Heat of transition

Solid carbon monoxide exists in one of two aUotropes, a body-centered cubic or a hexagonal stmcture. The body-centered stmcture converts into the hexagonal stmcture at 62 K with a heat of transition of 0.632 kj/mol (0.151 kcal/mol) (5). The melting point at atmospheric pressure is 68.1 K and... 

Latent Heats of Transition (in the narrower sense), in which a system of substances A passes over into another system B at a definite temperature and pressure with absorption of heat ... 

Where pit denote the pressure and latent heat of transition in the system which does not contain the i-th phase. 

R. H. Sherman and W. F. Giauque, "Arsine. Vapor Pressure, Heat Capacity, Heats of Transition, Fusion, and Vaporization. The Entropy from Calorimetric and from Molecular Data", J. Am. Chem. Soc., 77, 2154-2160 (1955). 

The next development on water-oil isotherms was presented by Mohwald s group at the Max-Planck Institute in Berlin [21,22]. They investigated monolayers of dipalmitoyl phosphatidylethanolamine (DPPE) at interfaces of water and hydrocarbons -dodecane (C]2, -hexadecane (Cig), and bicyclohexyl (BCH). The transition pressure was increased and the molecular area at transition decreased in the order Cig—C12 BCH. Also the heat of transition was decreased in the same order, and was more strongly decreasing with... 

If we assume that the data of Figs. 22 and 23 can be treated by equilibrium thermodynamics, the discontinuities in the ESP versus temperature phase diagram should indicate the presence of a three-way equilibrium between bulk surfactant and two different film types in both homo- and hetero-chiral systems. The surface heats of transition (U) between the two film types in either system may be obtained by relation (15), where IT is the equilibrium... 

Fig. 2 Typical thermogram obtained using conventional differential scanning calorimetry on PNIPAM solution the temperature of maximum heat capacity (Tmax), the width of the transition at half-height (AT1/2), the heat of transition (AH), the difference in the heat capacity before and after the transition (ACp), and the demixing temperature (Tdem). (Adapted from Ref. [200])...

Ashton, J.G., Fink, H.L., Schumann, S.C. (1943a) The heat capacity, heats of transition, fusion and vaporization and the vapor pressures of cyclopentane. Evidence for a non-planar structure. J. Am. Chem. Soc. 65, 341-346. 

One of the most powerful techniques available for the evaluation of the solubility attributes of polymorphic or solvate species is solution calorimetry. Each distinct crystal phase is characterized by a well-defined heat of solution as it dissolves into a given solvent system, and the difference between the heats of solution of each phase in the same defined solvent system equals the heat of transition between them at the temperature of measurement. Solution calorimetry has been used to complement or to investigate a wide variety of crystal properties,... 

When one polymorph can be thermally converted to another, differential scanning calorimetry (DSC) analysis cannot be used to deduce the heat of transition between the two forms, and so solution calorimetry represents an alternative methodology. This situation was encountered when evaluating the polymorphs of losartan [140], Enthalpies of transition were obtained in water (A(A//sol) = 1.723 kcal/mol) and in A A-dimethylformarnide (A(A//S0 ) = 1.757 kcal/mol), with the equivalence in results demonstrating the quality of the results. Although enthalpy does not indicate stability, the authors deduced from solution calorimetry that form I was more stable than form II at ambient temperature. 

One would expect the enthalpy of sublimation (d) to be the largest of the four quantities cited. Molar heat capacities are quite small, on the order of fractions of a kilojoule per mole-degree. (Remember that specific heats have values of joules per gram-degree.) All of the heats of transition (or latent heats) are positive numbers and on the order of kilojoules per mole. Since the heat of sublimation is the sum of the heat of fusion and the heat of vaporization, AHsubl must be the largest of the three. 

The most common thermodynamic experiment is a calorimetry experiment such as experiment 13 in Experimental chapter. In this experiment the heat of transition or heat of reaction is determined. 

For each of the phase transitions, there is an associated enthalpy change or heat of transition. For example, there are heats of vaporization, fusion, sublimation, and so on. 

OK can be computed from heat capacity measurements [8] for each crystalline form from near 0 K to the transition temperature (368.6 K) and the heat of transition. The result is zero within experimental error. Hence, both rhombic and monoclinic sulfur ate assigned zero entropy at 0 K. 

Possible determinations from DSC or DTA measurements include (1) heat of transition, (2) heat of reaction, (3) sample purity, (4) phase diagram, (5) specific heat, (6) sample identification, (7) percentage incorporation of a substance, (8) reaction rate, (9) rate of crystallization or melting, (10) solvent retention, and (11) activation energy. Thus, thermo-calorimetric analysis can be a useful tool in describing the chemical and physical relationship of a polymer with respect to temperature. 

Ni(C2HgN2)3(N03)2 is quite different - the space group type and the lattice change at (ca. 106 K). The transition show discontinuity of the cell volume and, as expected, there is a latent heat of transition. Notably at the critical temperature the two phases are structurally different and therefore they are in equilibrium at that temperature. A minor hysteresis is observed. 

Heat of Transition is the quantity of heat liberated or absorbed when a substance changes from one allotropic crystal form to another... 

The Meissner effect is a very important characteristic of superconductors. Among the consequences of its linkage to the free energy are the following (a) The superconducting state is more ordered than the normal state (b) only a small fraction of the electrons in a solid need participate in superconductivity (c) the phase transition must be of second order that is, there is no latent heat of transition in the absence of any applied magnetic field and (d) superconductivity involves excitations across an energy gap. 

J.M. Rosen, J.R. Holden H.T. Simmons, Microscope 19, 151—6 (1971) (Crystal props of TNEtTNBu, transition temperatures, mp, heats of transition and fusion)... 

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