Enthalpy calorimetric measurements

An independent measurement of the energy difference between secondary and tertiary cations in solution is available from calorimetric measurement of the enthalpy... 

P. A. G. O Hare, "Thermochemistry of Uranium Compounds XV. Calorimetric Measurements on UCI4, UOiCl . and UCFF . and the Standard Molar Enthalpy of Formation at 298.15 K of UC14", J. Chem. Thermodyn.. 17. 611-622 (1985). 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

On the other hand, Arnett and his coworkers have reported both the enthalpies of the protonation (AHJ and the hydrogen bond (AHf) for acid-base reactions. They calculated Hj by measuring the association constants for the proton transfer (ionization) in a number of bases by using FSO3H as the acid and determined Ai/j by calorimetric measurements of the heat of dissolution of P-FC6H4OH in various hydrogen bond acceptors, including sulphoxides, in They have also tried to correlate and... 

Simple amides of this type are the bis(trimethylsilyl)amides M[N(SiMe3)2]2 (M = Cd and Hg) the essential thermodynamic data of which have been determined in calorimetric measurements of the heats of hydrolysis in dilute H2S04.146 Evaluation of the measured data yielded the standard enthalpies of formation AH° = —854(21)kJmoU1 and —834(9)kJmol-1 for M =Cd and Hg, respectively. Using subsidiary data, the average thermochemical bond energies E—(Cd—N) 144 and E(Hg—N) 108 kJ mol-1 were also obtained, i.e., the Cd—N bonds are considerably stronger than the Hg—N bonds. 

Table II shows the "heats of formation" of the conjugate phases, that is, the excess enthalpies for mixing the appropriate amounts of water and amphiphile (at the same initial temperature and pressure as the final system) to make a unit amount of the conjugate phase. Values labeled "calorimeter" and "phase volume," respectively, are based on the same set of calorimetric titrations. In the former case the phase composition was taken from the calorimetric measurements, and in the latter case the composition was taken from our phase-volume compositions. Literature values for the heats of formation are based on data from references 13-16.

Gorman-Lewis D, Fein JB, Jensen MP (2006) Enthalpies and entropies of proton and cadmium adsorption onto Bacillus subtilis bacterial cells from calorimetric measurements. Geochim Cosmochim Acta 70 4862-4873... 

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. 

Enthalpies of solution of anhydrous rare-earth chlorides in nonhy-droxylic solvents are also scant. What values exist are collected together in Table XII (178, 183, 218, 220-222), which includes alcohol and water values for ease of comparison. The values in this table are direct calorimetric measurements, performed on anhydrous trichlorides. The value for gadolinium trichloride in dimethylformamide is... 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

The kinetic techniques were densitometry and reaction calorimetry, and the electrical conductivity, K, was monitored for most systems the calorimetric measurements also yielded the enthalpies of polymerisation (AHp). Analysis of the polymers provided information on initial groups, DP, and DPD for many of the products. The determination of the quantity and origin of kinetically significant impurities is a feature of this work, because much of it was done with initiator concentrations, c0, between 10 4 and 10"3 mold"1, and the measured impurity levels, c , ranged from 10"4 down to 10"5 mold 1. 

Anion Binding. This discussion illustrates how valuable information on enthalpy changes of surface reactions (either from temperature dependence or from direct calorimetric measurements) are. Zeltner et al. (1986) have studied calorimetrically the surface complex formation of phosphate and salicylate on goethite. They show that these reactions are exothermic (at pH = 4) with AHadS values at low coverage ( 10 %) of ca. -24 kJ mol 1, they argue tentatively that these values indicate biden-tate surface complex formation. They also show that -AH decreases with increasing surface coverage. 

TABLE V. Calorimetrically measured enthalpies of exchange of pyridinium and bipyridium salts in... 

The values of Arto Awere calorimetrically measured or taken from the literature by the authors [143], The enthalpy of formation of mordenite is related with Ar//°(8.9) by... 

Despite the importance of mixtures containing steam as a component there is a shortage of thermodynamic data for such systems. At low densities the solubility of water in compressed gases has been used (J, 2 to obtain cross term second virial coefficients Bj2- At high densities the phase boundaries of several water + hydrocarbon systems have been determined (3,4). Data which would be of greatest value, pVT measurements, do not exist. Adsorption on the walls of a pVT apparatus causes such large errors that it has been a difficult task to determine the equation of state of pure steam, particularly at low densities. Flow calorimetric measurements, which are free from adsorption errors, offer an alternative route to thermodynamic information. Flow calorimetric measurements of the isothermal enthalpy-pressure coefficient pressure yield the quantity 4>c = B - TdB/dT where B is the second virial coefficient. From values of obtain values of B without recourse to pVT measurements. 

