Showing papers on "Enthalpy published in 1977"

Intrinsic acidities of substituted phenols and benzoic acids determined by gas-phase proton-transfer equilibriums

Abstract: Gas-phase proton-transfer equilibria: A I H + A2= A I + A2H, involving some 50 benzoic acids and phenols and various other standard acids with known acidities were measured with a pulsed electron beam high ion source pressure mass spectrometer. The equilibrium constants K were used to obtain the free energy changes and establish the relative and absolute gas-phase acidities of the benzoic acids and phenols. The gas-phase acidities of the substituted benzoic acids and phenols are compared with the corresponding aqueous acidities. The reasons for the very different attenuation of the substituent effects in water and Me2SO are examined. Ortho effects in the gas phase and specific hydrogen bonding effects are also discussed. The electron affinities of some phenolic radicals are determined from known bond dissociation energies and the gas-phase acidities. Linear free energy relationships have had a profound impact on the development of physical organic chemistry. Thus the classification of substituent effects on chemical reactions done in connection with Hammett type plots led to many of the now widely used concepts of bonding and electronic effects.] The ability of phenols and benzoic acids to ionize as Brqnsted acids in the experimentally easily accessible pH range made them the natural choice as references for free energy relationships.2 The vast majority of reaction types, including the reference reactions that have been successfully correlated by linear free energy relationship plots, are processes occurring in solution. Since ionic solvation is an integral and dominant part of the energetics of these reactions, interpretations of free energy correlations purely in terms of electronic arguments involving the reactants but not the solvent has been a perennial area of concern. One pertinent example is the finding that in most cases the differences in free energy of ionization of substituted phenols and benzoic acids are due to a change of ent r ~ p y . ' ~ Yet, i f solvation effects are not taken into account, electronic effects of the substituents should show up in the enthalpy and not in the entropy change. T o separate solvent effects from the intrinsic molecular effects of the substituents one needs to know the free energy and enthalpy change for the same process occurring in the dilute gas phase. Fortunately such data have recently become available from studies of gas-phase ion molecule reac t iok8 Information on the intrinsic acidities and basicities is obtained from studies of the gas-phase proton-transfer reactions A I H + A2= A I + A2H (1) In earlier work9*Io nly the sign of the free energy change could be obtained from the obsefved occurrence or nonoccurrence of the reaction in the forward direction. Many important results on gas-phase acidity orders were obtained with this technique.' ' I 3 More recently, the study of ion equilibriaI4 was extended to proton-transfer reactions in ICR mass spectrome t e r ~ , ' ~ high-pressure pulsed ion sources,16 and flowing afterglow apparatus.I7 The resulting determinations of K , AGO, and A H o have provided already many significant data. It is interesting to note that recentlyt* results from a b initio MO methods using relatively simple STO-3G basis sets have predicted energy changes (AE2) for the positive ion protontransfer reactions (eq 2) which are in very good agreement with gas-phase ion equlibria data ( A H * ) . Thus an important independent contribution for these reactions can be expected from theoretical calculations. On the other hand the negative ion systems (eq 1) are electron crowded systems in which electron repulsion and electron correlation are much more important. These systems require much more accurate calculations, larger basis sets, and electron correlation corrections. Therefore, unfortunately, numerous and good theoretical calculations of AE2 cannot be expected in the near A brief reportI9 on the gas-phase acidities of the monosubstituted benzoic acids was presented earlier from this laboratory. The present publication presents more extensive documentation of these early results and combines them with newly obtained results for the substituted phenols20 in a more comprehensive treatment of substituent effects on the acidity of benzoic acids and phenols. Experimental Section The acidity data obtained in this work were measured using a pulsed electron beam high-pressure mass spectrometer, which has been described previously.2' Briefly, a typical experiment is carried out as follows. A concentrated methanol solution containing a known molar ratio of the two acids of interest is prepared by dissolving accurately weighed amounts of the acids into methanol. This solution is injected with a syringe through a septum into a 5-L bulb containing I a tm of the buffer gas methane. NF3 (50 cm3) at I a tm are also injected into the bulb. The bulb and inlet system to the ion source are maintained at 150-200 "C to ensure complete vaporization of the rather involatile benzoic acids and phenols. Typically the bulb contains 1 atm of methane, 3 Torr each of the acids of interest, and I O Torr of NF3. After allowing sufficient time for mixing, this gaseous mixture is passed in slow flow through the ion source. The flow out of the bulb is controlled by means of an all metal valve. The gas mixture flows through I O mm i.d. tubing in and out of the ion source and then passes via a flow control capillary into a pump. The pressure in the flow system and ion source is maintained at some constant value in the 2-5 Torr range. Viscous flow results under these conditions. Therefore no fractionation of the minor components (Le., the acids) should occur and the partial pressure ratios of the acids in the ion source should be identical with those in the storage bulb. A confirmation of the expected lack of fractionation could be obtained by observing that the measured equilibrium constant K I changed by only 5% after 85% of the gas had flowed out of the 5-L storage bulb. The ratio of the neutral acids in the bulb was varied by a factor from 2 to 5 in separate experiments where new methanol solutions were prepared and injected. The equilibrium constants were found to be independent of the concentration ratios used. Ionization of the sample is accomplished by a short (-10 p s ) pulse of 2000-eV electrons, which creates some I O 7 ions and electrons. The N F3 undergoes dissociative electron attachment to produce F-. The fluoride ion rapidly abstracts protons from the benzoic acids and Journal of the American Chemical Society / 99:7 / March 30, I977 D ow nl oa de d by T A R B IA T M O D A R R E S U N IV o n A ug us t 1 9, 2 00 9 Pu bl is he d on M ar ch 1 , 1 97 7 on h ttp :// pu bs .a cs .o rg | do i: 10 .1 02 1/ ja 00 44 9a 03 2

