Direct Reaction Calorimetry

The progress of polymer degradation may be followed by a wide variety of techniques, some of them being mentioned at the right column in the Bolland-Gee scheme (Scheme 2). There are techniques that directly monitor some of the elementary reaction steps such as, for example, oxygen absorption (reaction 2), differential scanning calorimetry (DSC) (reaction 3), chemiluminescence (reaction 11) analytical and/or spectral methods of determination of hydroperoxides, etc. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

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. 

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

Constant-pressure calorimetry Constant-pressure calorimetry directly measures an enthalpy change (A/ ) for a reaction because it monitors heat flow at constant pressure AH=qp. 

Table 7 shows the electron contribution to the energy of activation determined from a GAUSSlAN-03 and the Gibbs energy of transient state calculated with the aid of a MOLTRAN program from the zero oscillation frequencies. Also, Table 7 shows the rate constan we determined for the symmetrical and asymmetrical reaction pathways and the experimental value borrowed from [33]. The latter value was obtained by the method of cryokinetic calorimetry (direct measurements on the rate constants of the reaction of TFE with pure ozone were performed at 90-150 K). The measurements showed that that the rate constant may be described as follows ... 

Morozova, Vladimirova, Stolyarova, and Nepomnyashchaya [74MOR/VLA] determined the enthalpy of formation of Ni3Sc2(cr) using direct reaction calorimetry. The obtained value is selected ... 

Baskin and Smith [70BAS/SM1] measured the enthalpy of formation USe(cr) and U ,Se4(cr) using direct reaction calorimetry and found minor amounts of P-USe2 as a by-product in one of their samples. In order to evaluate their results, the enthalpy of formation of P-USe2 was estimated to be - 426.8 kJ-moP . The measurements were reevaluated by the review as discussed in Section V.13.5.1.1 without using an estimated value, yielding extreme limits for the enthalpy of formation of P-USe2, - 477 kJ-mof... 

Direct Reaction Calorimetry (DRC) works slightly different. It is a form of calorimetric measurements where you synthesize the alloys directly in the crucible (container), where you also measure the heats. The method will be described very quickly here, for further reading one could consult Refs. [122, 123, 124]. In this method you have a crucible made of ceramic aluminium, in which several thermoelectric elements (usually made of a platinum alloy) are lodged. With the help of these one can then measure the heat of the reaction and get the mixing enthalpy from... 

Direct Calorimetry. Some reactions occur to completion without side reactions, and it is therefore possible to measure their AH values by causing the reactions to occur in a calorimeter. The neutralization of an aqueous solution of a strong acid by a solution of a strong base is an example of such a process, the reaction which occurs being... 

Kub] Adiabatic direct reaction calorimetry Integral enthalpy at 1292°C, whole composition range, a,y,a + y... 

There has only been one thermodynamic study of this system. [1983Kub] measured the heats of formation of alloys lying in the p phase field at 1323 and 1500K by direct reaction calorimetry. Only fluee ternary compositions were studied. 

The heats of formation of the crystalline adducts of pyridine with the triphenyls of B, Al, Ga and In were obtained by direct reaction calorimetry thus... 

A determination of AH29s by combustion calorimetry gave —43.9 0.4kcal/mole for a composition near TiC, 0 (Humphrey, 1951). The carbide was TiCo.996 (total carbon) with 0.60% impurities and was the same material used by Naylor (1946) for his heat capacity measurement. This becomes — 44.1 kcal/mole when a more recent value for the heat of formation of TiOj is used (Mah et al, 1957). The heat produced by the direct reaction between the elements above 1320° has been used to calculate a standard heat of formation of —45.5 + 4.6 kcal/mole for an unstated composition (Lowell and Williams, 1961). 

While calorimetric methods, direct reaction calorimetry or solution calorimetry using tin or aluminum as solvent have been performed in rare earth based alloys, this has not been the case in actinide-based alloys, where acid solution calorimetry has sometimes... 

The excess molar enthalpies of the BaF2-LiF-NdF3 system were measured in the temperature range 1220-1400 K principally by direct-reaction calorimetry with the direct drop method [16]. A high-temperature calorimeter operable up to 1800 K was used for excess molar enthalpies determinations it has been described several times [17]. 

As to the computation of reaction enthalpies and entropies, AH and AS , the same arguments apply if they have been obtained from the temperature dependence of the equilibrium constant. A different situation arises vdien AH is determined directly from calorimetry, say with a constant relative error 6. The standard entropy AS then has the standard error... 

In equilibrium measurements, there is the possibility of determining the reaction enthalpy AH directly from calorimetry and of combining it with logK (i.e., AG°) to get the reaction entropy, AS . This case, advantageous and simple from the statistical point of view, was only mentioned in a previous paper (149). Since that time, this experimental approach has been widely used (59, 62-65, 74-78, 134, 137, 138, 210, 211) hence, a somewhat more detailed mathematical treatment seems appropriate. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

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