Entropy order/disorder

Since we have reason to believe that the order-disorder situation in ionic co-spheres, overlapping and merging as in Fig. 69, could give rise to forces of attraction or repulsion, superimposed on the Coulomb forces, we may inquire whether the observed facts as to activity coefficients may be correlated with the known behavior of the ions as regards viscosity and entropy. 

Figure 2.14 The contribution from the order-disorder transition of CaCC>3(s) to the total Gibbs energy and entropy [25],...

An ordered arrangement of particles (atoms, ions, or molecules) has lower entropy (smaller disorder) than the same number of particles in random arrangements. Thus, the entropy of a pure substance depends on its state. The entropy of a system increases (becomes more disordered) with temperature, because the motion of particles becomes more chaotic at higher temperatures. See Figure 7.6 on the next page. 

If you examine the NaCl crystal structure closely, you will see that each Ch is surrounded by 6 Na, and each Na is surrounded by 6 Cl . The regular, repetitive structure has a high degree of order and low entropy low disorder. 

Mixing can be described in terms of free energy. Free energy has two terms one related to energy and the other related to order/disorder. The energy-related term is called enthalpy, H, and the order/disorder term is called entropy, S. 

Order-disorder transitions are generally associated with (i) positional disordering, (ii) orientational disordering or (iii) disordering of electronic (or nuclear) spin states. The configurational entropy due to disordering is given by... 

Most chemical reactions occur by a change in the configurational order (AS 0). Compared to fluids, crystalline reactants already have a low entropy and thus solid state reactions are normally exothermic In this sense, order-disorder reactions are in no way special, except that they occur in homophase crystals. 

That part of the entropy of a substance that is due to a disordered arrangement of the particles as opposed to a similar but ordered arrangement. The most clear-cut example is the order-disorder transition in binary alloys, in which virtually the whole entropy change is of this kind. The entropy change on fusion of a solid is largely due to entropy of disorder. 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

Ehrenfest s concept of the discontinuities at the transition point was that the discontinuities were finite, similar to the discontinuities in the entropy and volume for first-order transitions. Only one second-order transition, that of superconductors in zero magnetic field, has been found which is of this type. The others, such as the transition between liquid helium-I and liquid helium-II, the Curie point, the order-disorder transition in some alloys, and transition in certain crystals due to rotational phenomena all have discontinuities that are large and may be infinite. Such discontinuities are particularly evident in the behavior of the heat capacity at constant pressure in the region of the transition temperature. The curve of the heat capacity as a function of the temperature has the general form of the Greek letter lambda and, hence, the points are called lambda points. Except for liquid helium, the effect of pressure on the transition temperature is very small. The behavior of systems at these second-order transitions is not completely known, and further thermodynamic treatment must be based on molecular and statistical concepts. These concepts are beyond the scope of this book, and no further discussion of second-order transitions is given. 

The demonstration of change in order/disorder, that is, a change in entropy, is present in everyday life. For example, the liquid mercury in a thermometer is made of small compact molecules (Fig. 16.1). When it is heated the molecules move faster they push their neighboring molecules away, the volume of the liquid expands, and the disorder, hence the entropy, increases. When the thermometer is cooled the opposite happens. Since there is relatively little interaction between the mercury molecules, the process is completely reversible. Not every material behaves this way. 

In the experiment on the polypropylene sheet, what happened to the order/disorder and, hence, to the entropy ... 

The concepts of thermodynamics and kinetics are poorly defined in MESA. We do not understand the distribution of energies, the rates of encounter, or of order/disorder in the assemblies. The distribution of energy among the objects is unknown, and may vary from system to system. We have not measured the rates of encounter or the rates of collision within each encounter complex, and do not know how to measure the entropy of the system. 

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