Thermodynamics of electrolytes. 8. High-temperature properties, including enthalpy and heat capacity, with application to sodium chloride

Abstract: The ion-interaction type of equation which has proven so successful for activity and osmotic coefficents of both pure and mixed electrolytes near room temperature is extended for wide temperature ranges and for the enthalpy and heat capacity. An improved equation is obtained for the dielectric constant of water over a wide range of temperature and density; it is used to calculate the Debye-Hueckel parameters to 350/sup 0/C for enthalpy and heat capacity as well as for osmotic and activity coefficients. These equations are applied to the extensive array of thermodynamic data for aqueous sodium chloride from 0 to 300/sup 0/C. Good agreement is obtained and the equations thus provide a convenient analytical representation of the thermodynamic properties of NaCl(aq). Also the results indicate that at 300/sup 0/C sodium chloride is still a strong electrolyte with little tendency toward the formation of ion pairs.

Procedure for estimating the temperature of a hot-water component in a mixed water by using a plot of dissolved silica versus enthalpy

Abstract: A graphical method using a plot of dissolved silica versus enthalpy allows quick determination of the temperature of the hot-water component of a nonboiling thermal spring. The method is applicable to warm spring waters that either have not lost heat before mixing or have lost heat by separation of steam before mixing.

Structure and thermodynamics of liquid metals and alloys

Abstract: Ab initio calculations of the structure and the thermodynamic properties of simple liquid metals and alloys are presented. The basic ingredients of the theory are as follows: (a) An orthogonalized-plane-wave-based first-principles pseudopotential is used to describe the interatomic forces. The pseudopotential is optimized specifically for binary systems. The $X\ensuremath{\alpha}$ method is used to construct the electron-ion potential. (b) A system of hard spheres is used as a reference system for describing the liquid structure. Effective hard-sphere diameters are determined by a variational method based on the Gibbs-Bogolyubov inequality. The method of this paper yields encouraging results for the structure factors, the excess volume, and the enthalpy and entropy of formation for alloys with a nearly random distribution of the components. Difficulties arise where nonrandomness has to be expected. The theory provides a basis for a microscopic understanding of the thermodynamics of alloys of simple metals in terms of their electronic structure.

Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO2-Al2O3

Abstract: Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO/sub 2/--Al/sub 2/O/sub 3/ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO/sub 2/- and Al/sub 2/O/sub 3/-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of approx. = 1540/sup 0/C at a critical composition of approx. = 36 mol % Al/sub 2/O/sub 3/ was considered to be thermodynamically most probable; the gap extended from approx. = 11 to approx. = 49 mol % Al/sub 2/O/sub 3/ at 1100/sup 0/C. SiO/sub 2/-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.

