Exothermic solution processes

Solubility will decrease with increasing temperature for an exothermic solution process. This is the case for cerium(lll) sulfate—Ce2(S04)3—in water. 

How does the magnitude of compare with the magnitude of Wsoiute + Wsoiute foi" exothermic solution processes ... 

Figure 13.4 For exothermic solution processes the magnitude of AHmix will be larger than the magnitude of Affsoime + AHsohrent...

When a solution is diluted—that is, when more solvent is added to lower the overall concentration of the solute— additional heat is usually given off or absorbed. The enthalpy (or heat) of dilution (Affdu) is the heat change associated with the dilution process. If a certain solution process is endothermic and the solution is subsequently diluted, more heat will be absorbed by the same solution from the surroundings. In an exothermic solution process, more heat will be liberated if additional solvent is added to dilute the solution. Therefore, always be cautious when diluting a solution in the laboratory. 

If the solubility xb increases with increasing temperature, Asoi,b ° must be positive and the solution process is endothermic. A decrease of solubility with increasing temperature implies an exothermic solution process. These statements refer to a solid of low solubility see page 357 for a discussion of the general relation between the temperature dependence of solubility and the sign of the molar differential enthalpy of solution at saturation. 

You know that heating decreases the solubility of a gas, so dissolution of gases is exothermic. How do the values for the enthalpies of solution in Figure 2.13 support this idea of exothermic solution processes for gaseous solutes ... 

Solution Process. With the exception of fibrous triacetate, practically all cellulose acetate is manufactured by a solution process using sulfuric acid catalyst with acetic anhydride in an acetic acid solvent. An excellent description of this process is given (85). In the process (Fig. 8), cellulose (ca 400 kg) is treated with ca 1200 kg acetic anhydride in 1600 kg acetic acid solvent and 28—40 kg sulfuric acid (7—10% based on cellulose) as catalyst. During the exothermic reaction, the temperature is controlled at 40—45°C to minimize cellulose degradation. After the reaction solution becomes clear and fiber-free and the desired viscosity has been achieved, sufficient aqueous acetic acid (60—70% acid) is added to destroy the excess anhydride and provide 10—15% free water for hydrolysis. At this point, the sulfuric acid catalyst may be partially neutralized with calcium, magnesium, or sodium salts for better control of product molecular weight. 

Reality Check Notice that qieacu0n is negative, so the solution process is exothermic. That is reasonable since the temperature of the water increases. 

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. 

Solution polymerizafion. Highly exothermic reactions can be handled by this process. The reaction is carried out in an excess of solvent that absorbs and disperses the heat of reaction. The excess solvent also prevents the formation of slush or sludge, which sometimes happens in the bulk process when the polymer volume overtakes the monomer. The solution process is particularly useful when the polymer is to be used in the solvent, say like a coating. Some of the snags with this process its difficult to remove residual traces of solvent, if that s necessary the same is true of catalyst if any is used. This process is used in one version of a low-pressure process for high-density polyethylene and for polypropylene. 

Solution-based polymerization is the most common process used today for SAP manufacture. This process is efficient and generally has a lower capital cost base. The solution process uses a water-based monomer solution to produce a mass of reactant polymerized gel. The polymerization s own reaction energy (exothermic) is used to drive much of the process, helping reduce manufacturing cost. The reactant polymer gel is then chopped, dried, and ground to its final granule size. Any treatment to enhance performance characteristics of the SAP is usually accomplished after the final granule size is created. 

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. 

If the solution process is exothermic, the i-i and j-j attractions are smaller than the i-j attraction, i atoms attempt to have only j atoms as nearest neighbours, and j atoms attempt to have only i atoms as nearest neighbours - tendency toward compound formation. 

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. 

The hearth is sloping so that as the sodium sulfide and sodium carbonate (melting points 950 and 851°C) accumulate in the char on the hearth the mixture, called smelt (literally a molten rock), trickles out. The smelt is immediately dissolved in lime mud wash water for the first stage in fresh liquor preparation. Dissolving of the molten smelt in water is a noisy procedure from the large temperature differential between the smelt and the water, and from the exothermic nature of the solution process as well. 

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... 

If the solute-solvent attraction is stronger than the solvent-solvent attraction and solute-solute attraction, the solution process is favorable, or exothermic < 0). If the... 

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. 

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. 

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