Enthalpy spontaneity determination

For many reactions entropy effects are small and it is the enthalpy that mainly determines whether the reaction can take place spontaneously. However, in certain... 

Both these everyday processes are spontaneous, but whereas one process is endothermic, the other is exothermic. The energy and enthalpy of the system increase in one process, but these quantities decrease in the other process. This simple example demonstrates that analyzing energy changes and enthalpy changes is not enough to predict whether a process will occur spontaneously. We need a property other than energy and enthalpy if we hope to use thermodynamics to determine when a process will be spontaneous. 

The total energy of a system involves both enthalpy and entropy. Thus, whichever causes the greater change in overall energy during the reaction will be the one controlling the reaction and determining whether it is exothermic, endothermic, spontaneous, or not spontaneous [11]. 

Definition of enthalpy and entropy Definition of free energy Enthalpy (a measure of the change in heat content of the reactants and products) and entropy (a measure of the change in the randomness or disorder of reactants and products) determine the direction and extent to which a chemical reaction will proceed. When combined mathematically, they can be used to define a third quantity, free energy, which predicts the direction in which a reaction will spontaneously proceed. 

Two factors determine the spontaneity of a chemical or physical change in a system a release or absorption of heat (AH) and an increase or decrease in molecular randomness (AS). To decide whether a process is spontaneous, both enthalpy and entropy changes must be taken into account ... 

Free energy, G = H — TS, is a state function that indicates whether a reaction is spontaneous or nonspontaneous. A reaction at constant temperature and pressure is spontaneous if AG < 0, nonspontaneous if AG > 0, and at equilibrium if AG = 0. In the equation AG = AH — TAS, temperature is a weighting factor that determines the relative importance of the enthalpy and entropy contributions to AG. 

Since we already have tables of standard enthalpies and standard entropies, we can substitute these values into the free energy equation and determine the standard free energies of formation for substances. The standard free energies of formation tell you if a substance will form spontaneously if the constituent atoms are combined. The formula to determine the standard free energy change for a reaction is the same as Equation 17.8 for the enthalpy change (except Gs are substituted for the Hs) ... 

Using bond enthalpies, determine which of the following reactions (not balanced) are likely to be spontaneous. 

Values for the enthalpy and entropy of activation for the reaction catalyzed by 1F7 were determined from the temperature dependence of kcat. Apparently, the observed rate acceleration is due entirely to a lowering of the enthalpic barrier (15 kcal/mol versus 21 kcal/mol for the uncatalyzed reaction (24)), consistent with the notion that induced strain might be an important component of catalysis. The entropy of activation for the antibody-promoted reaction (-22 eu) is actually less favorable than for the spontaneous reaction (-13 eu) (23). This fact may reflect the need for some conformational change in the antibody binding pocket during catalysis. However, possible solvent effects make the interpretation of AS difficult. 

In thermodynamics it is the change in a certain function that is usually important. The change in enthalpy determines whether a reaction is exothermic or endothermic at constant pressure. The change in free energy determines whether a process is spontaneous at constant temperature and pressure. It is fortunate that changes in thermodynamic functions are sufficient for most purposes, because absolute values for many thermodynamic characteristics of a system (such as enthalpy or free energy) cannot be determined. 

This reaction is spontaneous mainly because of the change in the transformed entropy. Note the entropy increases because is produced. For most of these reactions, the enthalpy change at pH 7 determines whether the reaction goes to the right or the left, but sometimes the entropy change does, especially when hydrogen ions are produced by the reaction. Standard transformed thermodynamic properties are given for more enzyme-catalyzed reactions in Chapter 13. 

It is of interest to see the extents to which the standard transformed enthalpy and standard transformed entropy of reaction determine whether the reaction goes spontaneously to the right or the left under specified conditions. 

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