via
its integrand while the logarithmic functions were performed by applying a
suitable transformation of the differential calculus. The relations derived
relation, using the computer program, have enabled the design of appropriate
thermodynamic tables through which it is possible to determine the change in
entropy of arbitrary state changes in an efficient and rational manner,
without the use of calculus or finished forms. In this way, the change in the
entropy of a semi-ideal gas is determined for an arbitrary temperature
interval using the method which is analogous to that applied in determining
the change of internal energy and enthalpy or the amount of heat exchanged,
which was the goal of the work. Verification of the proposed method for both
the above functions was performed for a a few characteristic semi-ideal gases
where change c(T) is significant, for the three adopted temperature
intervals, for the characteristic change of state. This was compared to the
results of the classical integral and the proposed method through the
prepared tables. In certain or special cases, it is possible to apply the
presented method also in determining the change in entropy of the real gas.
Apart from that, the paper shows that the change in entropy for the observed
characteristic case can be represented or graphically determined using the
planimetric method of diagrams with suitably selected coordinates.
Specific heat of refractory carbides (ZrC0.95, HfC0.85 and Ta0.8Hf0.2C) in the temperature interval 2500-5000 K
