Enthalpy and entropy11/30/2023 This section should aid in resolving any uncertainties. For the purposes of this entry different terms may refer to what may seem to be the same effect, but that either a term is being used as a shorthand (isokinetic and isoequilibrium relationships are different, yet are often grouped together synecdochically as isokinetic relationships for the sake of brevity) or is the correct term in context. The misapplication of and frequent crosstalk between fields on this matter has, however, often led to the use of inappropriate terms and a confusing picture. In these contexts, the unambiguous terms are preferred. Many of the more popular terms used in discussing the compensation effect are specific to their field or phenomena. The empirical relations above were noticed by several investigators beginning in the 1920s, since which the compensatory effects they govern have been identified under different aliases. The correlation is significant because, for linear free-energy relationships (LFERs) to hold, one of three conditions for the relationship between enthalpy and entropy for a series of reactions must be met, with the most common encountered scenario being that which describes enthalpy–entropy compensation. The enthalpy and entropy compensate for each other because of their opposite algebraic signs in the Gibbs equation.Ī correlation between enthalpy and entropy has been observed for a wide variety of reactions. Similarly, for the second and third instances, in accordance with the Gibbs free energy equation, with which we derive the listed equations, Δ H scales proportionately with Δ S. An increase in A tends to compensate for an increase in E a,i, which is why we call this phenomenon a compensation effect. When the activation energy is varied in the first instance, we may observe a related change in pre-exponential factors. Where H i are the enthalpy changes and S i are the entropy changes. (iii) between the enthalpy and entropy changes of a series of similar reactions ( enthalpy–entropy compensation) Where H ‡ i are the enthalpies of activation and S ‡ i are the entropies of activation. (ii) between enthalpies and entropies of activation ( enthalpy–entropy compensation) Where the series of closely related reactions are indicated by the index i, A i are the preexponential factors, E a,i are the activation energies, R is the gas constant, and α and β are constants. (i) between the logarithm of the pre-exponential factors (or prefactors) and the activation energies ln A i = α + E a,i/ Rβ The compensation effect refers to the behavior of a series of closely related chemical reactions (e.g., reactants in different solvents or reactants differing only in a single substituent), which exhibit a linear relationship between one of the following kinetic or thermodynamic parameters for describing the reactions: Enthalpy–entropy compensation is a specific example of the compensation effect.
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