Temperature entropy/enthalpy effects

This fundamental relationship points out that the temperature at which crystal and liquid are in equilibrium is determined by the balancing of entropy and enthalpy effects. Remember, it is the difference between the crystal and... 

Equation 3.6.10. given above shows that intrinsic defect concentrations will increase with increasing temperature and that they will be low for high Enthalpy of defect formation. This arises because the entropy effect is a positive exponential while the enthalpy effect is a negative exponential. Consider the following examples of various types of compounds and the natural defects which may occur (depending upon how the compounds were originally formed) ... 

At high temperatures, does enthalpy or entropy have a greater effect on a reaction s Gibbs energy ... 

The Wilson equation is very effective for dilute composition where entropy effects dominate over enthalpy effects. The Orye-Prausnitz form of the Wilson equation for the activity coeflicient, as given in Table 5.3, follows from combining (5-2) with (5-41). Values of A ij < 1 correspond to positive deviations from Raoult s law, while values of A >1 correspond to negative deviations. Ideal solutions result from A, = 1. Studies indicate that Af, and ka are temperature dependent. Values of viJvjL depend on temperature also, but the variation may be small compared to temperature effects on the exponential term. 

If phase transitions in the solid phase occur additionally between the considered temperature and the melting temperature, these enthalpy and entropy effects have to be considered in the thermodynamic cycle (Figure 8.3). Neglecting the terms with the heat capacity differences, the following equation can be derived for simple eutectic systems ... 

Entropy, enthalpy, and free energy of reversible polymerization Arrhenius relationship for rate constants Subcritical damped oscillations during thermal polymerization Polyrate of terpolymerization of AMS-AN-Sty Enthalpy of random copolymers Effect of chain sequence distribution Entropy and free energy of copolymerization Copolymer composition with and without ceiling temperature effect... 

Before exploring why enthalpy-entropy compensation effects exist, it is instructive to delineate the temperature dependence of LFERs. Because temperature dependence has been mostly explored with regard to kinetics, we present this discussion below in terms of activation parameters, but it also applies to reaction free energies. 

The reason for the odd-even effect on the melting temperatures has usually been attributed to different positions of the ordered planar zigzag conformation resulting in different alignments of the carbonyl group.(65-68) It has been pointed out (70), as is evident from the table, that AH alternates with carbon number in an opposite manner to the melting temperature. Consequently enthalpy differences cannot be the cause of the alternation. The alternation must result from differences in the entropies of fusion. 

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