Disorder, chemical systems

Note that the entropy of a system cannot increase forever. Eventually, a maximum state of disorder is reached. When this happens, the system appears to have constant properties, even though changes are still taking place at the molecular level. We say that a chemical system is at equilibrium when it has constant observable properties. Therefore, equilibrium occurs when a system has reached its maximum entropy. In the next section, you will look more closely at the reactants and products of chemical systems and learn how equilibrium is measured. 

The second law of thermodynamics involves entropy, which for our purposes is a statistical measure of the degree of disorder in a chemical system. As an illustration, compare the arrangements of Na and Cl ions in both solid and liquid sodium chloride. 

The second law of thermodynamics states that the total entropy of a chemical system and that of its surroundings always increases if the chemical or physical change is spontaneous. The preferred direction in nature is toward maximum entropy. Moving in the direction of greater disorder in an isolated system is one of the two forces that drive change. The other is loss of heat energy, AH. 

The randomness or disorder of the components of a chemical system is expressed as entropy, S (Box 1-3). 

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. 

If affective disorder could be eliminated by the simple expedient of administering a chemical substance, did this not suggest that the basis of these disorders might take a chemical form And not only that might affect, mood, emotion, and the whole gamut of human conscious experience be translated into chemical terms At a stroke, the elusive "aetherial" Freudian psyche was replaced as the primary object of attention by the polyphasic physico-chemical system called the brain. Psychiatry came of age and took its place among the biological sciences. 

Fractals, Diffusion, and Relaxation in Disordered Complex Systems A Special Volume of Advances in Chemical Physics, Volume 133, Part A, edited by William T. Coffey and Yuri P. Kalmykov. Series editor Stuart A Rice. 

As we affirmed in our discussion of thermodynamics, systems naturally tend toward a state of maximum entropy. Physical and chemical systems alike favor disorder. It is their natural state. Gases expand. Dyes distribute. Mixtures do whatever they can to dilute. Solutions do not want to lose solvent and become concentrated because there is more entropy in being dilute. The vapor pressure of a solution is lowered so the solution will not lose solvent and become more concentrated. The boiling point is elevated for the same reason. There is also more entropy for a liquid solution than for a solid, so solution freezing is anti-entropic. Hence, solutions experience... 

Ngai, K. L., Casalini, R., Capaccioli, S., Paluch, M., and Roland, C. M. (2006) Adv. Chem. Phys. in Chemical Physics Part B, Fractals, Diffusion and Relaxation in Disordered Complex Systems, 133B, 497-582... 

Adv. Chem. Phys. in Chemical Physics Part B, Fractals, Diffusion and Relaxation in Disordered Complex Systems, 133B, 497-582... 

The free energy of a chemical system can be defined as G = H — TS, where H is the bond energy, or enthalpy, of the system T is its temperature in degrees Kelvin (K) and S is the entropy, a measure of its randomness or disorder. If temperature remains constant, a reaction proceeds spontaneously only if the free-energy change AG in the following equation is negative ... 

A measure of the randomness of a chemical system is its entropy. The entropy of a substance is represented by the symbol S°. A random, or disordered, system is characterized by high entropy a well-organized system has low entropy. 

What do we mean by disorder in chemical systems Disorder is simply the absence of a regular repeating pattern. Disorder or randomness increases as we convert from the solid to the liquid to the gaseous state. As we have seen, solids often have an ordered crystalline structure, liquids have, at best, a loose arrangement, and gas particles are virtually random in their distribution. Therefore gases have high entropy, and crystalline solids have very low entropy. Figures 8.3 and 8.4 illustrate properties of entropy in systems. 

---