Hydration, heat of

The heat of hydration is approximately —70 kj /mol (—17 kcal/mol). This process usually produces no waste streams, but if the acrylonitrile feed contains other nitrile impurities, they will be converted to the corresponding amides. Another reaction that is prone to take place is the hydrolysis of acrylamide to acryhc acid and ammonia. However, this impurity can usually be kept at very low concentrations. American Cyanamid uses a similar process ia both the United States and Europe, which provides for their own needs and for sales to the merchant market. 

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Crystalline anhydrous borax takes up some water from moist air even at 300°C. It becomes anhydrous near 700°C and melts at 742.5°C. The heat of hydration to borax has been calculated as 161 kJ/mol (38.5 kcal /mol) of Na20 2B202 (73,82). The heat of fusion has been reported as 81.2 kJ/mol (19.4 kcal/mol) (17). 

Type IV (Low Heat of Hydration). Type IV is used where the rate and amount of heat generated from hydration have to be minimised, ie, large dams. Compared to Type I, Type IV Pordand cement has only about 40 to 60% of the heat of hydration during the tirst seven days and cures at a slower rate. In large stmctures such as dams where the heat of hydration cannot be readily released from the core of the stmcture, the concrete may cure at an elevated temperature, and thermal stresses can build up in the stmcture because of nonuniform cooling that weakens the stmcture. U.S. production of Type IV Pordand cement is less than 1%. 

Supersulfiated cement (82) has a very low heat of hydration and low drying shrinkage. It has been used in Europe for mass concrete constmction and especially for stmctures exposed to sulfate and seawaters. 

Estimates of the heat of hydration of Li+(g) give values near 520kJ mol compared with... 

All M cations of this triad are diamagnetic and, unless coordinated to easily polarized ligands, colourless too. In aqueous solution the Cu ion is very unstable with respect to disproportionation (2Cu v - Cu + Cu(s)) largely because of the high heat of hydration of the divalent ion as already mentioned. At 25°C, K (= [Cu ][Cu ]-2) is large, (5.38 0.37) x 10 1mol , and standard reduction potentials have been calculated to be ... 

Hydratstionsw me, /. heat of hydration. Hydratbildung /. hydrate formation, hydration. 

In this equation, AH is not known, but Eucken (as quoted by von Stackelberg48) suggested, AH = 0, and comparison of experimental and calculated heats of hydrate formation30 certainly supports a low value of AH. The variation of the composition of a gas hydrate along the three-phase line ice-hydrate-gas will therefore be small. (The variation along the three-phase line hydrate-aqueous liquid-gas is larger, cf. Section III.C.(l).)... 

For the gas hydrates it is not possible to make an entirely unambiguous comparison of the observed heat of hydrate formation from ice (or water) and the gaseous solute with the calculated energy of binding of the solute in the ft lattice, because AH = Hfi—Ha is not known. If one assumes AH = 0, it is found that the hydrates of krypton, xenon, methane, and ethane have heats of formation which agree within the experimental error with the energies calculated from Eq. 39 for details the reader is referred to ref. 30. 

Example 2. The heat of hydration of a salt may be calculated from the change of dissociation pressure with temperature BaCl2. 2 H20 — BaCl2 + 2 H20... 

G. Brodale and W. F. Giauque, "The Heat of Hydration of Sodium Sulfate. Low Temperature Heat Capacity and Entropy of Sodium Sulfate Decahydrate", J. Am. Chem. Soc., 80, 2042-2044 (1958). 

Figure 8-12. Contributions to heats of hydration A = B = bond weakening due to steric activity of configuration C = LFSE as multiples of Dq D = LFSE, allowing for variation in Dq E = exchange energy F = A + B + D + E.

If the temperature of dry saturated steam is increased, then, in the absence of entrained moisture, the relative humidity or degree of saturation is reduced and the steam becomes superheated (Fig. 20.5). During sterilization this can arise in a number ofways, for example by overheating the steamjacket (see section 4.2.2), by using too dry a steam supply, by excessive pressure reduction during passage of steam from the boiler to the sterilizer chamber, and by evolution of heat of hydration when steaming... 

