The Cations and Anions

If and r are the radii of the cation and anion respectively then by applying Pythagoras s theorem to triangle ABC we find that... 

The stoichiometry between two reactants in a precipitation reaction is governed by a conservation of charge, requiring that the total cation charge and the total anion charge in the precipitate be equal. The reaction units in a precipitation reaction, therefore, are the absolute values of the charges on the cation and anion that make up the precipitate. Applying equation 2.3 to a precipitate of Ca3(P04)2 formed from the reaction of Ca and P04 , we write... 

Because it is impossible to vary single ion concentrations independently, the activity coefficient of an electrolyte is a function of activity coefficients of the cation and anion of the electrolyte. For example, for 1 1 electrolytes the relationship is... 

Without some additional relationship it is impossible to resolve y into and "y. By introducing an extrathermodynamic assumption as this additional relationship, it becomes possible to estimate single ion transfer activity coefficients. A widely used assumption is that the transfer activity coefficients of the cation and anion of tetraphenylarsonium tetraphenylboride, Ph4As BPh4, are equal, i.e.,... 

The price of ionic liquids is determined by many parameters, such as personnel, overheads, and real production costs. One can imagine that production on a small scale would be mostly determined by the personnel cost and little by the material cost. On a large scale, the material cost should become more important and mainly determine the price of an ionic liquid. This means that the price of a large-scale commercial ionic liquid should be dictated by the price of the cation and anion source. 

The question arises as to how the B-coefficient for any solute is to be divided among the cation and anion. Before attempting to solve this problem, we could, of course, begin by assigning an arbitrary value to some species of ion (we could, for example, assign the value zero to K+) then the values to be assigned to all the other monovalent ions can readily... 

Strategy First identify the elements. Then (using Table 2.2 as necessary) identify the charges of the cation and anion. Finally, balance positive and negative charges. 

As pointed out in Chapter 2, when an ionic solid dissolves in water, the cations and anions separate from each other. This process can be represented by a chemical equation in which the reactant is the solid and the products are the positive and negative ions in water (aqueous) solution. For the dissolving of MgCl2, the equation is... 

A salt is an ionic solid containing a cation other than H+ and an anion other than OH or O2-. When a salt such as NaCl, K2CO3, or A1(N03)3 dissolves in water, the cation and anion separate from one another. 

If you know how the cation and anion of a salt affect the pH of water, it is a relatively simple matter to decide what the net effect will be (Example 13.12). 

Selective solvation of the cations and anions takes place if the molar fraction xs of the solvent S differs from that in the vicinity of the ions ... 

The crystal structure (Figure 3.31) shows the cations and anions to be stacked alternately [80] with a Pt—Pt separation of 3.25 A. 

The critical coefficients derived above correspond to the case when the metal ion m is the minority ion, and the cation + and anion - of the supporting electrolyte form the majority ions. The critical potential coefficients can be also obtained when the cation + of the supporting electrolyte is taken as the minority ion and the metal ion m and the anion - form the majority ions. Such equations are easily obtained if the subscripts m and + are converted to + and m, respectively, in Eqs. (50) and (51) for constant Qm and M+-,... 

In an ionic solid, the coordination number means the number of ions of opposite charge immediately surrounding a specific ion. In the rock-salt structure, the coordination numbers of the cations and the anions are both 6, and the structure overall is described as having (6,6)-coordination. In this notation, the first number is the cation coordination number and the second is that of the anion. The rock-salt structure is found for a number of other minerals having ions of the same charge number, including KBr, Rbl, MgO, CaO, and AgCl. It is common whenever the cations and anions have very different radii, in which case the smaller cations can fit into the octahedral holes in a face-centered cubic array of anions. The radius ratio, p (rho), which is defined as... 

In an electrolyte solution, each formula unit contributes two or more ions. Sodium chloride, for instance, dissolves to give Na+ and Cl ions, and both kinds of ions contribute to the depression of the freezing point. The cations and anions contribute nearly independently in very dilute solutions, and so the total solute molality is twice the molality of NaCl formula units. In place of Eq. 5a we write... 

Diborane reacts with ammonia to form an ionic compound (there are no other products). The cation and anion each contain one boron atom, (a) Predict the identity and formula of each ion. (b) Give the hybridization of each boron atom, (c) Identify the type of reaction that has occurred (redox, Lewis acid-base, or Bronsted acid-base). 

FIGURE 16.21 The compounds in parts (a) and ib) are coordination isomers. In these compounds, a ligand has been exchanged between the cationic and anionic complexes. 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

FIG. 4 Nomialized concentration distribution of a 0.1 molar 1 1 electrolyte in an uncharged cylindrical pore of radius five times the diameter of the ions. The dashed line, solid up-triangles, and solid down-triangles are the neutral solvent particles, cations, and anions, respectively, in an SPM model with 0.3 solvent packing fraction. The open symbols are for the cations and anions in the RPM model. 

A sodium chloride crystal contains equal numbers of Na" cations and Cl anions packed together in an alternating cubic array. Figure 2-24 illustrates a portion of the sodium chloride array. Electrical forces hold the cations and anions in place. Each Na cation attracts all the nearby Cl anions. Likewise, each Cl anion attracts all its Na neighbors. Positive cations and negative anions group together in equal numbers to make the entire collection neutral. 

To verify that two of the compounds are ionic, we can identify the cations and anions that they contain. CaCl2 is made up of Ca cations and Cl anions. Co(N03)2 contains nitrate (NO3 ) anions and Co cations. 

The names of the cations and anions identify the chemical formulas, including the charges of each ion. The charges, in turn, dictate the ion ratio that must appear in the chemical formula. 

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