Latent heat transformations

Phase transformations in the solid state (like those in iron), too, have latent heats. They may be small, but with sensitive equipment for measuring cooling curves or heating curves, they are easily detected. 

AH = latent heat of transformation = equilibrium temperature (absolute). 

Latent heat The quantity of heat that is absorbed or released in an isothermal transformation of phase, in kj kg C b Latent heat of vaporization The heat added during an isothermal change of phase from liquid to gas. 

The integral terms representing AH and AH can be computed if molal heat capacity data Cp(T) are available for each of the reactants (i) and products (j). When phase transitions occur between T and Tj for any of the species, proper accounting must be made by including the appropriate latent heats of phase transformations for those species in the evaluation of AHj, and AH terms. In the absence of phase changes, let Cp(T) = a + bT + cT describe the variation of (cal/g-mole °K) with absolute temperature T (°K). Assuming that constants a, b, and c are known for each species involved in the reaction, we can write... 

For example, in the case of the reversible isothermal transformation of ice to water at the melting point (273 K), the heat gained by the ice will be the latent heat of fusion (A//f = 6(X)6 J mol" ) and a corresponding quantity of heat will be lost by the surrounding, and... 

Lag in the system 509 Langmuir-Hinshelwood kinetics 321 Laplace transformation 80, 536 Latent heat of vapourisation 517 Least squares 112 Level control 509... 

Latent heat of vaporization The amount of heat required to transform a specified mass of liquid water into steam or the amount of heat that must be removed to transform a specified mass of steam into liquid water. 

LANGMUIR TROUGH AND BALANCE LAPLACE TRANSFORM LARMOR PRECESSION LASER-FLASH KINETIC ANALYSIS LATENT ACTIVITY LATENT HEAT Latent heat of fusion LATENT HEAT Latent heat of vaporization LATENT HEAT Lateral binding proteins,... 

At particular critical points (Tq, Pc) on the phase diagram of a substance, two phases can be found in thermodynamic equilibrium. Therefore, upon application of a pressure or a temperature gradient, a transformation occurs from one phase into the other. This is a phase transition, in many aspects similar to a transformation implying the change of aggregation state. However, the extent of the changes in a solid to solid transformation is much smaller. For example, latent heat or latent volumes associated with the transformations are quite small, sometimes even difficult to detect. 

The latent heat ol fusion of sodium hydroxide per gram is 400 cals. potassium hydroxide, 28-6 cals. rubidium hydroxide, 15 8 cals. and csesium hydroxide, 107 cals. The corresponding values in calories per mol. are 1602, 1606, 1614, and 1609 cals, respectively. The heat of transformation of sodium hydroxide per gram is 24 7 cals. potassium hydroxide, 27T cals. rubidium hydroxide, 16 8 cals. and csesium hydroxide, 11 8 cals. The corresponding values in calories per mol. are 990,1522,1702, and 1763 cals, respectively. 

The pf-relations of the six different forms of ammonium nitrate are illustrated diagrammatically in Eig. 85. M. Bellati and R. Romanese, and U. Behn measured the changes in volume at atm. press, at the four transition points. M. Bellati and R. Romanese have also measured the latent heats of the transformation... 

This solution, which corresponds to negligible sensible heat effects, can be used to start the numerical integration for any finite mi, since, in Eq. (99), Uy (0) = 0. The case mi = is well approximated, for example, by the melting of steel originally at room temperature. Here mi = 27, so that latent heat effects play a very small role. Some modification of the transformed variables is necessary to get the equations into convenient form for numerical integration. 

If a substance undergoes a transformation from one physical stale to another, such as a polymorphic transition, the fusion or sublimation of a solid, or the vaporization of a liquid, the heat adsorbed hy the substance during the transformation is defined as the latent heat of transformation (transition, fusion, sublimation or vaporization). It is equal in the enthalpy change of the process, which is the difference between the enthalpy of the substance in the two states at (he temperature of the transformation. For the purpose of thcrmochemical calculations, i( is usually reported as a molar quantity with die units of calories (or kilocalories) per mule (or gram formula weight). The symbol L or AH. with a subscript i.f (or in), s. and n is commonly used and the value is usually given at the equilibrium temperature of the transformation under atmospheric pressure, or at 25 C. 

For a substance undergoing one phase transformation, with a latent heat Ah,. at a temperature. the enthalpy change between two temperatures, / i and 7i. such that I] < T, < 7). is given by... 

This effect can be simply demonstrated by wetting the back of your hand and blowing on it. The airstream releases the latent heat of vaporization, causing the temperature of the water on the skin to drop. As the water is transformed to the vapor state, it consumes heat, which it derives from the remaining water on the skin. The net effect is one of cooling. 

It is well established that all the known martensitic transitions, before Nitinol, occurs through crystallographic transformation [11], As described above [10], single crystal X-ray diffraction work shows a crystallographic distortion (rather than transformation) and therefore is a second-order transformation [12], This conclusion has the support of heat capacity investigation [13] that gave a latent heat of transition AH >... 

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