Exothermic process free energy change

Due to the complicated kinetics for both processes no attempt was made in ref. 83 to treat the data quantitatively. It was estimated, however, that the back electron transfer reaction is slower by about 3 orders of magnitude than that of the forward electron transfer. At the same time, the free energy change for the forward reaction (AG° = - 0.4 eV) is smaller than that for the back electron transfer (AG° = — 1.7 eV). This decrease of the reaction rate at large exothermicity was attributed [83] to the decrease of the Franck-Condon factors with increasing J in the situation when J > Er (see Chap. 3, Sect. 5). 

Weller 171> has pointed out that the rate constants for electron transfer to fluorescent aromatics are diffusion-controlled when the process is exothermic but are proportional to the free energy change when the process is endothermic. Here (D/D+) is the donor oxidation potential, positive by convention, and E(A jA)... 

The sequence of enthalpy changes where replacement of a condensed alkali metal counterion by a heavier alkali metal counterion is an exothermic process is now in the same sequence as the observed equilibrium data. (, ) For the interaction of alkali metal cations with sulphated polyelectrolytes this is K>Na>Li. Thus in the case of the sulphated polyanions the enthalpy change and the equilibrium, free energy change, follow the same sequence. The sequence of enthalpy changes seems to imply that the bond strength between the condensed alkali metal cation and the dextran sulphate increases in the sequence Li[Pg.360]

The reaction is exothermic, but not favored entropically. The process may be spontaneous at low enough temperatures for the TAS term to outweigh the enthalpic contribution to the free-energy change. 

This means that in a spontaneous process, which requires a negative free energy change (see Eq. 1.12), the enthalpy of adsorption must be negative in order to compensate the loss of entropy. In other words, the process must be exothermic of an amount of heat evolved at least as high as the decrease of the T AaS term. 

Once an exothermic decomposition is initiated, usually by application of heat to raise the temperature, the energy that is released may maintain the higher temperature and thus cause the reaction to continue until all material is converted or until the reaction is stopped by forced cooling. The change in the Gibbs free energy during such a process (at constant temperature and pressure) is ... 

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