Hydration, heat number

Why Do Hydration Heats of Transition-Metal Ions Vary Irregularly with Atomic Number ... 

Again in the case of the lyotropic number interpolation the accuracy is dependent on the accuracy of the measurement of the lyotropic number, though the sum of the hydration heats only changes slowly with lyotropic number. It must also be stressed that the lattice energies obtained in this way are not theoretical lattice energies in that they are not based on any model of the crystal. Rather they are empirical or experimental in that they are based on a combination of empirical hydration enthalpies and experimental enthalpies of solution. 

When many ionic compounds are crystallized from a water solution, they include individual water molecules as part of their crystalline structure. If the substances are heated, this water of crystallization may be driven off and leave behind the pure anhydrous form of the compound. Because the law of multiple proportions also applies to crystalline hydrates, the number of moles of water driven off per mole of the anhydrous compound should be a simple whole-number ratio. You can use this information to help you determine the formula of the hydrate. 

Figure 4. A complete reaction scheme up to two hydration particles. Numbers over reaction arrows represent the heats of reactions in kcal/mol. Fractions means reaction for cis-/trans-conformers, respectively.

The hydration heat results of the binders in concretes are presented in Table 3. The results shows that with the increase of FA replacement levels, the hydration heat release rate was slowed down and the hydration heat quantity was gradually reduced. The reduction in hydration heat is attributed to the decrease of the hydration particle numbers in per binder paste with the addition of FA. The rate and quality of hydration heat of both the 25% FA and 30% FA mixes were lower than those of the ternary mixes of FA and GGBS at all the test ages. It can be concluded that the addition of FA is better than GGBS for controlling hydration heat of HPCs. 

KBr pellet or Nujol mull), powder XRD, or thermogravimetric analysis to determine the number of waters of hydration (heat the sample at 10°Cmin from room temperature to 700°C under air or nitrogen atmosphere). 

Characterize your sample by one or more of the following methods elemental analysis (P and/or W), UV-vis spectroscopy (2x 10 M in 0.1 M HCl), FTIR (KBr pellet or Nujol mull), MAS P NMR (single-pulse excitation, 8.0 ps pulse, recycle delay of 10 s, spin rate of 5 kHz, and 64 acquisitions), or thermogravimetric analysis to determine the number of waters of hydration (heat the sample at 10°C min from room temperature to 700°C under air or nitrogen atmosphere). 

Antimony trioxide is insoluble in organic solvents and only very slightly soluble in water. The compound does form a number of hydrates of indefinite composition which are related to the hypothetical antimonic(III) acid (antimonous acid). In acidic solution antimony trioxide dissolves to form a complex series of polyantimonic(III) acids freshly precipitated antimony trioxide dissolves in strongly basic solutions with the formation of the antimonate ion [29872-00-2] Sb(OH) , as well as more complex species. Addition of suitable metal ions to these solutions permits formation of salts. Other derivatives are made by heating antimony trioxide with appropriate metal oxides or carbonates. 

Anhydrous An anhydrous material does not contain any water molecules. Many substances occur naturally as hydrates, compounds that have a specific number of water molecules attached to them. This water can often be removed by heating and/or vacuum to give the anhydrous material. Anhydrous materials can absorb water from their surroundings and find use as dessicants. Examples include those packets of silica gel you find in some consumer goods, as well as dehumidifying sachets used in clothes closets. When an anhydrous material reacts with water, this could release a large amount of heat, possibly leading to a heat or pressure buildup that could result in an explosion. 

Although not strictly binary compounds, hydroxides are conveniently classified here between hydrated salts, since both release water on heating and incorporate certain common features of behaviour, and oxides, which are the usual residual product. A number of hydroxide decomposition studies have extended measurements to consider the relationship with subsequent higher temperature changes in the product oxide. 

Many ionic compounds can have water molecules incorporated into their solid structures. Such compounds are called hydrates. To emphasize the presence of discrete water molecules in the chemical structure, the formula of any hydrate shows the waters of hydration separated from the rest of the chemical formula by a dot. A coefficient before H2 O indicates the number of water molecules in the formula. Copper(II) sulfate pentahydrate is a good example. The formula of this beautiful deep blue solid is C11SO4 5 H2 O, indicating that five water molecules are associated with each CuSOq unit. Upon prolonged heating, CuSOq 5 H2 O loses its waters of hydration along with its color. Other examples of hydrates include aluminum nitrate nonahydrate, A1 (N03)3 9 H2 O,... 

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... 

Ions not solvated are unstable in solutions between them and the polar solvent molecules, electrostatic ion-dipole forces, sometimes chemical forces of interaction also arise which produce solvation. That it occurs can be felt from a number of effects the evolution of heat upon dilution of concentrated solutions of certain electrolytes (e.g., sulfuric acid), the precipitation of crystal hydrates upon evaporation of solutions of many salts, the transfer of water during the electrolysis of aqueous solutions), and others. Solvation gives rise to larger effective radii of the ions and thus influences their mobilities. 

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. 

When the solvent is evaporated, a solid is obtained that contains the hydrated aluminum ion and chloride ions. This solid can be described as [A1(H20)6]C13, although the number of water molecules may depend on the conditions. When this solid is heated, water is lost until the composition [A1(H20)3C13] is approached. When heated to still higher temperature, this compound loses HC1 rather than water ... 

A large number of inorganic compounds crystallize as hydrates. One of the most familiar examples is copper sulfate pentahydrate, CuS04-5H20. Like most hydrates, when this material is heated it loses water, but because all of the H20 molecules are bound in different ways, some are lost more easily than others. Therefore, as the solid is heated the reactions observed first are... 

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. 

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