An ions in aqueous solutions

Consider the standard enthalpy for formation AHf of an ion in aqueous solution from a metal, corresponding to the process ... 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

State symbols Symbols used in chemical equations to denote whether a reactant or product is a solid (s), a liquid (I), a gas (g), or an ion in aqueous solution (aq). 

Because oC the difficulty of deteimining the amount of hydration of an ion in aqueous solutions, chemists have been slow to accept these formulas the older formulas are ZnOa , AlOo", etc. 

Molecular Derivation of the Model. The molecular theory of surface complexes is a special case of a multispecies lattice model. The surface complexes thus are interpreted as molecular species immobilized (relative to the 10-ps time scale for diffusive motion of an ion in aqueous solution) on an array of M sites that represent surface functional groups. If there are two different surface species on the sites (e.g., a protonated and an unprotonated surface hydroxyl group) and if each site has z nearest neighbors, then any distribution of the two species (call them A and B) over the sites must satisfy the general conditions (23)... 

Whereas absolute thermodynamic values for individual ions cannot be measured directly, they can be estimated on the basis of theory. Unfortunately, owing to the large number of interactions involved, the theoretical treatment of an ion in aqueous solution is very difficult. The following absolute values are generally agreed to be not far from the actual values, for the proton ... 

The standard partial molar volume of an ion in aqueous solution, Vf, is the actual volume to be assigned to the ion in the solution (at infinite dilution). It is the sum of its intrinsic volume, Tnntr", and the electrostriction that the ion has caused in the water around it, yieiec , the latter being a negative quantity. The volume of a bare unhydrated ion, A%Np,l i)r, cannot represent its intrinsic volume and must be enlarged to account for the void spaces between the water molecules and the ion and among themselves in order to represent the intrinsic volume of the ion in the solution. A factor of A = 1.213 was proposed by Mukerjee (1961) for the alkali metal and the halide ions, producing ... 

However, in many applications the essential space cannot be reduced to only one degree of freedom, and the statistics of the force fluctuation or of the spatial distribution may appear to be too poor to allow for an accurate determination of a multidimensional potential of mean force. An example is the potential of mean force between two ions in aqueous solution the momentaneous forces are two orders of magnitude larger than their average which means that an error of 1% in the average requires a simulation length of 10 times the correlation time of the fluctuating force. This is in practice prohibitive. The errors do not result from incorrect force fields, but they are of a statistical nature even an exact force field would not suffice. 

Samal et al. [25] reported that Ce(IV) ion coupled with an amide, such as thioacetamide, succinamide, acetamide, and formamide, could initiate acrylonitrile (AN) polymerization in aqueous solution. Feng et al. [3] for the first time thoroughly investigated the structural effect of amide on AAM polymerization using Ce(IV) ion, ceric ammonium nitrate (CAN) as an initiator. They found that only acetanilide (AA) and formanilide (FA) promote the polymerization and remarkably enhance Rp. The others such as formamide, N,N-dimethylformamide (DMF), N-butylacetamide, and N-cyclohexylacetamide only slightly affect the rate of polymerization. This can be shown by the relative rate (/ r), i.e., the rate of AAM polymerization initiated with ceric ion-amide divided by the rate of polymerization initiated with ceric ion alone. Rr for CAN-anilide system is approximately 2.5, and the others range from 1.04-1.11. 

It is clear that the use of (18) would point toward the conclusion that, for an atomic ion in aqueous solution, the contributions from these two... 

In the case of a singly charged atomic ion in aqueous solution we have estimated the mutual potential energy between the ion and an adjacent water molecule when they are of nearly the same size, and have found the value to be about four times as great as the mutual potential energy of two adjacent water molecules. We conclude then that in the vicinity of an atomic ion the water structure will have to build itself round the ion, insofar as this is possible. 