Calorimetric studies of 7000 series aluminum alloys: I. Matrix precipitate characterization of 7075

Abstract: Both differential scanning calorimetry (DSC) and hot stage transmission electron microscopy were used to characterize the solid state reactions accompanying heating of the highest strength (T651) and overaged (T7351) tempers of 7075 aluminum alloy. Each of the observed endothermic or exothermic reactions that occurs over the 20 to 500°C temperature range has been ascribed to the dissolution or formation of a specific precipitate. The dissolution of each matrix phase,i.e., GP zones, ή and ή, can be characterized by a distinguishable dissolution temperature, dissolution enthalpy, activation energy, and activation entropy. The values of activation energy and activation entropy for the dissolution of the initial matrix precipitate of these phases indicate that the relative stability of the matrix precipitates is primarily influenced by the entropy rather than the energy term. This investigation provides a basis for the use of DSC analysis in the rapid, quantitative identification of the matrix microstructure of 7075 aluminum alloy as well as in the characterization of the matrix microstructure of other 7000 series aluminum alloys.

Charge density effects in ion exchange. Part 1.—Heterovalent exchange equilibria

Abstract: A thermodynamic study is made on the effect of surface charge density on the sodium–cesium equilibrium in three clay minerals, Otay montmorillonite, hectorite and a series of Camp Berteau montmorillonite samples with reduced layer charge.It is shown that the standard free energy, enthalpy and entropy changes for the reversible displacement of sodium by cesium ions increase linearly with surface charge density. The opposite effect of charge density on cesium selectivity in zeolites and clay minerals is discussed in terms of electrostatic and hydration effects and the swelling properties of clays.

Enthalpy of mixing of poly(2,6‐dimethyl phenylene oxide) and polystyrene

Abstract: Various compatibility criteria for polymer‐polymer blends are considered and their shortcomings discussed. An unambiguous thermodynamic criteria of compatibility, the enthalpy of mixing ΔHm, is obtained by application of Hess’s law to heat‐of‐solution data for blends and their components. The enthalpy of mixing for PPO/PS blends is determined by utilizing the Hess’s‐law approach. Consideration is given to the excess glass enthalpy contribution to ΔHm. Consistent with the known compatibility of PPO/PS blends, a small negative ΔHm is found for all blend compositions.

Estimation of the migration enthalpy and entropy for cation vacancy motion in alkali halides with the NaCl-type structure

Abstract: A method is proposed for estimating the free energy of cation vacancy migration in alkali halides with NaCl structure from macroscopic parameters. It is based on the calculation of the electrical and the dilatation work during the migration of the cation. The dilatation work can be calculated from the elastic constants and the migration volume corresponding to free-cation motion. By considering anharmonic effects the free energy is calculated as function of temperature. In the high-temperature region the temperature-independent part of the migration free energy and its temperature derivative give values which agree with the published values of the migration enthalpy and entropy.

Thermodynamic consistency of vapor pressure and calorimetric data for argon, krypton, and xenon

Abstract: A new two‐parameter vapor pressure equation has been derived which, unlike the Salter equation, is shown to be equally applicable to quantum or classical solids and even liquids. The condensed phase enthalpies and entropies are given directly by the fitted parameters with accuracies comparable to those which have been claimed for existing independent calorimetric measurements. Recent vapor pressure data for the solid and liquid phases of argon, krypton, and xenon are analyzed in this manner, and the results are compared with the available calorimetric data. New values for the cohesive energy at T=0 are also derived for these substances.

A study of hydrogen bonding in methanol vapor by measurement of thermal conductivity

Abstract: The thermal conductivity of methanol vapor was measured in a modified thick hot‐wire cell between 307 and 345 K at pressures ranging from approximately 50 torr up to a maximum of 1 atm. Analysis of the experimental data indicates that a tetrameric species is the principal source of the observed enhancement of the thermal conductivity of the vapor due to association reactions. Equilibrium constants for the reaction 4CH3OH? (CH3OH)4 were determined from the thermal conductivity data. The enthalpy of formation of the methanol tetramer is −23.73 kcal/mole and the entropy of formation is −80.92 cal/mole K.

