Enthalpy change sign, determining

For many reactions, AH is a large negative number the reaction gives off a lot of heat In other cases, AH is positive heat must be absorbed for the reaction to occur. You may well wonder why the enthalpy change should vary so widely from one reaction to another. Is there some basic property of the molecules involved in the reaction that determines the sign and magnitude of AH ... 

There seem to be no direct calorimetric determinations of enthalpies of solution of rare-earth tribromides in nonaqueous solvents,3 and very few reports on the temperature variation of solubilities whence solution enthalpies might be roughly estimated. The most detailed set of data concerns cerium tribromide in pyridine (257). In this system there exists a series of solvates (cf. Section IH,C,2), but sufficient solubilities were determined for the estimation of enthalpies of solution of each solvate. These enthalpies are included in Fig. 3, which shows an extraordinary zig-zag variation of solubility with temperature. The actual values of enthalpies of solution cannot be accurate, but at least it is clear that they change sign and magnitude in an eccentric manner. 

In the definition of corrosion given above, the word "spontaneous" was used in the thermodynamic sense, i.e., a chemical reaction for which the free enthalpy change, AG, is negative. Therefore, one can ascertain whether a postulated corrosion reaction can lead to corrosion by determining the sign of the free enthalpy change of the reaction. 

In many situations we will find it valuable to know the sign and magnitude of the enthalpy change associated with a given chemical process. As we see in the following sections, AH can be either determined directly by experiment or calculated from known enthalpy changes of other reactions. 

In order to determine the sign of the contribution to the enthalpy change associated with the HtpO process, we need to know the difference in the stabilization effects of M and D on the structure of the solvent. 

To determine the sign of AStotai = ASsys + ASsurr/ we need to calculate the values of ASsys and ASsurr. The entropy change in the system equals the standard entropy of reaction and can be calculated using the standard molar entropies in Table 17.1. To obtain ASsurr = —AH°/T, first calculate AH° for the reaction from standard enthalpies of formation (Section 8.10). 

The word driven" is used here to imply that either the entropy or enthalpy term is dominant in determining the sign of AC. There is no force due to enthalpy or entropy responsible for driving spontaneous changes. 

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