Calorimetrically determined enthalpies

The report of Gmndnes and Christian (14) presents another interesting complication from solvent effects. The reported enthalpy of formation of the (CHg)3N—SOg adduct in the gas phase is — 9.1 0.4 k. cal mole i, while that measured in heptane is — 11.03 0.3. The cause of this discrepancy is not understood and before drawing any definite conclusions about the nature of the solvent effects in this system, it would be informative to have a calorimetrically determined enthalpy in... 

There is a report of calorimetrically-determined enthalpies of reaction of methyl- and ethyhnagnesium bromides with some ketones in ether solution at 15 °C. The reaction shown in equation 13, which results in an exotherm of —202.3 kJmoU, produces an alkoxymagnesium bromide that also appears as a product of a different reaction in Table 4. Using the enthalpy of formation of MeMgBr and the liquid phase enthalpy of formation of acetone, the enthalpy of formation of the f-butoxymagnesium bromide is calculated as —783.5 kJmol . This is within 10 kJmoU of the value reported in Table 4. 

Solid phosgene exists in three crystalline forms, identified by the following melting temperatures and calorimetrically-determined enthalpies of fusion solid I, T, = 145.37 K ... 

This chapter (no. VI) contains a tabulation of calorimetrically determined enthalpy values accompanying conformational changes of macromolecules in solution (26 references) and their transition temperatures. 

Tabulated are single-ion entropies of about 110 diatomic and polyatomic ions in water Gibbs energies, enthalpies, and entropies of hydration of monatomic ions at 25 C partial molar volumes of about 120 common ions at 25 C ionic partial molar heat capacities of ions Gibbs energies of transfer of inorganic electrolytes from HjO to 020 and calorimetrically determined enthalpies of solution of salts in H2O and 020. 

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 ... 

There seem to be no direct calorimetric determinations of enthalpies of solution of rare-earth tribromides in nonaqueous solvents,3 and very few reports on the temperature variation of solubilities whence solution enthalpies might be roughly estimated. The most detailed set of data concerns cerium tribromide in pyridine (257). In this system there exists a series of solvates (cf. Section IH,C,2), but sufficient solubilities were determined for the estimation of enthalpies of solution of each solvate. These enthalpies are included in Fig. 3, which shows an extraordinary zig-zag variation of solubility with temperature. The actual values of enthalpies of solution cannot be accurate, but at least it is clear that they change sign and magnitude in an eccentric manner. 

I. Tomaszkiewicz, G. A. Hope, C. M. Beck II, P. A. G. O Hare. Thermodynamic Properties of Silicides. VI. Pentamolybdenum Trisilicide (Mo Sf). Fluorine Combustion Calorimetric Determination of the Standard Molar Enthalpy of Formation at the Temperature of 298.15 K. J. Chem. Thermodynamics 1997, 29, 87-98. 

Calorimetric Determination of Enthalpies of Binary and Multicomponent Mixtures of Components Encountered in SNG Processes"... 

In this section, you learned how to calculate the enthalpy change of a chemical reaction using Hess s law of heat summation. Enthalpies of reaction can be calculated by combining chemical equations algebraically or by using enthalpies of formation. Hess s law allows chemists to determine enthalpies of reaction without having to take calorimetric measurements. In the next section, you will see how the use of energy affects your lifestyle and your environment. 

We have found a direct calorimetric determination of both K and AH° to be capable of producing much more reliable thermodynamic data than infrared or ultraviolet-visible spectroscopy. Furthermore, things that react extensively have enthalpies but may not have convenient changes in absorption bands. The procedure for the simultaneous determination of K and AH° from calorimetric measurements has been described (20—22). 

Handa, Y.P. (1986b). Calorimetric determinations of the compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of xenon and krypton. J. Chem. Thermodynamics, 18 (9), 891-902. 

There is one other report in the literature of a measurement of the enthalpy of formation of an organomagnesium halide. The enthalpy of reaction of magnesium with methyl iodide in ether was calorimetrically determined as —273.6 0.8 kJmoH Using a recent enthalpy of formation for liquid methyl iodide of —13.6 0.5 kJmoU, the enthalpy of formation of methylmagnesium iodide is —287.2 kJmoU. The exchange (equation 8) is thus 11.2 kJmoU endothermic. 

Gill, S. J., N. F. Nichols, and I. Wadso, Calorimetric Determination of Enthalpies of Solution of Slightly Soluble Liquids II. Enthalpy of Solution of Some Hydrocarbons in Water and Their Use in Establishing the Temperature Dependence of Their Solubilities. 

In principle, the calorimetric determination of the fraction of crystallinity of a semicrystalline polymer is simple. One needs only to know (1) the enthalpy change required to convert the sample to the completely... 

As we described earlier, the calorimetric determination of log K allows one to also get ArH for reaction (15.37). The values reported by Izatt and his colleagues were obtained in an aqueous solution with an ionic strength of 0.1. Izatt reports that enough measurements were made in more dilute solutions to show that the enthalpy of dilution to the infinitely dilute solution (the standard state) is small and can be ignored. Hence, we will assume that the ArH values reported are the standard state ATH° values. Thus we have available, ArG°, obtained from equation (15.42), and ArS ° obtained from equation (15.43). 

A comparison of the different reaction enthalpies reveals that the calorimetric determination shows a large difference between ArHi and ArH2 (90 kj mol-1), whereas ArHi and ArH2 obtained by the infrared method differ by only 10 kj mol-1. The combined protocol (difference = 40 k mol-1) lies in between. In particular, the results by the calorimetric method appear to be unreasonable. 

CALORIMETRIC DETERMINATION OF THE STANDARD TRANSFORMED ENTHALPY OF REACTION... 

The (non-combustion calorimetrically) experimentally determined enthalpies of formation for benzvalene [31], semibullvalene [32], and triquinacene [33] are 363, 308, and 224 kJ mol1 respectively. Assuming linear dependence of enthalpy of formation on any of these counts (r2 = 0.98), we derive a value for the enthalpy of formation of tetrahedrane of 437 kJ mol1. The equivalent non-statistical derivation is to assume thermoneutrality for the reaction... 

Calorimetric Determination of the Standard Transformed Enthalpy of Reaction... 

---