Classical reaction solution calorimetry

One of the limitations of "classical reaction-solution calorimetry (either relying on temperature change or heat flux measurements) ) as a tool for obtaining information on the energetics of chemical bonds, stems from the fact that reactions seldom involve the formation or the cleavage of an individual bond. Therefore, the enthalpy of reaction usually reflects a difference between bond dissociation enthalpies. A second drawback of the method is that the bond enthalpy balance can be masked by solvation effects. 

In organometallic reactions it is often possible to make measurements or reasonable estimates of vaporization and solution enthalpies, in order to derive the enthalpy of reaction in the gas phase. Yet, the situation where only one bond dissociation enthalpy is unknown is rather uncommon. Despite these shortcomings, classical reaction-solution calorimetry, being a well tested, inexpensive, reliable technique, has provided an outstanding contribution to our present knowledge in the area of transition metal organometallic thermochemistry. 

Results obtained by classical and non-classical reaction-solution calorimetry are described in the present paper. 

The application of classical and non-classical (photoacoustic) reaction-solution calorimetry to probe the energetics of metal-ligand bonds in organometallic systems is briefly analysed and illustrated by thermochemical results involving two families of compounds. The classical reaction-solution studies enabled the discussion of the systematics of metal-carbon bond enthalpies in several complexes M(t) -C H ) L. The photoacoustic studies addressed the effect of phenyl goups on the energetics of silicon-hydrogen bonds in phenyl-substituted silanes. 

A "non-classical" type of reaction-solution calorimetry has been developed in recent years (2,3). In this new method, the enthalpy of a reaction is not measured with thermometers or... 

One may be somewhat disappointed by the error bars in A0bs7/ and Ar/7, which are larger then most obtained by classical calorimetric methods, such as reaction-solution or combustion calorimetry. However, the comparison is unfair because PAC deals with species that have lifetimes smaller than a microsecond, not amenable to those classical methods. Although the quality of the photoacoustic measurements is rather good, as shown by the correlations obtained (see figure 13.7), a realistic error in the ratio of the slopes (0bs) is 1-2%, which implies an uncertainty in A0bsH of 4 to 8 kJ mol-1. This error bar is particularly serious when the reaction quantum yield is low (recall equation 13.15). 

In the last few paragraphs the describing adjectives were selected with care suitable, reliable, but never correct, exact or true. The reaction calorimetry as applied in safety engineering must not be mistaken for the classical calorimetry known from physical chemistry. Most of the measurements are conducted with comparably highly concentrated solutions and consciously using material directly obtained fix>m the plant or development laboratory. Many theoretical approaches of physical chemistry, however, are valid only if the experiments are conducted in very diluted systems. Especially the heat of reaction, which, as has already been mentioned several times, must be looked upon as a gross value in those safety related experiments, is a property extremely sensitive to the presence of impurities or similar influences. 

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