Ionic reactions, entropy changes

If the change in heat capacity in a chemical reaction is equal to zero, the enthalpy of the reaction is independent of temperature, and the equilibrium constant of the chemical reaction can be readily calculated over a range of temperature without making an integration, as described in Section 3.7. In general, the enthalpy of a chemical reaction is a function of temperature and ionic strength. When ArG° and ArH° are known, the standard reaction entropy ArS° can be calculated... 

LaMer has emphasized that E probably changes with temperature, but present experimental methods are not often accurate enough to distinguish the difference. An entropy of activation must exist if E depends on temperature and the activated molecules and the normal, reacting molecules have different specific heats. Ionic reactions particularly are apt to involve large specific heats which may change in reaction. The entropy of activation may arise in part from specific steric factors. 

Solvation entropy changes are large if the reactions involve ionic charges. If opposite electric charges are created the contribution to the... 

The standard heats of formation of ions, as recorded in Table XL, may be combined with the free energies of formation and the entropies, in the same table, to calculate thermodynamic quantities for a variety of ionic reactions. Further, these data may be utilized in conjunction with those given in Tables V, XV, XIX and XXIV to determine standard free energy, entropy and heat content changes for reactions involving both ions and neutral molecules. 

Charles (1954) has compared the entropy of ethylenediamine-tetraacetate (EDTA) complex formation for several elements, including zinc, and finds the entropy change to be a linear function of the partial molal entropy of the complexed metal. In Fig. 10, the entropy changes in the formation of ammonia (Williams, 1954), ethylenediamiue (En) (Davies et al., 1954), and EDTA (Charles, 1954) complexes of Zn, Cu, and Cd are plotted as a reciprocal of the ionic radius minus a term containing the molecular weight (Powell and Latimer, 1951), as a measure of the partial molal entropy of the cations. The AS° values plotted are for the reaction ... 

The van t Hoff plot in Figure 3.2-20 yielded the enthalpy and entropy changes for the reaction to be (0.8 0.3)kcal/mol and (0.7 0.8)cal/(Kmol), respectively. This indicates that the heterobimetallic bond formation process in eq (3.2-5) is nearly thermoneutral, and that there is little ionic character to the Mn-Co bond in (CO)5Mn-Co(CO)4. 

From the value of the equilibrium constant and its temperature variation one calculates free energies, enthalpies, and entropies for the reactions. As an illustration of the results obtained we give in Table XVIII thermodynamic values for ionic equilibria in benzyl acetate and t-amyl acetate. The remarkable aspect of these results is the positive AS values obtained for the formation of the protonated dimers of benzyl acetate and r-amyl acetate and the BzAc C3H3 association complex. Reactions involving the association of two entities result in the loss of three translational degrees of freedom, which corresponds to an entropy change of — (30-40)... 

IONIC CLUSTERING EQUILIBRIA—DETERMINATION OF ENTHALPY AND ENTROPY CHANGES OF CLUSTERING REACTIONS... 

Gibbs free energy change of reaction [J/mol] Entropy change of reaction [J/mol K] Faraday constant [96,485 C/mol] oxygen flux density [mol/cm s] surface exchange coefficient [cm/s] partial oxygen pressure [bar] gas constant [8.314 J/mol K] ionic conductivity [S/cm] electronic conductivity [S/cm] temperature [K]... 

This energy cost is partially offset by an increase in entropy as we form two particles out of one. But we have to be careful when we estimate entropy changes because there are invdsible partidpants in most reactions, induding this one. These indsible participants are the solvent molecules.They will have to be disordered and reordered in any ionic reaction in solution, and the net change in entropy may not be at all easy to estimate. 

This enthalpy change of solution is positive, that is, the reaction is endothermic. This reaction proceeds because a large positive entropy change occurs during the dissolving process (Chapter 15). Many ionic compounds have enthalpies of solution that are negative, that is, the reactions are exothermic. 

Let us now try to sketch the behaviour of the enthalpy, entropy and free energy (AfG) as a function of the salt concentration. Upon increasing the ionic strength, a change from exothermic to endothermic behaviour is observed. This means that at low ionic strength the complex formation is enthalpically favourable, but it becomes entropically driven when the salt concentration is increased. This may be explained considering the following reaction ... 

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