As with pure steam the properties of binary steam mixtures can be obtained from flow calorimetric measurements of the enthalpy of the mixture. With steam + n-alkane binaries, for which the enthalpies of both components are known, it is more sensible to measure the excess enthalpy directly rather than measure the large total enthalpy of the mixture to determine a small excess quantity. Extrapolation of the excess enthalpy Hp at pressure p to zero pressure yields = XjX2p(2< >j2 jj (f )... 

Flow calorimetric measurements of the excess enthalpy of a steam + n-heptane mixture over the temperature range 373 to 698 K and at pressures up to 12.3 MPa are reported. The low pressure measurements are analysed in terms of the virial equation of state using an association model. An extension of this approach, the Separated Associated Fluid Interaction Model, fits the measurements at high pressures reasonably well. 

Calorimetric measurements, when combined with the normally available room temperature thermodynamic properties, give values for free energy, enthalpy, heat capacity and even volume at high temperatures. 

Despite the above enunciated difficulties in obtaining pure nitroso compounds for conventional calorimetric measurements, gas-phase chemists have been active in devising methods for obtaining the C—NO bond enthalpy of monomeric nitroso species. In principle—and in practice—the desired enthalpy of formation of RNO may be obtained if we know the enthalpy of formation of both NO and the organic radical78. One such method consists of directly determining the rates of gas-phase reaction 54... 

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. 

Enthalpy of Conformational Transition of a Protein from Indirect Calorimetric Measurements [8]... 

Figure 4.1. Thermodynamic cycles, which illustrate states of enzyme ATCase during calorimetric measurements of enthalpies accompanying the binding of progressively increasing quantities of the substrate analog PALA. At the outset, the ATCase is 100% in the T conformation at the conclusion of the transformation, the ATCase (PALA)6 is 100% in the R conformation and has six bound PALA molecules. When the extent of binding of PALA is between 0 and 6, the extent of conformational conversion is between 0% and 100%. The horizontal broken arrow at the top of the diagram indicates the process that is accompanied by the enthalpy that we want to know. The vertical broken arrow at the right represents a pure...

We can calculate AH from thermal data alone, that is, from calorimetric measurements of enthalpies of reaction and heat capacities. It would be advantageous if we could also compute AS from thermal data alone, for then we could calculate AG or Ay without using equilibrium data. The requirement of measurements for an equilibrium state or the need for a reversible reaction thus could be avoided. The thermal-data method would be of particular advantage for reactions for which AG or AT is very large (either positive or negative) because equilibrium measurements are most difficult in these cases. 

Two different crystalline forms of benzothiophene have been described [18]. The one that is the stable form at low temperatures is labeled I and the other II. Calorimetric measurements down to 12 K have been made with each crystalline form. At the normal transition temperature, 261.6 K, the molar enthalpy of transition (I = II) is 3010 J moI . Some additional thermodynamic data obtained by these investigators are given in Table 11.6. Is Crystal II a perfect crystal at 0 K ... 

Once activity coefficients have been determined at one temperature by one of the methods mentioned above, calorimetric measurement of enthalpies of dilution can be used to determine activity coefficients at other temperatures. 

Calorimetric measurements in solution support these observations A relative enthalpy scale for different NHCs coordinated to ruthenium(II) [Eq. (40)] shows that NHCs have a higher donor capability than the best phosphine donor ligands. The only exception is the sterically extremely demanding l,3-di(l-adamantyl)imida-zolin-2-ylidene (Table III). The enthalpies for the formation of sulfur adducts... 

Newton et al. (1980) calorimetrically measured the enthalpy of the NaAl-Si30g-CaAl2Si208 mixture at P = 970 K and P = 1 bar and found a positive excess enthalpy reproduced by a subregular Margules model ... 

Although in crystalline phases determination of the enthalpy of formation from the constituent elements (or from constituent oxides) may be carried out directly through calorimetric measurements, this is not possible for molten components. If we adopt as standard state the condition of pure component at T = 298.15 K and P = bar , it is obvious that this condition is purely hypothetical and not directly measurable. If we adopt the standard state of pure component at P and T of interest , the measurement is equally difficult, because of the high melting temperature of silicates. 

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