Molecular disorder random system

Consider two liquid substances that are rather similar, such as benzene and toluene or water and ethylene glycol. When moles of the one are mixed with B moles of the other, the composition of the liquid mixture is given by specification of the mole fraction of one of them [e.g., Xa, according to Eq. (2.2)]. The energy or heat of the mutual interactions between the molecules of the components is similar to that of their self interactions, because of the similarity of the two liquids, and the molecules of A and B are distributed completely randomly in the mixture. In such mixtures, the entropy of mixing, which is a measure of the change in the molecular disorder of the system caused by the process of mixing the specified quantities of A and B, attains its maximal value ... 

To decide whether we need to worry about AS0 with regard to any particular reaction, we have to have some idea what physical meaning entropy has. To be very detailed about this subject is beyond the scope of this book, but you should try to understand the physical basis of entropy, because if you do, then you will be able to predict at least qualitatively whether AH° will be about the same or very different from AG°. Essentially, the entropy of a chemical system is a measure of its molecular disorder or randomness. Other things being the same, the more random the system is, the more favorable the system is. 

What do a melting ice cube and the reaction of barium hydroxide octahydrate have in common The common feature of these and all other spontaneous processes that absorb heat is an increase in the amount of molecular disorder, or randomness, of the system. The eight water molecules rigidly held in the Ba(OH)2 8 H20 crystal break loose and become free to move about in the aqueous liquid product similarly, the rigidly held H20 molecules in the ice lose their crystalline ordering and move around more freely in liquid water. 

Entropy, denoted by S, is a state function that measures molecular disorder, or randomness. The entropy of a system (reactants plus products) increases (AS is positive) for the following processes phase transitions that convert a solid to a liquid or a liquid to a gas reactions that increase the number of gaseous molecules dissolution of molecular solids and certain salts in water raising the temperature of a substance expansion of a gas at constant temperature. 

This increase in entropy is a measure of the increased randomness, or molecular disorder, in the system due to the expansion. The second law of thermod5mamics, therefore, is really about probability Under the influence of motional energy, a system tends to assume the state of maximum probability. 

During the flow through the capillary the macromolecules are orientated to the direction of the force action. According to the Boltzmann s theory, any process of molecular orientation corresponds to an entropycal state lower than that characteristic of the entirely random state. Hence, the polymer entropy at the capillary outlet is lower than the initial one. On the other hand, the brownian motion tends to disorder the system and in case of a slow flow this process can prevail preventing from the orientation. But in case of the rapid flow of very big macromolecules the orientation effect is quite marked. 

The tendency of things to get "messed up" is common in everyday life. You may rake the leaves on your lawn into an orderly pile, but after a few windy days the leaves are again scattered randomly. The reverse process is nonspontaneous the wind never blows the randomly disordered leaves into a neatly arranged pile. Molecular systems behave similarly Molecular systems tend to move spontaneously to a state of maximum randomness or disorder. 

Molecular randomness, or disorder, is called entropy and is denoted by the symbol S. Entropy is a state function (Section 8.3), and the entropy change AS for a process thus depends only on the initial and final states of the system ... 

Minerals are particularly challenging for modellers as they are often formed under conditions of high temperature and/or pressure and may contain several randomly-distributed cations and water. Nevertheless modelling solids at the temperatures and pressures found in the Earth s core and mantle and in the core of other planets is possible whereas it is not always possible to reproduce these conditions experimentally. Molecular dynamics is particularly suited to disordered systems and high temperature simulations and the advances in computer hardware offer an opportunity for an expansion in modelling of minerals. 

Anisotropic molecules show optically isotropic behavior in the bulk when they are disordered and randomly oriented, for instance in solutions or liquid crystal above the transition temperature. Under the influence of a strong beam, the induced dipole moment of the molecules feels a torque that tends to orient the molecule. The reorientation of the molecular dipoles induces a change in the refractive index. The typical values for molecular susceptibilities and the time-responses vary depending on the type of systems. For small anisotropic molecular systems, x 10 esu, with a time response 10 s. However, in the nematic phase, liquid crystal molecules are strongly correlated, resulting in much higher values, x 10 esu,... 

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