Small Enthalpy Research Articles

The heat of formation of the single-phase NiSn electrodeposit

The heat of formation of tin-nickel electrodeposit was measured by solution calorimetry. A single phase obtained between 47·5 and 60·3 at.% Ni had a heat of formation (at 298 K) best represented as a linear function of composition. The constant partial molar enthalpies were, for nickel, −11·6 kJ/g atom, and for tin, −47·4 kJ/g atom; the estimated error is 5% or more. The alloy, not a phase appearing in the accepted phase diagram, decomposes slowly above 575 K with a small positive enthalpy change for the reaction considered at 298 K. It can be argued that the alloy is in fact a phase stable at its usual temperature of formation 340 K. The volume change on formation of the alloy is constant within the single-phase composition range and is numerically quite large (−1·3±0·1 cm3/g atom at 298 K). It is believed that this is the first study made of an alloy intermetallic compound which can only be prepared at relatively low temperatures.

Thermodynamics of polystyrene solutions. Part 2.—Polystyrene and ethylbenzene

The volume change on mixing polystyrene with ethylbenzene has been determined directly at 25°C. It is negative and amounts to ca. 0.30 % of the total volume at ϕ2= 0.5. Osmotic pressures have been measured at 10°, 35°, and 60°C in the concentration range ϕ2= 0.09 to 0.27. The temperature coefficient of the reduced residual chemical potential χ is close to zero; hence the heat of dilution is very small. The entropy of dilution is positive but small. The statistical thermodynamic theory employed in the preceding paper accounts for the sign and magnitude of the excess volume. The experimental partial molar entropies are much smaller than the combinatory contribution alone would allow. This finding and the small negative enthalpy of dilution indicated by recent work of Palmen are shown to arise mainly from equation-of-state terms of the newer theory. The parameter X12 representing the exchange enthalpy is positive and small, as should be expected for this system. The observed increase in χ with concentration is well reproduced by the theory. Thermal expansivities and thermal pressure coefficients of ethylbenzene have been determined at atmospheric pressure from 10° to 90°C.

Imidazole Catalyses in Aqueous Systems. IV. Bimolecular and Michaelis-Menten-Type Catalyses of a Phenyl Ester Hydrolysis by Some Hydrophobic Imidazole Derivatives

Abstract Catalytic hydrolyses of p-acetoxybenzoic acid by several imidazole derivatives were conducted at 15–40°C in aqueous systems using a pH-stat. Imidazole compounds containing the naphthalene ring catalyzed the hydrolysis following the Michaelis-Menten kinetics as in enzymatic reactions, while catalysis by less hydrophobic imidazoles followed the second-order kinetics. Enhancement of the overall catalytic efficiency in the Michaelis-Menten pathway was estimated to be ten times at most as compared with the bimolecular pathway. The thermodynamic and activation parameters were obtained at 15–40°C for the Michaelis-Menten pathway. The binding of catalyst and substrate was characterized by small positive enthalpy changes (ΔH≅+3.5 kcal/ mol) and large positive entropy changes (ΔSu≅+28 eu), indicating that the complex formation was ascribable to hydrophobic interactions. The intra-complex product formation was characterized by not unreasonable ΔH≠ values and extraordinarily large negative ΔS≠ (≅−50 eu) values. The unexpectedly small rate enhancement in the Michaelis-Menten pathway was attributed to the unfavorable ΔS≠ term, which suggested that the structure of the Michaelis complex was quite different from that of the transition state of the product formation.

Electromigration and thermomigration in gamma-uranium

Atom transport produced by large direct currents (electromigration) and thermal gradients (thermomigration) was measured in gamma-uranium by observation of local dilatations induced in high purity rods which were heated with a.c. or d.c. in a vacuum of 10 −8 Torr. or better. Both radial and longitudinal dimensional changes, the latter obtained from displacements of surface markers, were measured in order to determine the atom flux. Rates of dimensional change were observed constant during the 8–12 days duration of the experiments. In the d.c. experiments, where the electromigration effect dominated, mass transport was observed toward the anode with Z ∗ ƒ equal to − 1·6 ± 0·1; here f is the tracer diffusion correlation factor. The small negative value for Z ∗, the effective charge, indicates a dominance of the electron ‘wind’ force over the combined forces of the hole ‘wind’ and the electrostatic field. In the a.c. experiments mass transport was observed toward the cooler regions with Q ∗ f equal to + 4.7 ± 0.5 kcal mole . If one adopts the Wirtz model for a vacancy mechanism, the thermomigration results imply a relatively large migration enthalpy and a correspondingly small formation enthalpy; the latter leads to a vacancy fraction at melting of about 2·7 per cent. Semi-log plots of DZ ∗ ƒ and DQ ∗ ƒ vs. 1 T were found linear with ‘activation energies’ of 27·2 ± 0.6 and 23·0 ± 1.6 kcal mole , respectively. The former agrees with the activation energy obtained from tracer experiments while the latter is slightly less. A possible explanation is that Q ∗ has some temperature dependence, which is not too surprising in view of the large absolute thermoelectric power of gamma-uranium.