Values for the heats of hydration of a number of ions that were calculated by the aforementioned methods on the basis of theoretical models and experimental data are reported in Table 7.2. We see that there is a certain general agreement, but in individual cases the discrepancies are large, due to inadequacies of the theoretical concepts used in the calculations. 

TABLE 7.2 Heats of Hydration of Individual Ions (kj/mol) and Hydration Numbers h. 

The fact that the water molecules forming the hydration sheath have limited mobility, i.e. that the solution is to certain degree ordered, results in lower values of the ionic entropies. In special cases, the ionic entropy can be measured (e.g. from the dependence of the standard potential on the temperature for electrodes of the second kind). Otherwise, the heat of solution is the measurable quantity. Knowledge of the lattice energy then permits calculation of the heat of hydration. For a saturated solution, the heat of solution is equal to the product of the temperature and the entropy of solution, from which the entropy of the salt in the solution can be found. However, the absolute value of the entropy of the crystal must be obtained from the dependence of its thermal capacity on the temperature down to very low temperatures. The value of the entropy of the salt can then yield the overall hydration number. It is, however, difficult to separate the contributions of the cation and of the anion. 

Organic materials, particularly if fibrous with adsorbed or absorbed moisture present, may char or ignite in contact with the stabilised liquid form because of the very high heat of hydration (2.1 kJ/g) and formation of hot oleum which then functions as an oxidant. 

In this cycle, U is the lattice energy, AH+ and AH are the heats of hydration of the gaseous cation and anion, and AHs is the heat of solution. From this cycle, it is clear that... 

As defined earlier, the lattice energy is positive while the solvation of ions is strongly negative. Therefore, the overall heat of solution may be either positive or negative depending on whether it requires more energy to separate the lattice into the gaseous ions than is released when the ions are solvated. Table 7.7 shows the heats of hydration, AH °hy(, for several ions. 

Cations in aqueous solutions have an effective radius that is approximately 75 pm larger than the crystallographic radii. The value of 75 pm is approximately the radius of a water molecule. It can be shown that the heat of hydration of cations should be a linear function of Z /r where is the effective ionic radius and Z is the charge on the ion. Using the ionic radii shown in Table 7.4 and hydration enthalpies shown in Table 7.7, test the validity of this relationship. 

Compounds of beryllium and aluminum are substantially covalent as a result of the high charge -to-size ratio, which causes polarization of anions and very high heats of hydration of the ions ( —2487kJ mol-1 for Be2+ and — 4690kJ mol-1 for Al3+). 

One of the consequences of the lanthanide contraction is that some of the +3 lanthanide ions are very similar in size to some of the similarly charged ions of the second-row transition metals. For example, the radius of Y3+ is about 88 pm, which is approximately the same as the radius of Ho3+ or Er3 +. As shown in Figure 11.8, the heats of hydration of the +3 ions show clear indication of the effect of the lanthanide contraction. 

FIGU RE 11.8 Heat of hydration of + 3 lanthanide ions as a function of ionic radius. 

The heat of hydration of an ion is related to its size and charge (see Chapter 7). However, in this case the aqua complex that is formed causes the d orbitals to be split in energy, and if the metal ion has electrons in the d orbitals, they will populate the t2g orbitals, which have lower energy. This results in... 

Hydration of the metal ions produces an enthalpy change that is commensurate with the size and charge of the ion with the addition of the number of Dq units shown in the weak field column in Table 17.4. For d°, d5, and d10 there is no additional stabilization of the aqua complex since these cases have no ligand field stabilization. Figure 17.10 shows a graph of the heats of hydration for the first-row + 2 metal ions. 

FIGURE 17.10 Heat of hydration of -I- 2 metal ions of the first transition series. 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

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