Endothermic processes solution process

Figure 7.7 Exothermic (top) and endothermic (bottom) solution processes.

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... 

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. 

This is a form of Henry s law, where the Henry s law constant is simply the product of the vapor pressure of solute 2 multiplied by the energy correction term exp(Ae/ 7 ). For oleophilic substances, which do not like water, mixing is an endothermic process and the term Ae/kT is positive, so that Henry s law constant is much higher than the vapor pressure of the pure substance. 

A novel complex, Mo[N( Bu) (Ar)]3 140, was prepared and shown to activate the triple bond in molecular nitrogen in a stoichiometric fashion. However, the complex, when used for alkyne metathesis, does not affect the expected transformation, but undergoes a vigorous endothermic process in a CH2CI2 solution. The resultant solution... 

In every solution in which there are ions, i.e. in every electrolyte, the ions are combined with molecules of the solvent, producing "solvate ions . Since the dissociation into solvated ions is usually an endothermic process, the formation of ions, free or solvated, is only possible if the process is associated with an increase in entropy. In actual fact, a diluted solution always has a considerable entropy, because the ions are distributed over a large volume of the solvent. It will be clear that dissociation into ions, at a given temperature and volume, will be more complete, the lower the total energy connected with the formation of the ions. 

Another process solution is that of ammonium sulfate. The dissolution of (NH4)2S04 is endothermal, and its solubility and dissolving rate are greatly affected by temperature, so that the use of hot water is needed. First put a certain amount of (NH4)2S04, and then hot water into an ordinary beaker, quickly agitate the mixture to speed up dissolution of (NH4)2S04, and a limpid, transparent solution is obtained. 

Recall that, if y > 1, increase in T results in decrease in yt. Thus the left-hand side of the above equation becomes negative and hence Hf is positive - endothermic. In a similar way one may find that, if y, < 1, then the solution process is exothermic. If the solution process in an i-j binary system is endothermic, the i-i and j-j attractions are greater than the i-j attraction, i atoms attempt to have only i atoms as nearest neighbours - tendency toward phase separation or clustering. 

A. The decline in water temperature indicates that the net solution process is endothermic (requiring heat). A temperature increase supplying more heat will favor the solution and increase solubility according to Le Chatelier s principle. 

Why, then, does a solution ever form You may recall from general chemistry that whether a reaction is spontaneous or not depends on both the enthalpy and the entropy of that process. Entropy (S) is a measure of disorder. Processes that increase the disorder in a system (AS > 0) are favored. Although most solution processes are endothermic (disfavored by enthalpy, AH > 0), the solution is more disordered than the separate solute and solvent (favored by entropy, AS > 0). Therefore, as long as the process is not too endothermic, the favorable entropy change will cause the solute to dissolve. Whether the process is likely to be too endothermic can be estimated by examining the... 

For uptake of solute from solution by porous solids the rate will be endothermic rather than exothermic if intraparticle transport is the rate-limiting mechanism. Because diffusion is an endothermic process while adsorption is exothermic, rate of uptake of solute by porous solids will often increase with increasing temperature while for the same system the equilibrium position of adsorption or adsorption capacity will decrease with increasing temperature. 

Similar arguments can be made to explain why nonpolar solutes dissolve in nonpolar solvents. In this case AH°0 n is expected to be small, because the endothermic and exothermic terms in the solution process are expected to be similar in size. Thus the expected positive value of AS°0in again would furnish the driving force for the solution process. These arguments suggest that AS 0jn provides the principal driving force for the behavior summarized by the rule like dissolves like. ... 

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