Endothermic solution process

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... 

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. 

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... 

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... 

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. ... 

By plotting the logarithm of the solubility in moles per liter vs. the reciprocal of the absolute temperature, the differential heat of solution can be calculated as the slope (-A/7s/2.3037 ). A positive heat of solution indicates an endothermic solubilization process (i.e., absorbs heat). Therefore, an increase in temperature increases solubility. A negative value indicates an exothermic solubilization process (i.e., emits heat) and a differential heat of solution near zero indicates that solubility is not significantly influenced by temperature. 

Dimethylaniline (bp 193°) is occasionally used as a solvent for the preparation of Grignard reagents. The main impurity is monomethyl-aniline, which may be removed by treatment with a small amount of acetic anhydride. The presence of secondary amine is indicated by a temperature rise on addition of acetic anhydride, and in the absence of this impurity, the solution process is endothermic. 

When a solute is dissolved in a solvent, heat change generally occurs. A dissolution process may be exothermic or endothermic. Exothermic processes emit energy as heat. Endothermic processes absorb energy as heat. Temperature rises in an exothermic process, but falls in an endothermic one. When lithium chloride (LiCl) dissolves in water, the solution gets warmer and the temperature goes up. We can say that the dissolution of lithium chloride is exothermic. (Figure 6). 

When a solid in a cold pack dissolves in water, the process takes place spontaneously. What causes the solution process to be spontaneous in spite of the fact that it is endothermic ... 

The heat of solution of an ionic compound in water is the sum of the lattice energy of the compound and the heat of hydration. The relative magnitudes of these two quantities determine whether the solution process is endothermic or exothermic. The heat of dilution is the heat absorbed or evolved when a solution is diluted. 

For simplicity, we can imagine the solution process taking place in three distinct steps (Figure 12.2). Step 1 is the separation of solvent molecules, and step 2 entails the separation of solute molecules. These steps require energy input to break attractive intermolecular forces therefore, they are endothermic. In step 3 the solvent and solute molecules mix. This process can be exothermic or endothermic. The heat of solution is given by... 

You may wonder why a solute dissolves in a solvent at all if the attraction for its own molecules is stronger than the solute-solvent attraction. The solution process, like all physical and chemical processes, is governed by two factors. One is energy, which determines whether a solution process is exothermic or endothermic. The second factor is an inherent tendency toward disorder in aU natural events. In much the same way that a deck of new playing cards becomes mixed up after it has been shuffled a few times, when solute and solvent molecules mix to form a solution, there is an increase in randomness, or disorder. In the pure state, the solvent and solute possess a fair de-... 

Figure 12.3 shows the temperature dependence of the solubility of some ionic compounds in water. In most but certainly not all cases, the solubility of a solid substance increases with temperature. However, there is no clear correlation between the sign of A/ZsoIj, and the variation of solubility with temperature. For example, the solution process of CaCl2 is exothermic, and that of NH4NO3 is endothermic. But the solubility of both compounds increases with increasing temperature. In general, the effect of temperature on solubility is best determined experimentally. 

As you know, some solution processes are endothermic and others are exothermic. Provide a molecular interpretation for the difference. 

Ammonium nitrate (NH4NO3) dissolves spontaneously and endothermically in water. What can you deduce about the sign of AS for the solution process ... 

For solid and liquid solutes, the impact of temperature depends on whether the solution process requires or releases heat. Endothermic and exothermic reactions will be discussed in more detail in Competency 5.1. The following brief analysis is applicable for the effect of temperature on solutions. 

The second problem, which needed to be resolved was the heat of solution, AH. For solutions with very similar molecules such as benzene and toluene, or hexane and octane, the heat of mixing is near zero. For various pairs of molecules, for example, acetone and chloroform, significant heat is evolved (AH < 0) and the solution process is said to be exothermic. For other pairs of molecules, heat is absorbed (AH > 0) and the solution process is called endothermic. 

Will there be a lower or an upper consolute point if the solution process for two liquids is exothermic For an endothermic process ... 

Thermodynamic Principles [14.14]. Forces of attraction act between the molecules of the pure components and between the different molecules in the solution. If the forces of attraction in the solution are greater than those in the pure components, dissolution is accompanied by a decrease in the internal energy of the system. The process is exothermic and heat is released. If, however, the forces of attraction between the molecules of the pure components are greater than those in the solution, the internal energy of the system is increased with absorption of heat. In a closed system, this endothermic dissolution process is accompanied by cooling. In open systems heat is absorbed from the surroundings. 

Solution processes are typically accompanied by changes in enthalpy. For example, when NaCl dissolves in water, the process is slightly endothermic, = 3.9 kj/mol. We... 

When ammonium chloride dissolves in water, the solution becomes colder, (a) Is the solution process exothermic or endothermic (b) Why does the solution form ... 

The enthalpy of solution of KBr in vrater is about +198 kj/mol. Nevertheless, the solubility of tCBr in water is relatively high. Why does the solution process occur even though it is endothermic ... 

A solution composed of an equal numher of moles of acetone and chloroform has a vapor pressure of 250 torr at 35 "C. (a) What would he the vapor pressure of the solution if it exhibited ideal behavior (b) Use the existence of hydrogen bonds between acetone and chloroform molecules to explain the deviation from ideal behavior, (c) Based on the behavior of the solution, predict whether the mixing of acetone and chloroform is an exothermic (AHjoin < 0) or endothermic > 0) process. 

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