Entropy with temperature

Figure 7.9. Variation of (a) the Gibbs energy, (b) ttie enthalpy and (c) the entropy with temperature (scaled to the nearest-nei bour interaction energy J ) for the complex structure A15. Comparison between BWG CVM in the tetrahedron approximation ( ) and the Monte Carlo method (—) (Turchi and Finel 1992).

Temperature and Pressure Relationships 20a. Variation of Entropy with Temperature.—The equation (19.18), i.e.,... 

This is a relation between the change of entropy with temperature and the change of internal energy with temperature in a reversible process in which the volume is maintained constant... 

If CP can be regarded as constant the change of entropy with temperature is given by... 

This definition is sound, but is not strictly applicable in practice. Note that in this case, even d < 0 => dt/ = 0 holds. This behavior is in connection with the monotony of entropy with temperature. We will explain this issue elsewhere. 

The entropy change in a body of 75.3 kg of water, when its temperature is raised from 36.6 to 41.2°C, is calculated using the formula for the reversible change in entropy with temperature change. We will again use the differential d, which is as small change - in both temperature and in entropy - as we desire it to be. We write... 

Figure 15.35 Variation of entropy with temperature for a pure substance undergoing meiting and boiiing... 

As we can see, the temperature not only has a direct effect but also indirectly affects the Gibbs free energy through the functional variation of enthalpy of formation and entropy with temperature. 

The variation of entropy with temperature can also be expressed using the specific heat capacity Cp. From classical thermodynamics... 

Usually, for relatively narrow temperature ranges, we can consider constant standard enthalpy and entropy with temperature. 

To obtain standard entropy at temperature T, we consider the law of variations of entropy with temperature, that is ... 

For a hydrogen/air fuel cell operating at 60°C with reactant gases at atmospheric pressure and with liquid water as a product, calculate the theoretical cell potential taking into account the changes of reaction enthalpy and entropy with temperature (Equations 2-22 and 2-23). 

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