Direct calorimetric methods

In this section we deal with the first of the physical effects which impinge on reactivity — the influences which heats of reaction and bond dissociation energies have on the course of chemical reactions. Both heats of reaction and bond dissociation energies are enthalpy values that are experimentally determined by thermochemical methods, in the first case usually by direct calorimetric methods, in the second by more indirect techniques 22). 

Direct calorimetric method or temperatnre dependence of eqnilibrium constants can be nsed to measnre enthalpies and entropies of acid-base reactions. Calorimetric techniqnes allow obtaining an interesting quantification and evaluation of the gas-solid interactions and more details on use of data from these measurements will be given in the following section. 

In a thorough review of calorimetric studies of clathrates and inclusion compounds, Parsonage and Staveley (1984) presented no direct calorimetric methods used for natural gas hydrate measurements. Instead, the heat of dissociation has been indirectly determined via the Clapeyron equation by differentiation of three-phase equilibrium pressure-temperature data. This technique is presented in detail in Section 4.6.1. 

Equation (24.4) shows that the heat of mixing can be calculated from the temperature variation of the activity coefficients. This may be done by measuring the partial vapour pressures of the solution at different temperatures. It is generally preferable, however, to employ a direct calorimetric method, by which h is found without having to employ the activity coefficients. 

The heat of association, in aqueous solution, of fluoride ion with metal ions of the Mn(Il)-Zn(II) series has been measured by a direct calorimetric method at / = 0.5 M and... 

Among direct calorimetric methods there are two main modifications adiabatic [65-72] and conduction [73-78] calorimeters. Adiabatic calorimeters have been widely employed to determine enthalpies of phase changes for volatile compounds, whereas conduction calorimeters are among the most accurate methods for compounds with low vapour pressures. 

Generally, it is not possible to measure, by direct calorimetric methods, the strength of a bond formed when two radicals or atoms are joined together. It is more convenient to measure the bond dissociation energy. Experimental methods for determining bond dissociation energies in diatomic and polyatomic molecules have been described in detail elsewhere. The two methods which... 

The values of and of TS for CF + CH4 shown in Table 1 were estimated by combining the measurements of G made by Thorp and Scott over the temperature range 105.5 to 110.5 K with the recent value of this function measured at 98 K by Simon and Knobler and are thus subject to considerable uncertainty. It is to be hoped that FT for this important binary system will be measured by a direct calorimetric method in the near future. 

In contrast to the stable hydrocarbons, where the standard enthalpy of formation is based on one or another of the direct calorimetrical methods, values for radicals come from all sorts of very difficult measurements ranging from photoionization mass spectroscopy to measurements of reaction rates. It is no surprise that the results are more contentious and less accurate. In Table 1.3, the uncertainty ranges can be seen to be typically an order of magnitude greater than those for stable hydrocarbon values except where the Active Thermochemical Tables can help. 

To discover the origin of the departures, Boissonas determined the heats of solution of these systems by a direct calorimetric method and... 

The direct calorimetric method for measurement of for multicomponent gas mixtures (i > 2) may still be tedious, but it is much simpler, more accurate, and less time-consuming than the indirect methods. 

Consider the possible causes of predominant formation of the NisTa compound from five possible componds according to the phase diagram (Fig.4) [12]. Firstly, this compound is the strongest, as is seen from Table 2. Enthalpy of its formation measured by the direct calorimetric method is -81.6 kJ/mole at 1200 C [15] whereas according to [13] AEf =-l 16.8 kJ/mole at 900 1050 C. At the same time the thermodynamic characteristics of the NisTa compound were not determined. 

Reaction calorimetry is the experimental determination of the enthalpy changes accompanying chemical reactions by direct methods using calorimeters. It is the principal means by which enthalpies of formation of pure chemical compounds are determined. With the exception of certain binary compounds, chiefly oxides, it is impractical to measure the enthalpy of formation of a compound from its elements directly, and it is necessary to determine the enthalpy of a reaction involving the compound in which the enthalpies of the other reactants and products are all known, and then to apply Hess s law. Occasionally, enthalpies of formation can be derived from the study of equilibria (as measured by the e.m.f.s. of electrochemical cells, dissociation pressures, etc.) by means of second- or third-law methods, or from electron impact experiments, but such indirect approaches are outside the scope of the present review, which is confined to the discussion of experimental procedures used in direct calorimetric methods. 

Heat of Adsorption from Direct Calorimetric Methods... 

Heats of adsorption derived from direct calorimetric methods are based on the measurement of the heat evolved when a known amount of gas is allowed to adsorb onto a clean surface. A clean surface is a solid surface kept in high vacuum conditions after having been activated either in vacuo in order to eliminate (either totally or partially) the surface contaminants, or in controlled atmosphere/conditions. 

In the following, the thermodynamic features of the direct calorimetric methods for measuring the heat of adsorption will be discussed. The integral heat 2 is the heat evolved when n moles are adsorbed at constant T in a closed gas-solid system. Because no volume work is done, according to the First Law, the change in internal energy during the adsorption of moles ... 

The chemical potential difference —ju may be resolved into its heat and entropy components in either of two ways the partial molar heat of dilution may be measured directly by calorimetric methods and the entropy of dilution calculated from the relationship A i = (AHi —AFi)/T where AFi=/xi —/x or the temperature coefficient of the activity (hence the temperature coefficient of the chemical potential) may be determined, and from it the heat and entropy of dilution can be calculated using the standard relationships... 

