Entropy changes accompanying chemical reactions

To move into the arena of biochemistry, where reactants are transformed into products, we need to estabiish procedures for using the tabuiated vaiues of absoiute entropies to caicuiate entropy changes associated with chemicai reactions to assess the spontaneity of a bioiogicai process, we need to see how to take into account entropy changes in both the system and the surroundings. 

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First we will consider the entropy changes accompanying chemical reactions that occur under conditions of constant temperature and pressure. As for the other types of processes we have considered, the entropy changes in the surroundings are determined by the heat flow that occurs as the reaction takes place. However, the entropy changes in the system (the reactants and products of the reaction) can be predicted by considering the change in positional probability. 

Thus the absolute entropies of elements and compounds can be. established. These can be used to determine the entropy changes accompanying chemical reactions. 

So knowing the states of the reactants and products in a chemical reaction should allow us to predict whether the reaction is accompanied by an increase or a decrease in entropy. Consider, for example, the reaction 2Na(s) + Cl fg) -> 2NaCl(s). We know that the entropies of solids are very much smaller than the entropies of gases and, because this reaction results in a decrease in the number of moles of gaseous molecules (from 1 to 0), the entropy will decrease. Similarly, we would predict an increase in entropy for the reaction CaCOjfs) -> CaO(s) + CO g) because there is an increase in the number of moles of gaseous molecules (from 0 to if However, the entropy change for the reaction CaSiOjfs) CaO(s) + SiO s) is difficult to predict because the reactants and products are solids and are likely to have very similar entropy values. All we can say is that the entropy change is likely to be small. 

Entropy changes accompanying a particular chemical reaction v, A, 0 are... 

It follows that, if in a chemical reaction a molecule is only partly modified, the part of the molecule which remains unchanged contributes in the same way to the total entropy of both the reactants and of the products. As a consequence, the entropy change accompanying a given reaction is simply the difference between the contribution to the total entropy of those groups which are modified in the reaction. 

The entropy change accompanying a chemical reaction is defined as the difference between the sum of the entropies of all products and the sum of the entropies of all reactants. Thus for a reaction... 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

We use a short version of the seven-step method. The problem asks for the entropy and enthalpy changes accompanying a chemical reaction, so we focus on the balanced chemical equation and the thermodynamic properties of the reactants and products. 

The key concept of entropy as an assessment of dispersal of matter and of energy is carefully developed to provide a firm foundation for later ideas including heat changes that accompany chemical and physical changes, prediction of reactions, and chemical bond stability. Throughout this chapter, many fundamental terms are rigorously defined, discussed, and illustrated for use throughout later studies of chemistry. 

For a chemical reaction, any change of the entropy of the system provoked by a reaction is accompanied by a change of the entropy in the surrounding. If the generated (or consumed) heat is reversibly transferred from or into the system, the change of the entropy becomes zero, which is expressed by the free enthalpy (Cibbs function) as ... 

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