Entropic change

The adiabatic expansion and compression serve only to change the temperature of tire gas widrout heat being absorbed or evolved, i.e. iso-entropic changes. The heat changes are therefore only related to the work which is done during the isothermal stages, which is given by... 

If AG is equal to 0, the process is at equilibrium, and there is no net flow either in the forward or reverse direction. When AG = 0, A.S = H/T, and the enthalpic and entropic changes are exactly balanced. Any process with a nonzero AG proceeds spontaneously to a final state of lower free energy. If AG is negative, the process proceeds spontaneously in the direction written. If AG is positive, the reaction or process proceeds spontaneously in the reverse direction. (The sign and value of AG do not allow us to determine how fast the process will go.) If the process has a negative AG, it is said to be exergonic, whereas processes with positive AG values are endergonic. 

Entropy is often described as a measure of disorder or randomness. While useful, these terms are subjective and should be used cautiously. It is better to think about entropic changes in terms of the change in the number of microstates of the system. Microstates are different ways in which molecules can be distributed. An increase in the number of possible microstates (i.e., disorder) results in an increase of entropy. Entropy treats tine randomness factor quantitatively. Rudolf Clausius gave it the symbol S for no particular reason. In general, the more random the state, the larger the number of its possible microstates, the more probable the state, thus the greater its entropy. 

A significant increase in the specific maximum entropic change was achieved by substituting the fumaric acid with the much smaller formic acid, as reported in Table 4.1 where data for a selection of networks based on gadolinium... 

Consider the case of pure methanol for which the values of Cp and a are known. Using a specific volume of 0.00127 m3/kg, a temperature of 25°C (298 K), and a compression pressure of 1000 bar, Equation 13.2 predicts the eluent temperature will increase approximately 15°C assuming adiabatic conditions. In actual practice, the increase in eluent temperature entering the column will be lower than this upper limit due to thermal losses in the pump, connecting tubing, and injection system, as well as entropic changes (AS A 0). 

As experimentally demonstrated above, in the complexation thermodynamics involving cationic species as guests and ionophores as hosts, the entropic change TAAS, induced by altering cation, ligand, or solvent, is proportional to the enthalpic change AAH. This correlation immediately leads to an empirical Eq. 14 with a proportional coefficient a, integration of which affords an extrathermodynamic relationship between TAS and AH. Thus, Eq. 15 is the quantitative expression of the observed compensation effect ... 

Because of the short timescale for the optical transition, solvent dipole orientations in the initially formed excited state are the same as in the ground state and there is no entropic change. For a self-exchange reaction, the contribution to AG is A0/4 as noted above. 

The adsorbed layers between the particles may interpenetrate and so give a local increase in the concentration of polymer segments. Depending on the balance between polymer-polymer and polymer-dispersion medium interactions, this may lead to either repulsion or attraction by an osmotic mechanism. Enthalpic and entropic changes will be involved. If interpenetration takes place to a significant extent, elastic repulsion will also operate. 

As Roylance [25] pointed out, in contrast to the instantaneous nature of the energetically controlles elasticity, the conformational or entropic changes are processed whose rates are sensitive to the local molecular mobility. This mobility is influenced by a variety of physical and chemical factors, such as molecular architecture, temperature, or the presence of absorbed fluids which may swell the polymer. Often a simple mental picture of free volume - roughly, the space available for molecular segments to act cooperatively so as to carry out the motion or reaction in question-is useful in intuiting these rates. 

Lafont V, Armstrong AA, Ohtaka H, Kiso Y, Amzel LM, Freire E. Compensating enthalpic and entropic changes hinder binding affinity optimization. Chem. Biol. Drug Design 2007 69 413-422. 

Owing to the entropic changes that take place in a viscoelastic system perturbed by a force field, the response does not vanish when the perturbation field ceases. A consequence of this fact is that the deformation depends not only on the actual stress but also on the previous stresses (mechanical history) undergone by the material in the past. Under the linear behavior regime, the responses to different perturbations superpose. Let us assume that the stresses Aa(0i) and A(t(02) are applied on the material at times 0j and 02, respectively. This stress history is shown schematically in Figure 5.12. The response is given by... 

The distinctions between these two pure types may also be viewed from entropic considerations and the nature of the driving mode. The entropic change in an order-disorder phase transition is mainly configurational, and the driving mode is of diffusive (nonpropagating) nature. On the contrary, for a displacive phase transition, the entropy change is mainly vibrational, with an associated underdamped soft phonon. 

These results suggest that the N-terminal dipeptide is the trigger and that the glycine residues are the hinges for the trypsinogen-to-trypsin transition. Further work will be necessary to follow the dynamics of the transition and to estimate the entropic change involved in going from a disordered to an ordered structure. 

The lattice model of Flory-Huggins calculates the entropic changes of the mixing process. The enthalpy effects are considered by the interaction parameter % ... 

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