The integral heat of mixing is, of course, the quantity directly measured in the calorimetric method However, the heat change on diluting a solution of the polymer with an additional amount of solvent may sometimes be measured in preference to the mixing of pure polymer with solvent In either case, the desired partial molar quantity AHi must be derived by a process of differentiation, either graphical or analytical. 

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

Table 5 lists equilibrium data for a new hypothetical gas-phase cyclisation series, for which the required thermodynamic quantities are available from either direct calorimetric measurements or statistical mechanical calculations. Compounds whose tabulated data were obtained by means of methods involving group contributions were not considered. Calculations were carried out by using S%g8 values based on a 1 M standard state. These were obtained by subtracting 6.35 e.u. from tabulated S g-values, which are based on a 1 Atm standard state. Equilibrium constants and thermodynamic parameters for these hypothetical reactions are not meaningful as such. More significant are the EM-values, and the corresponding contributions from the enthalpy and entropy terms. 

Most thermochemical calorimetric methods are used to determine enthalpy changes of chemical reactions. The reaction may give the enthalpy of interest directly or may represent a step in a thermodynamic cycle needed to obtain an enthalpy of interest. These techniques are also very suitable for direct determination of enthalpy of mixing in the liquid state or indirect determination of enthalpy of mixing in the solid state. Calorimetric methods for studies of chemical reactions involving solids can be divided into three main categories ... 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

AH values for various monomers. The AS values fall in a narrower range of values. The methods of evaluating AH and AS have been reviewed [Dainton and Ivin, 1950, 1958], These include direct calorimetric measurements of AH for the polymerization, determination by the difference between the heats of combustion of monomer and polymer, and measurements of the equilibrium constant for the polymerization. The overall thermodynamics of the polymerization of alkenes is quite favorable. The value of AG given by... 

Three types of measurements are usually used for the study of thermomechanical behaviour of polymers the temperature changes resulting from the instantaneous loading of the sample, the temperature dependence of the stress or force and direct calorimetric measurements of heat effects in various deformation modes. Detailed discussion of the first and second types of thermomechanical measurements may readily be found in the literature and, therefore, we confine ourselves to a brief description of typical procedures. As to the third method, we will describe it in... 

The most dramatic effects of Lewis bases in organolithium chemistry are observed in polymerization reactions. Aside from colligative property measurements, there is little direct quantitative information on the nature of the organolithium-base interactions responsible for the observed effects. The calorimetric method has been used also to examine the fundamental nature of the interaction of bases with polymeric organolithium compounds 83,88,89). Information is now available on the ground-state interaction of bases with poly(styryl)lithium (PSLi), poly(isoprenyl)lithium (PILi) and poly(butadienyl)lithium (PBDLi). 

The entropy of transition, AS can also be obtained directly from an integration of (ACp/T)dT. As shown in Figure 16.8, the calorimetric method also provides a direct measure of the change in the heat capacity, AtransQ. of the two macrostates at Tm. That is... 

Titration calorimetry has been successfully employed in the determination of thermodynamic parameters for complexation (Siimer et al., 1987 Tong et al., 1991a). The technique has the advantage of employing direct calorimetric measurements and has been proposed as the most reliable method (Szejtli, 1982). It should be noted that the information derived from multistep series reactions is macroscopic in nature. In contrast to spectrophotometric methods that provide information concerning only the equilibrium constant(s), titration calorimetry also provides information about the reaction enthalpy that is important in explaining the mechanism involved in the inclusion process. 

The study and control of a chemical process may be accomplished by measuring the concentrations of the reactants and the properties of the end-products. Another way is to measure certain quantities that characterize the conversion process, such as the quantity of heat output in a reaction vessel, the mass of a reactant sample, etc. Taking into consideration the special features of the chemical molding process (transition from liquid to solid and sometimes to an insoluble state), the calorimetric method has obvious advantages both for controlling the process variables and for obtaining quantitative data. Calorimetric measurements give a direct correlation between the transformation rates and heat release. This allows to monitor the reaction rate by observation of the heat release rate. For these purposes, both isothermal and non-isothermal calorimetry may be used. In the first case, the heat output is effectively removed, and isothermal conditions are maintained for the reaction. This method is especially successful when applied to a sample in the form of a thin film of the reactant. The temperature increase under these conditions does not exceed IK, and treatment of the experimental results obtained is simple the experimental data are compared with solutions of the differential kinetic equation. 

For a single reaction, the determination of reactant accumulation can be done directly by using calorimetric methods the conversion is replaced by the thermal conversion defined by... 

Method C The calorimetric back titration method, the sample of engine oil was dissolved in a mixture of toluene-isopropanol-water (50 49.5 0.5,v/v) and 0.25 M HC104 in isopropanol-toluene (1 1) was used as the titrant the excess acid was titrated by 1 M (CH3)4NOH in isopropanol. Method D The determination of the TAN and AH was performed by direct calorimetric titration. (CH3)4NOH in isopropanol was used as the titrant and a toluene-isopropanol-water (50 49.5 0.5,v/v) was used as the solvent... 

---