Soft-sphere model for liquid metals

Abstract: A semi-empirical soft-sphere model of fluids is modified for application to the thermodynamic properties of liquid metals. Enthalpy, volume, and sound speed are computed as functions of temperature for 13 metals and compared with experimental data. Critical points and coexistence curves are also computed and compared with experimental data, where these have been measured. Strengths and weaknesses of the model are discussed.

PVT surface and thermodynamic properties of isopentane

Abstract: A number of correlational studies presents both vapor liquid coexistence and superheated vapor thermodynamic properties of n-butane. The present compilation is part of a general revision of the j-Tables of API Research Project 44 (31) for C/sub 1/-C/sub 5/ alkanes. Selected values of vapor pressure, specific volume, enthalpy, and entropy are presented for saturated liquid and vaporous n-butane from the normal boiling point (272.67/sup 0/K) to the critical point (425.16/sup 0/K). Tabular data show the following: (1) physical and conversion constants used in this investigation; (2) critical constants of n-butane; (3) saturated properties of n-butane; (4) comparison of enthalpies of vaporization; (5) comparison of saturated liquid enthalpies; (6) comparison of saturated liquid entropies; (7) constants for the DR-BWR equation; (8) condensed thermodynamic properites of n-butane; (9) compressibility factors from corresponding states; and (10) second virial coefficients. (37 refs.)

Thermodynamics and mechanism of hydrophobic interaction

Abstract: .In the mixed solvent, 0.1 mole fraction ethanol-water, long-chain decyltrimethylammonium carboxylates form ion pairs. Ion-pair association constants (and hence the free energy of ion-pair formation) can be measured conductometrically. It is possible to separate the hydrophobic from the electrostatic contribution to the free energy of ion-pair formation by systematically varying the hydrocarbon chain length. We report measurements of the free energy of hydrophobic interaction (ΔG°HI) over the temperature range 278-328 K. The value of ΔG°HI becomes more negative (stronger hydrophobic interaction) with increasing temperature. The temperature coefficient of ΔG°HI was used to calculate the enthalpy (ΔH°HI) and entropy (ΔS°HI) of hydrophobic interaction. At low temperature the entropic contribution to the free energy is the larger but ΔH°HI, dominates at temperatures above c. 324 K. The volume change of hydrophobic interaction was similarly estimated from the volume change of ion-pair formation. We obtained values of apparent molar volume of the decyltrimethylammonium carboxylates (over a range of concentrations) from very precise density measurements. These could then be combined with the appropriate ion-pair association constant (from the conductance measurements) to give the partial molar volumes of the free ions and the ion pair. Hydrophobic interaction was found to be accompanied by a substantial increase in volume amounting to 10.2 ± 0.3 ml mol-1 for each pair of interacting methylene groups. Our results support the view that hydrophobic interaction occurs with a further ordering of water molecules over and above that which exists in the hydrophobic hydration layer surrounding an isolated hydrophobic molecule.

Diffusional aspects of non-stoichiometry a model for UCx

Abstract: The self-diffusion properties of carbon in are described in terms of an f.c.c. lattice gas model in which the C2 groups attract only when they are nearest neighbours. A modified Monte Carlo method is used to calculate the self diffusion coefficient as a function of x and temperature. The diffusion coefficient shows a maximum at about the composition UC1.2 for the four temperatures studied: 2380, 2500, 2600 and 2650 K. The activation enthalpy for diffusion increases non-uniformly from approximately 54 kcals mole−1 (reference point) at compositions close to UC to about 75 kcals mole−1 for compositions near UC2. The functional form of this variation is in satisfactory agreement with existing experimental data.

Approximate Relationships for the Thermodynamic Properties of Hydrocarbon-Air Combustion Products

Abstract: -A model is presented by means of which the enthalpy (h) and density (p) of the products of hydrocarbonair combustion can be calculated as functions of pressure (P), temperature (T). and equivalence ratio (φ) for a given C: H ratio of the fuel, and N: 0 ratio of Ihe oxidizer. By direct differentiation, the partial derivatives of h and p with respect to P, T, and φ can also be calculated. Results of calculations using this model are compared with results from a detailed thermochemical equilibrium calculation incorporating over 40 species, and it is shown that accuracy of the approximate model is acceptable over most conditions of interest in typical combustion calculations for a range of practical fuels. The principal advantage of this method is computational speed; it thus has obvious utility in, for example, internal combustion engine performance calculations.