Phase equilibria of dodecylhexaoxyethylene glycol monoether in water

Low-angle X-ray diffraction has been used to characterize the two lyotropic mesomorphic phases (neat phase and middle phase) which are formed in the dodecylhexaoxyethylene glycol monoether + water system. The occurrence of these two anisotropic mesomorphic phases does not affect the behaviour of the thermodynamic excess functions which have been interpreted in terms of solute-water interactions. Careful measurements of water vapour pressure have been necessary to establish the limited extent of the two phase co-existence regions which separate homogeneous mesomorphic phase regions from isotropic solution in this system. Volume and heat capacity measurements as a function of temperature across mesomorphic phase boundaries have confirmed that the neat phase/isotropic solution and the middle phase/isotropic solution transformations are first-order phase transitions with very small volume and enthalpy changes.

Thermomigration in solid metals

Atom transport produced by large direct currents (electromigration) and thermal gradients (thermomigration) was measured in gamma-uranium by observation of local dilatations induced in high purity rods which were heated with a.c. or d.c. in a vacuum of 10−8 Torr. or better. Both radial and longitudinal dimensional changes, the latter obtained from displacements of surface markers, were measured in order to determine the atom flux. Rates of dimensional change were observed constant during the 8–12 days duration of the experiments. In the d.c. experiments, where the electromigration effect dominated, mass transport was observed toward the anode with Z∗ƒ equal to − 1·6 ± 0·1; here f is the tracer diffusion correlation factor. The small negative value for Z∗, the effective charge, indicates a dominance of the electron ‘wind’ force over the combined forces of the hole ‘wind’ and the electrostatic field. In the a.c. experiments mass transport was observed toward the cooler regions with Q∗f equal to + 4.7 ± 0.5 kcalmole. If one adopts the Wirtz model for a vacancy mechanism, the thermomigration results imply a relatively large migration enthalpy and a correspondingly small formation enthalpy; the latter leads to a vacancy fraction at melting of about 2·7 per cent. Semi-log plots of DZ∗ƒ and DQ∗ƒ vs. 1T were found linear with ‘activation energies’ of 27·2 ± 0.6 and 23·0 ± 1.6 kcalmole, respectively. The former agrees with the activation energy obtained from tracer experiments while the latter is slightly less. A possible explanation is that Q∗ has some temperature dependence, which is not too surprising in view of the large absolute thermoelectric power of gamma-uranium.

Enthalpy stabilized superconducting magnets

An essential requirement for complete stabilization of a superconducting magnet is adequate circulation of liquid helium in the winding, resulting in a low average current density, in partially stabilized magnets, the size of the cooling channels is reduced, but access for liquid helium is still considered necessary. With enthalpy stabilization, however, the need for helium penetration into the winding is eliminated, resulting in better packing of the conductors and a higher average current density. A small enthalpy stabilized magnet has been tested that was fully impregnated with epoxy resin and operated at a current density above 104A/cm2. The basis of the design of such magnets is discussed, with particular reference to the choice of conductor size and its effect on average current density in the magnet. Possible applications of this technique are also discussed.

Light Scattering from a Chemically Reactive Fluid. I. Spectral Distribution

The spectral distribution of light scattered by fluctuations about equilibrium in a chemically reactive fluid is calculated from the time dependence of the fluctuations predicted by the linearized hydrodynamic equations of irreversible thermodynamics. The theory treats a system with an arbitrary number, n, of independent chemical reactions which are coupled to the hydrodynamic processes involving viscosity and thermal conductivity. However, diffusion and any frequency dependence of the transport coefficients are neglected. The scattered light as a function of frequency has a central component and two side or Brillouin components. The central component is analyzed as a sum of n + 1 Lorentzian lines, one associated with a thermal diffusion relaxation process and n lines associated with the relaxation of the chemical reactions. In the limit of zero angle of scattering the widths of all Lorentzian lines associated with transport properties (thermal conductivity, viscosity, etc.,) vanish, and the entire width of the central component is related to the chemical relaxation processes. In fact, in this limit each chemical process contributes a Lorentzian line whose half-width is the reciprocal of the relaxation time for that process. The analysis of the widths for all the Lorentzian lines in the central component is given in detail for small but nonzero scattering angles and for the case of reactions having small enthalpy and volume changes for all scattering angles. The effect of chemical reaction rates on the position and width of a Brillouin or phonon component is also analyzed and parallels the treatment of ultrasonic dispersion and attenuation of sound waves.

