Reaction-solution calorimetry

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

There is general agreement that static-bomb combustion calorimetry is inherently unsatisfactory to determine enthalpies of formation of organolead compounds2,3. Unfortunately, as shown in Table 6 only three substances have been studied by the rotating-bomb method. The experimentally measured enthalpies of formation of the remaining compounds in Table 6 were determined by reaction-solution calorimetry and all rely on AH/(PbPh4, c). 

Reaction calorimetry in solution has been used19) to measure the heat of reaction between halogens and the compounds [Co3(CX)(CO)9](X = Cl, Br). The enthalpies... 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Figure 8.2 Scheme of a typical calibration circuit used in isoperibol reaction-solution calorimetry. P power supply S switch R- electrical resistance inside the calorimetric vessel (F in figure 8.1) R2 standard resistance. 

Various substances and reactions have been used to test the accuracy of reaction-solution calorimeters [3 9,40]. The solution of tris(hydroxymethyl)amin-omethane (THAM orTRIS) in 0.1 mol dm-3 HCl(aq), which was first proposed by Wadso and Irving [133] in 1964 and recommended by the Standards Committee of the U.S. Calorimetry Conference in 1966 [134,135], is perhaps the most widely used method. Several problems encountered in the use of the THAM+HC1 (aq) reaction to assess the accuracy of reaction-solution calorimeters have been... 

Johnson et al. [143] also studied the dehydrated form of mordenite by reaction-solution calorimetry. Their results and the foregoing enthalpy of formation data lead to Af//°(Cao.289Nao.36iAlo.94oSi5.06oOi2.ooo, cr) = —5661.7 4.8 kJ mol-1. From the enthalpies of formation of both forms of mordenite and the enthalpy of formation of liquid water already quoted (—285.830 0.040 kJ mol-1), it is possible to conclude that at 298.15 K, the enthalpy of dehydration of mordenite, which corresponds to the reaction... 

Johnson et al. [143] used low-temperature adiabatic calorimetry and high-temperature drop calorimetry to obtain the heat capacity of both forms of mordenite as a function of the temperature. These results and the results of the reaction-solution calorimetric studies discussed herein, enabled the tabulation of the thermodynamic properties (C°, S°, Af H°, and Af G°) of mordenite from 0 K to 500 K and dehydrated mordenite from 0 K to 900 K. 

The enthalpy of the reverse of reaction 10.17, the cis - trans isomerization reaction is thermally activated and thus can be determined by isoperibol reaction-solution calorimetry (however, because the reaction is slow, a catalyst must be used). These experiments were also made by Dias et al. and led to -49.1 1.0 kJ mol-1 for reaction 10.18. 

A primary use of titration calorimetry is the determination of enthalpies of reaction in solution. The obtained results may of course lead to enthalpies of formation of compounds in the standard state by using appropriate thermodynamic cycles and auxiliary data, as described in chapter 8 for reaction-solution calorimetry. Moreover, when reactions are not quantitative, both the equilibrium constant and the enthalpy of reaction can often be determined from a single titration run [197-206], This also yields the corresponding ArG° and ATS° through equations 2.54 and 2.55. 

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). 

R. R. Hung, J. J. Grabowski. Enthalpy Measurements in Organic Solvents by Photoacoustic Calorimetry A Solution to the Reaction Volume Problem. J.Am. Chem. Soc. 1992,114, 351-353. 

Thermochemistry has been closely associated with techniques such as reaction-solution and combustion calorimetry, although it has long been recognized that thermochemical information can also be obtained from noncalorimetric methods, such as equilibrium and kinetics experiments. 

Methods DBBC = Databook MS = mass spectrometry RB = rotating-bomb combustion calorimetry RS = reaction-solution calorimetry SB = static-bomb combustion calorimetry. 

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