Isoperibol reaction-solution calorimetry

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry 

To obtain AT/icp it can be assumed that the calorimetric system, with an energy equivalent , is first brought from the reference temperature 7r to the initial temperature Tx with a corresponding enthalpy change of i(7i — 7r). The reaction is initiated at 7], and the temperature of the system varies from 7) to 7f. The enthalpy change associated with this step is f AT on- + A//,-. Here, 

The value of s (e, or sr) is usually determined by electrical calibration (note that contrary to combustion calorimetry, it is not common practice to separate the initial and final energy equivalents of the calorimeter into the contribution of the reference calorimeter, e0, and those of the contents present in the initial, C1, and, final, ecf, states see section 7.1). In the case of the calorimeter in figure 8.1, a current I is passed trough the resistance F for a known period of time t and the potential change V across F is measured. Then  

Ideally, the energy equivalents e and f should be measured over the same temperature range of the reaction ran, to avoid errors from their variation with temperature and to achieve maximum compensation for errors in the calibration of the temperature sensor [26,128,129], These errors are, however, frequently negligible in the temperature ranges involved, and the measurement of or f is normally performed outside the Jj - Tf interval. This procedure saves time because there is no need to readjust the initial temperature of the calorimeter between the calibration and main experiment runs. It is therefore a common practice, even when an exothermic reaction is studied, to measure before the reaction and ef after the reaction and adjust the experimental conditions so that Jr is the midpoint between J] and Tf. In this case, the temperature of the thermostatic 

C after period of the first calibration experiment and fore period of the reaction experiment D main period of the reaction experiment E after period of the reaction experiment and fore period of the second calibration experiment F main period of the second calibration experiment G after period of the second calibration experiment. 

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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. 

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. 

The conceptual basis of isoperibol solution calorimetry is that the reaction heat dJfect, represents a quantitative measure of the amount of product fortpjetftat is,... 

Another area where titration calorimetry has found intensive application, and where the importance of heat flow versus isoperibol calorimetry has been growing, is the energetics of metal-ligand complexation. Morss, Nash, and Ensor [225], for example, used potenciometric titrations and heat flow isothermal titration calorimetry to study the complexation of UO "1" and trivalent lanthanide cations by tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA), in aqueous solution. Their general goal was to investigate the potential application of THFTCA for actinide and lanthanide separation, and nuclear fuels processing. The obtained results (table 11.1) indicated that the 1 1 complexes formed in the reaction (M = La, Nd, Eu, Dy, andTm)... 

Low-temperature heat capacities of the solid coordination compounds Zn(Leu)S04 l/2H20(s) and Zn(His)S04T/2H20(s) (Leu = Leucine and His = Histidine) were measured by a precision automated adiabatic calorimeter over the temperature range between T = 78 K and T = 371 K. Di and coworkers [228,229] determined the initial dehydration temperature of the coordination compounds by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation with the reduced temperatures (x), [x = f (T)], by a least-squares method. Enthalpies of dissolution of both the complexes were determined by isoperibolic solution-reaction calorimetry. 

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