The enthalpy of isomerisation of methyl isocyanide

Abstract: A controlled thermal explosion in which methyl isocyanide isomerises quantitatively to methyl cyanide has been studied in a calorimeter at 300 K. The enthalpy of isomerisation ΔH = −23.70 ± 0.14 (2...

Calorimetric study of the glassy state. XIII. Thermodynamic properties of normal and deuterated orthoboric acid crystals.

Abstract: Thermal properties of normal and deuterated orthoboric acids were studied through the measurements of heat capacity in the temperature range from 13 to 370 K by using an adiabatic calorimeter and differential thermal analysis curves above room temperature. For both crystals, the heat capacity anomaly was found around 290 K in the heat capacity values dependent upon the thermal history of the specimen; i.e., the endothermic or exothermic enthalpy relaxation was observed in this temperature range. This behavior is of characteristic to the glass transition and is considered to be ascribed to the freezing-in phenomenon of the motion of rearrangement of the protons in hydrogen bondings. The enthalpy relaxation curves were analyzed with the exponential law and the characteristic time constant toward the equilibrium state was longer for enthalpy-excessive side than for enthalpy-deficient side. The glass transition temperature at which the endothermic relaxation time becomes 1 ks is 296.6 K for normal orthoboric ...

Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

Abstract: The evolution of enthalpy of As2Se3 glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As2Se3 is unlike previously studied glasses whose heat capacities are more dependent on thermal history.

Cesium-133 nuclear magnetic resonance study of the complexation of cesium tetraphenylborate by 18-crown-6 in pyridine solutions

Abstract: Cesium-133 nuclear magnetic resonance was used as a probe of the interaction of Cs/sup +/ with the macrocyclic polyether 18-crown-6 (18C6) in pyridine solutions. A study of the /sup 133/Cs chemical shift as a function of crown/Cs/sup +/ mole ratio showed that the complexation reaction occurs in two steps--formation of a stable 1:1 complex (K/sub 1/ approx. 10/sup 6/ at 298/sup 0/K) followed by the addition of a second ligand molecule to form a sandwich compound, Cs/sup +/(18C6)/sub 2/ (K/sub 2/ = 44 +- 2 at 298/sup 0/K). The thermodynamic functions for the second step are: (..delta..G/sup 0//sub 2/)/sub 298/ = -2.58 +- 0.02 kcal mol/sup -1/, ..delta..H/sup 0//sub 2/ = -5.8 +- 0.2 kcal mol/sup -1/, and ..delta..S/sup 0//sub 2/ = -10.7 +- 0.6 cal mol/sup -1/ deg/sup -1/. Line-shape analysis of the /sup 133/Cs resonance at various temperatures gave rate parameters for the decomplexation of the 1:1 complex as follows: ..delta..H/sup double dagger/ = 7.8 +- 0.1 kcal mol/sup -1/, ..delta..S/sup double dagger/ = -14.3 +- 0.4 cal mol/sup -1/ deg/sup -1/ and (..delta..G/sup double dagger/)/sub 298/ = 12.02 +- 0.02 kcal mol/sup -1/.

On the phase transition of ammonium hexafluoroferrate(III).

Abstract: The heat capacities of (NH4)3[FeF6] crystal have been measured from 15 to 350 K with an adiabatic calorimeter. A heat capacity anomaly was observed at 267.02±0.05 K. The enthalpy and entropy changes of the transition are 6490±500 J mol−1 and 24.8±1.9 J K−1 mol−1, respectively. This value of entropy was discussed in terms of a model involving orientational disorder of the ammonium as well as hexafluoroferrate(III) ions. Based on the assumption of the average fcc symmetry of the crystal and internal rigidity of the ions, the proposed model predicts the transition entropy equal to R In 16=23.05 J K−1 mol−1 which is in close agreement with the experimental value. Temperature dependent splitting of the Raman band due to the internal vibration of the anion was found to be consistent with the model. The excess heat capacity follows the prediction of the Landau theory of phase transitions of the second kind. Properties of the present and the similar crystals including ammonium and alkali salts of hexafluoro-compl...