Endothermic solution

Sorption. The word sorption will be used for the total amount of gas taken up by a given solid. This includes adsorption on the surface as well as absorption into the interior of the structure. The latter may be true endothermic solution or may be exothermic solution, which infers compound formation such as the formation of metal hydrides when hydrogen is involved. 

It must be expected that the effect on the heat of absorption will be particularly pronounced at higher pressures and in the middle temperature range, let us say between —170° and 200°. The reason for this is that absorption, which is a slow activated process, will affect the fast adsorption at low temperatures very little. Again at the very high temperatures absorption will be less than adsorption because of its lower heats. On the other hand, at the very high temperatures another absorption process, the endothermic solution, can possibly enter the picture, although this effect is undoubtedly small at low pressures, let... 

Dissolution is endothermic, solution becomes cold (and hence its apphca-tion in freezing bath) elevates the boiling point of water by 1°, 7.5°, 28.5° and 70°C at 10, 40, 80 and 95% (w/w) concentrations, respectively vapor pressure of saturated solution, 11.2 torr at 20°C. 

Fig. 4.11 Schematic potential energy curves for activated and non-activated chemisorption of hydrogen on a clean metal surface and exothermic or endothermic solution in the bulk. A more pronounced minimum just below the surface allows for subsurface hydrogen (onedimensional Lennard-Jones potential, Somorjai (1987) Ref [33]).

Endothermic Solute + solvent + heat —> solution Exothermic Solnte + solvent solntion + heat... 

An endothermic solution process is the basis for commercially available instant cold packs. Water is sealed in a thin plastic bag and placed inside a larger, stronger bag together with a quantity of solid solute (NH4CI or NH4NO3). When the inner bag is broken... 

The enthalpy effect might be positive (endothermic solution/mixture) or negative (exothermic solution/mixture) depending on the ratio tijns, i.e., the concentration of the total system. Unfortunately, in some of the older literature, the definition of the sign of the so-called (integral) heat of solution is reversed, compared to the enthalpy, occasionally causing some confusion. In principle, the enthalpy effect depends also on pressure. However, in the case of condensed systems this pressure dependence is relatively small. All values in this handbook usually refer to normal pressiue. Hoa and Hog are the molar enthalpies of pure solvent A and pure copolymer B and and Hg the partial molar enthalpies of solvent and copolymer in the solution/mixture. 

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

Equation (8.49) accounts only for endothermic mixing. It is not too surprising that we are thus led to associate exothermic values with more specifically chemical interactions between solvent and solute as opposed to the purely physical interactions we have been describing in this approximation. 

In the Godrej-Lurgi process, olefins are produced by dehydration of fatty alcohols on alumina in a continuous vapor-phase process. The reaction is carried out in a specially designed isothermal multitube reactor at a temperature of approximately 300°C and a pressure of 5—10 kPa (0.05—0.10 atm). As the reaction is endothermic, temperature is maintained by circulating externally heated molten salt solution around the reactor tubes. The reaction is sensitive to temperature fluctuations and gradients, hence the need to maintain an isothermal reaction regime. 

Other Technologies. As important as dehydrogenation of ethylbenzene is in the production of styrene, it suffers from two theoretical disadvantages it is endothermic and is limited by thermodynamic equiHbrium. The endothermicity requites heat input at high temperature, which is difficult. The thermodynamic limitation necessitates the separation of the unreacted ethylbenzene from styrene, which are close-boiling compounds. The obvious solution is to effect the reaction oxidatively ... 

The reaction in equation 6 requires six Faradays to produce one mole of chlorate. The reaction is endothermic, AH = 224 kcal/mol (53.5 kcal/mol) of chlorate or 2.43 kWh/kg. In practice, it takes about 5 kWh of energy to produce a kilogram of sodium chlorate. The remaining energy is lost to electrolyte solution resistance and heat. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

If the addition of liquid to the tank causes an average endothermic or exothermic heat of solution, q J/kg (Btii/lb) of m eiip, it may be included by adding q,/co to both the numerator and the denominator of the left side. The snbscrmt 0 refers to the makeup. 

In given work the possibilities enumerated above of varieties of thermal analysis used to reseai ch of solid solutions of hydrated diphosphates with diverse composition. So, for example, the results of differential-thermal analysis Zn Co j P O -SH O showed, that it steady in the time of heating on air to 333 K. A further rise of temperature in interval 333 - 725 K is accompanied with the masses loss, which takes place in two basic stages, registered on crooked TG by two clear degrees, attendant to removal 4,0 and 1,0 mole H O. On crooked DTA these stages dehydration registers by two endothermic effects. In interval 603 - 725 K on crooked DTA is observed an exothermal effect. 

In classical examples of kinetics, such as the hydrolysis of cane sugar by acids in water solution, the reaction takes hours to approach completion. Therefore Whilhelmy (1850) could study it successfially one and a half centuries ago. Gone are those days. What is left to study now are the fast and strongly exothermic or endothermic reactions. These frequently require pressure equipment, some products are toxic, and some conditions are explosive, so the problems to be solved will be more difficult. All of them require better experimental equipment and techniques. 

At room temperature, pure N2H4 and its aqueous solutions are kinetically stable with respect to decomposition despite ihe endothermic nature of the compound and its positive free energy of formation ... 

The reaction mechanism of the DNA (cytosine-5)-methyltransferase-catalyzed cytosine methylation was investigated at the MP2 and DFT levels [98JA12895]. This system has been modeled by 1-methylcytosine 117, methylthiolate, and trimethylsulfonium. The cytosine methylation is initiated by an attack of the anionic methylthiolate at Cg of the cytosine ring (Scheme 77). The formation of the methylthiolate adduct 118 of the neutral 117 was found to be endothermic in the gas phase and in solution. However, the MP2 and DFT results differ... 

The effect of a temperature change on solubility equilibria such as these can be predicted by applying a simple principle. An increase in temperature always shifts the position of an equilibrium to favor an endothermic process. This means that if the solution process absorbs heat (AHsoin. > 0), an increase in temperature increases the solubility. Conversely, if the solution process is exothermic (AH < 0), an increase in temperature decreases the solubility. 

Because the enthalpy of solution is positive, there is a net inflow of energy as heat when the solid dissolves (recall Fig. 8.23b). Sodium chloride therefore dissolves endothermically, but only to the extent of 3 kj-mol-1. As this example shows, the overall change in enthalpy depends on a very delicate balance between the lattice enthalpy and the enthalpy of hydration. 

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