Endothermic dissolution reaction

Eq. 6.27 implies that K will increase or decrease with temperature depending on the sign of AH, i.e., if AH > 0 (endothermic dissolution reaction), K will increase, while for AH < 0 (exothermic dissolution reaction), K will decrease. 

As in the Raschig process, aqueous caustic reacts with chlorine to make sodium hypochlorite solution. The urea solution is prepared by dissolving urea in water with the addition of steam to provide the heat needed for the endothermic dissolution. The temperature is kept at about 5°C for 43 percent urea solution. Glue is added at a ratio of 0.5g/Uter of solution to inhibit side reactions. The urea and hypochlorite solutions are added to the hydrazine reactor at a ratio of 1 4, and the reaction temperature is allowed to rise to 100°C. The crude product contains approximately 35 g N2H4/liter and can be refined in the same steps as used for the Raschig process. 

In contrast to the partial pressure, temperature rise does not generally contribute to the increase of the solubility. According to the principle of the smallest constraint (Le Chatelier), only endothermic dissolutions, i.e. reactions, which need additional heat, are favored (e.g. dissolution of silicates, aluminosilicates, oxides, etc.). Yet the dissolution of carbonates and sulfates is an exothermic reaction. Therefore the solubility of carbonates and sulfates is less favorable with increasing temperature. 

Solubility is temperature dependent. By Le Chatelier s principle it can either increase or decrease with increasing temperature depending on whether the dissolution reaction is endothermic or exothermic. 

Remarkably, Butler found the dissolution of oil-like groups to be exothermic. Therefore, the reverse reaction for loss of hydrophobic hydration must be endothermic. The reaction attending the inverse temperature transition to hydrophobic association is indeed endothermic as shown by differential scanning calorimetry data, specifically the middle curve in Figure 8.1 and the curves in Figure 7.1. 

In metals, dissolved hydrogen is usually found in atomic form. Its solubility typically reaches values = 10 - 10 and the dissolution reaction is endothermic. According to Le Chatelier s principle, the solubility therefore increases with the temperature. 

Endothermic reactions are much less common. The endothermic dissolution of ammonium nitrate in water is the basis of the instant cold packs that are included in some first-aid kits. They consist of a plastic envelope containing water dyed blue (for psychological reasons) and a small tube of ammonium nitrate, which is broken when the pack is to be used. 

We next consider the synthesis and chemical reactions of the oxides of chlorine. Because the compounds are strongly endothermic and have large positive free energies of formation it is not possible to prepare them by direct reaction of CI2 and O2. Dichlorine monoxide, CI2O, is best obtained by treating freshly prepared yellow HgO and CI2 gas (diluted with dry air or by dissolution in CCI4) ... 

A process that releases heat into the surroundings is called an exothermic process. Most common chemical reactions—and all combustions, such as those that power transport and heating—are exothermic (Fig. 6.8). Less familiar are chemical reactions that absorb heat from the surroundings. A process that absorbs heat is called an endothermic process (Fig. 6.9). A number of common physical processes are endothermic. For instance, vaporization is endothermic, because heat must be supplied to drive molecules of a liquid apart from one another. The dissolution of ammonium nitrate in water is endothermic in fact, this process is used in instant cold packs for sports injuries. 

KEY TERMS endothermic reaction enthalpy of reaction dissolution enthalpy of formation... 

Figure 7 shows the result of warming the reaction mixture. At approximately 10°C a rapid heat evolution occurs (adiabatic temperature rise of 44.33°C). It was reasoned that this exotherm was a composite of an endotherm resulting from dissolution of the crystallized sodium borohydride and a larger exotherm corresponding to the reduction of the ester, POX-C. 

Semibatch Reactors Some of the reactants are loaded into the reactor, and the rest of the reactants are fed gradually. Alternatively, one reactant is loaded into the reactor, and the other reactant is fed continuously. Once the reactor is full, it may be operated in a batch mode to complete the reaction. Semibatch reactors are especially favored when there are large heat effects and heat-transfer capability is limited. Exothermic reactions may be slowed down and endothermic reactions controlled by limiting reactant concentration. In bioreactors, the reactant concentration may be limited to minimize toxicity. Other situations that may call for semibatch reactors include control of undesirable by-products or when one of the reactants is a gas of limited solubility that is fed continuously at the dissolution rate. 

The endotherm that peaked at 868°C with a corresponding DTG peak at 870°C coincides with a significant dissolution of CaO as indicated by XRD relative peak heights. It is speculated that the reaction peaked at 816°C for the coarse batch (seen in the soda ash-silica-calcite mixture) was suppressed in the fine particle sizes because of the silicacious nature of the liquid phase. The endotherm that peaked at 868°C would then represent a shift to higher temperature for this reaction, with corresponding reduction in solid CaO. The weight loss associated with this endotherm would correspond to decreased CO2 solubility in a liquid phase enriched in calcia. 

That means, the higher the P(C02), the more CaC03 can be dissolved. Yet, the solubility of C02 as gas in water depends on the temperature The higher the temperature the lower the gas solubility. Consequently, initially, the calcite solubility increases with temperature due to the endothermic reaction of CaC03° complexation, but with increased temperature the exothermic reaction of CaC03(s) dissolution and the significantly reduced C02 dissolution decrease the total calcite solubility. 

Hydroxypropylated CDs are statistically substituted derivatives because hydroxy-propylation does not result in selective substitution as with methylation. While the reaction proceeds, the reactivity of the hydroxyl group changes, and this results in a mixture of products with various degrees of substitution. Their dissolution is endothermic so there is no decrease in solubility with increasing temperature [6,13]. It is necessary to note that degree of substitution in hydroxypropylated CDs is inversely correlated with their inclusion capability [14,15]. Hydroxyalkylated CDs are commercially available as tablets, ocular collyrs [16], and excipients under the trademarks Encapsin and Molecusol. 

Note that dissolution of acid phosphates is an endothermic reaction that cools the slurry, while the subsequent reaction between dissolved ions is an exothermic reaction. The net acid-base reaction is, however, exothermic and generates heat. In the case of formation of MgKP04 6H20, for example, the initial cooling of the ceramic is significant and aids in... 

The excess ammonium nitrate dissolves in the water produced in the reaction, (b) The dissolution process is very endothermic. If the flask is placed on a wet wooden block, the water freezes and attaches the block to the flask. 

Thus, because the reaction of hydrogen and oxygen to form water produces heat as a product, an increase in temperature that corresponds to an increase in system heat will favor the reverse reaction or the dissociation of water. Similarly, because the dissolution of potassium nitrate consumes heat (is endothermic, as is the dissolution of most minerals), it is favored by an increase in temperature. Further, Le Chatelier s principle tells us that because of the large reduction in volume (AV° = - 36.8 L) that accompanies the formation of liquid water from gaseous hydrogen and oxygen, an increase in pressure favors the forward reaction. 

X 102 (qj. q 7q jjcal) of heat energy were absorbed by the dissolution process because the solution lost (— sign) 7.0 X 102 cal of heat energy to the system. The reaction is endothermic. 

A major problem with Equation 2.13 is that it leads to the heat of mixing being positive, that is, always endothermic, absorbing heat. Many solutions are exothermic, that is, heat is evolved when the solution is formed. There have been various efforts to model exothermic heats of mixing. These are generally dated to Dolezalek (34) who considered dissolution to represent processes similar to chemical reactions. 

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