It will be recalled that in Fig. 28 we found that for the most mobile ions the mobility has the smallest temperature coefficient. If any species of ion in aqueous solution at room temperature causes a local loosening of the water structure, the solvent in the co-sphere of each ion will have a viscosity smaller than that of the normal solvent. A solute in which both anions and cations are of this type will have in (160) a negative viscosity //-coefficient. At the same time the local loosening of the water structure will permit a more lively Brownian motion than the ion would otherwise have at this temperature. Normally a certain rise of temperature would be needed to produce an equal loosening of the water structure. If, in the co-sphere of any species of ion, there exists already at a low temperature a certain loosening of the water structure, the mobility of this ion is likely to have an abnormally small temperature coefficient, as pointed out in Sec. 34. 

The conclusions are evidently relevant to the amount of entropy lost by ions in methanol solution—see Table 29. If, however, the expression (170) is used for an atomic ion, we know that it is applicable only for values of R that are large compared with the ionic radius—that is to say, it will give quantitative results only when applied to the solvent dipoles in the outer parts of the co-sphere. The extent to which it applies also to the dipoles in the inner parts of the co-sphere must depend on the degree to which the behavior of these molecules simulates that of the more distant molecules. This can be determined only by experiment. In Table 29 we have seen that for the ion pair (K+ + Br ) and for the ion pair (K+ + Cl-) in methanol the unitary part of ASa amounts to a loss of 26.8 e.u. and 30.5 e.u., respectively, in contrast to the values for the same ions in aqueous solution, where the loss of entropy in the outer parts of the co-sphere is more than counterbalanced by a gain in entropy that has been attributed to the disorder produced by the ionic field. 

Methyl bromide is a compound in which the chemical bonds are predominantly covalent. An aqueous solution of methyl bromide does not conduct electricity, hence it does not form ions (such as CH and Br ions) in aqueous solutions. 

Concept used in sophisticated scaling models, whereby certain ions in aqueous solution are said to associate in pairs (e.g., CaS04, CaHC03-). These ion pairs are then deducted from the total analytical value, to provide an estimate of the free ion content available for seed crystal scaling or growth agglomeration and deposition. 

In an aqueous solution, solute molecules or ions require a certain amount of time to migrate through the solution. The rate of this migration sets an upper limit on how fast reactions can take place, because no reaction can take place faster than the ions can he supplied. This limit is known as the diffusion-controlled rate. It has been found that the diffusion rate for hydrogen ions is about three times as fast as that for other ions in aqueous solution. Explain why this is so. 

The copper product is known as blister copper because of the appearance of air bubbles in the solidified metal. In the hydrometallurgical process, soluble Cu2+ ions are formed by the action of sulfuric acid on the ores. Then the metal is obtained by reducing these ions in aqueous solution either electrolytically or chemically, by using an inexpensive reducing agent that has a more negative standard potential than that of copper, such as hydrogen or iron (see Section 14.3) ... 

Ion chromatography can be used in unique ways and by appropriate modification can often be applied to the separation of mixtures where the components themselves do not ionize or do not normally produce interactive ions in aqueous solution. A good example of this type of separation is afforded by the analysis of saccharide mixtures using ion exchange interactions. An illustration of such a separation is given in figure 15. 

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as a substance that gives up a proton differs little from that of Arrhenius. However, the same is not true of their definition of a base as a substance capable of accepting protons which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston Rychtman, 1982) ... 

Dissolution of an ionic salt is essentially a separation process carried out by the interaction of the salt with water molecules. The separation is relatively easy in water because of its high dielectric constant. Comparison of the energies needed to separate ions of NaCl from 0-2 nm to infinity shows that it takes 692-89 kJ mol" in vacuum, but only 8-82 kJ moF in aqueous solution (Moore, 1972). Similar arguments have been used to try to estimate solvation energies of ions in aqueous solution, but there are difficulties caused by the variations in dielectric constant in the immediate vicinity of individual ions. 

Neutron scattering has been used for studying the state of solvation of ions in aqueous solution (Enderby et al., 1987 Salmon, Neilson Enderby, 1988). These studies have shown that a distinct shell of water molecules of characteristic size surrounds each ion in solution. This immediate hydration shell was called zone A by Frank Wen (1957) they also postulated the existence of a zone B, an outer sphere of molecules, less firmly attached, but forming part of the hydration layer around a given ion. The evidence for the existence of zone B lies in the thermodynamics of... 

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