UV spectroscopic determination of the chlorine monoxide (ClO) ∕ chlorine peroxide (ClOOCl) thermal equilibrium constant

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228 and 301 K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third-law fit of the equilibrium values determined from the experimental data provides the expression Keq=2.16×10-27e8527±35K/T cm3 molecule−1 (1σ uncertainty). A second-law analysis of the data is in good agreement. From the slope of the van't Hoff plot in the third-law analysis, the enthalpy of formation for ClOOCl is calculated, ΔHf∘(298K)=130.0±0.6 kJ mol−1. The equilibrium constant results from this study suggest that the uncertainties in Keq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.

UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228–301 K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third law fit of the equilibrium values determined from the experimental data provides the expression: Keq = 2.16 × 10−27 e(8533 ± 25 K/T) cm3 molecule−1. A second law analysis of the data deviates minimally: Keq = (2.06 ± 1.27) × 10−27 e(8546 ± 123 K/T) cm3 molecule−1. From the slope of the van't Hoff plot in the third law analysis, the enthalpy of formation for ClOOCl is calculated, ∆H◦f (298 K) = 129.9 ± 0.6 kJ mol−1. The equilibrium constant results from this study suggest that the uncertainties in Keq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.

Twin Tandem Bicycle Pedal Motion - Using X-rays to Resolve Disorder Thermodynamic Parameters

During our research into polar crystals for organic electronic applications, we synthesized E,E-1-[2-(4-nitrophenyl)ethenyl]-4-[2-(2,4-dimethoxyphenyl)ethenyl]benzene, which has a polar crystal structure (space group Pc) and displays the typical bicycle pedal motion, as studied in detail by Harada and Ogawa [1], in one of its ethenylic links. A van `t Hoff plot of the logarithm of the population ratio versus 1/T, however, showed a kink instead of being a straight line, which led us to conclude that an unusual phase transition was occurring in this material [2]. In the mean time we have crystallized the same material in a second, centrosymmetric polymorph (space group P-1). There, the asymmetric unit consists of two complete molecules, and they display the same kind of bicycle pedal motion, but this time in all four different ethenylic linkers. Every one of these population differences increases with temperature, so that four van `t Hoff plots can be constructed for this structure. Two of these behave normally, the other two display a kink, just like the van `t Hoff plot of the pedaling ethenylic link in the other polymorph of this molecule. This is, to the best of our knowledge, the first instance of a structure where four different dynamic equilibria can be resolved simultaneously, and only the second example in which van `t Hoff plots for the thermodynamics of dynamic disorder are not linear, indicating an unusual type of phase transition linked only to the dynamics of the molecules in the crystal.

Thermodynamic Characterization on Hydrogen Absorption and Desorption Reactions of Lithium – Silicon Alloy

Lithium hydride LiH is one of the attractive hydrogen storage materials, because it stores 12.7 mass% of H2. However, H2 desorption reaction occurs over 600 °C due to the large enthalpy change of H2 desorption Ho = 181 kJ/mol H2. The purpose of this work is to control the enthalpy change of LiH to much lower value by a mechanical alloying with Si, where the Li-Si alloy is thermodynamically more stable than Li. The alloy was synthesized from Li granule and Si powder by a mechanical alloying method. The H2 absorption and desorption properties were characterized by High-Pressure Differential Scanning Calorimetry and Thermogravimetry - Differential Thermal Analysis - Mass Spectroscopy, and X-ray diffraction measurement. Pressure - Composition - Isotherm measurements were performed at 400, 450, and 500 °C to estimate the enthalpy change. From the results, it was confirmed that reversible H2 ab/desorption reactions of the Li-Si alloy were expressed as 7LiH + 3Si ↔ (3/7)Li12Si7 + (13/7)LiH + (18/7)H2 ↔ Li7Si3 + (7/2)H2 (theoretically 5.0 mass% H2) at 400 °C. From van’t Hoff plot obtained by the results of PCI measurements, the enthalpy change of the former reaction was estimated to be Ho = 103 kJ/mol H2, which is lower than that of LiH.

Resolution of the Ambiguity of van’t Hoff Plots by the Effect of Pressure on the Equilibrium

The change in the Gibbs free energy function, ΔG, of chemical reaction is determined by the difference between the heats respectively released to and absorbed from the environment, and separation of the enthalpy and entropy changes that these changes represent cannot be achieved without specific hypotheses as to their relations. The determination of the enthalpy of reaction by the plot of ΔG/T against 1/T (van’t Hoff plot) implicitly assumes that the enthalpy ΔH and entropy ΔS are temperature independent, and this assumption leads to very large errors when this is not the case and ΔH « TΔS. It is therefore inapplicable to the reactions of molecules, such as proteins, that have thermally activated local motions. The concepts offered previously by the author to relate the entropy and enthalpy changes in protein associations are reviewed briefly and applied to account for the temperature dependence of ΔH and ΔS. It is shown that two different values of the enthalpy computed in that manner correspond to each value of the apparent van’t Hoff enthalpy, but that the choice between the two is easily made by reference to the volume change on reaction. The enthalpies of association of subunit pairs of seven oligomers are all found to be positive and much more uniformly related to the size of the intersubunit surface than those previously assigned by use of the classical van’t Hoff plot.