The solvolysis of 2-trimethylsilylpyridine by alcohols and by water

2-Trimethylsilylpyridine, unlike the 3- and 4-isomers, reacts with water, methanol, or ethanol to give pyridine and trimethylsilanol or the alkoxytrimethylsilane. The relative rates of reaction are 740, 120, and 1 in water, methanol, and ethanol respectively. No detectable reaction occurs with higher alcohols. Acids inhibit the reaction, and bases have very little effect. These solvolyses have small activation enthalpies and high negative activation entropies, and the solvent isotope effect is very small. The most likely reaction mechanism involves rate-determining attack on the silicon by the oxygen of the hydrogen-bonded solvent, i.e. nucleophilic substitution with electrophilic assistance via a 5-centred cyclic activated complex.

The solvolysis of 2-pyridyltrimethyl-silane, -germane, and -stannane by alcohols and by water

2-Pyridyltrimethyl-stannane and -germane, like the silane, react with water or methanol to give pyridine and trimethyltin(germanium) hydroxide or trimethyltin(germanium) methoxide. The relative rates of hydrolysis of 2-Me3MC5H4N (M = Si 1, Ge ∼ 10–4, Sn 22) are consistent with a reaction mechanism involving rate-determining nucleophilic substitution by the oxygen of the solvent on the metal through a 5-centred cyclic activated complex. Small activation enthalpies, high negative activation entropies, and the absence of a solvent isotope effect support this mechanism.

Thermodynamics of Chelation by Tetracyclines

Thermodynamic data were determined for the complexation of five tetracycline analogs with cupric ion. Free energy values were calculated from data previously published by the authors. Enthalpy determinations were made in a Dewar calorimeter. Entropy values were determined from ΔG° and ΔH° at 25°. For the formation of 1:1 complexes, ΔH° was approximately — 12 Kcal./mole and ΔS° was approximately — 4 e.u. for the three chlor-analogs studied (4-epi-chlor-, demethylchlor-, and chlortetracycline), while a ΔH° of — 7.6 Kcal./mole and a ΔS° of+10.2 e.u. was obtained for tetracycline. It is suggested that all of the analogs studied form inner sphere 1:1 complexes with cupric ion but that the chlorine at C-7 compels the three chlorderivatives to undergo a more severe conformational change than tetracycline in order to reach a favorable configuration for chelation. All of the above analogs showed a small negative enthalpy change and a large positive entropy change for 2:1 complex formation. It is suggested that all four analogs form outer sphere or ion-pair complexes during 2:1 complexation.

THERMOCHEMISTRY OF ANION MIXTURES IN SIMPLE FUSED SALT SYSTEMS. I. SOLUTIONS OF MONOVALENT CHLORIDES AND BROMIDES IN THE CORRESPONDING NITRATES

Calorimetric measurements were performed on the heats of solution and of dilution for monovalent chlorides and bromides dissolved in the corresponding liquid nitrates. It is found that all the studied alkali halide-alkali nitrate systems exhibit small positive enthalpies of mixing, with the largest effects observed for the sodium systems. It is suggested that these results may be explained in terms of a packing effect which is due to the repulsion between the second nearest neighbor anion cores. Corresponding data for the silver and thallium salts are more complex.They cannot be explained in terms of the second nearest neighbor interactions.

Aspect rhéologique de la dureté

The standard (p∘ = 0.1 MPa) molar enthalpies of formation in the crystalline phases of ortho, meta and para-terphenyl isomers, at T = 298.15 K, were derived from the standard molar energies of combustion, measured by mini-bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the dependence of the vapour pressure of the crystals with the temperature, thus deriving their standard molar enthalpies of sublimation by means of the Clausius–Clapeyron equation. Combining the standard molar enthalpies of formation and sublimation of the crystalline terphenyls, the standard molar enthalpies of formation in the gaseous state, at T = 298.15 K, were derived for the three isomers.The results show small but detectable isomerization enthalpies between the terphenyls, indicating the following relative enthalpic stabilities: m- > p- ≈ o-terphenyl.