Complex-ion equilibria

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

The equilibrium constant for the formation of a complex ion is called a formation constant (or stability constant) and given the symbol Kf. A typical example is... 

Table 15.4 lists formation constants of complex ions. In each case, Kt applies to the formation of the complex by a reaction of the type just cited. Notice that for most complex ions listed, Kf is a large number, 10s or greater. This means that equilibrium considerations strongly favor complex formation. Consider, for example, the system... 

Formation constant (Kf) Equilibrium constant for the formation of a complex ion from the corresponding cation and ligands, 422-425,639 Formic acid, 595... 

Only two types of complex ions are present in fluoride solutions NbOF52 and NbF6 the equilibrium of which is described by Equation (47). Higher concentrations of HF lead to the formation of NbF6 ions. 

The third step consisted of the direct investigation of IR emission spectra for a wide range of concentrations. The investigation showed the tendency of the metals to reduce their coordination number when moving from solid to molten state. This property of the melt depends on the equilibrium between two types of complex ions, MeF72 and MeF6 ... 

This equilibrium covers the presence of all inferred types of complex ions. The equilibrium constant of Equation (72) can be written as... 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

It should be noted that in addition to changes in K2NbF7 or K2TaF7 concentrations that afford control over the complex structure and electrolysis parameters, the cation type also affects the equilibrium between the complex ions. The heptacoordinated complexes become increasingly dominant when progressing along the cation series from Li to Cs. 

Higher temperatures, increased K2TaF7 concentrations and other factors mentioned above shift the equilibrium in Equation (174) to the left and lead to the formation of coarser tantalum particles. Form this point of view, it can be concluded that smaller hexacoordinated complexes, TaF6 lead to the formation of coarser tantalum powder, whereas the predominant presence of larger heptacoordinated complexes ions initiates the formation of finer particles. 

As an example of the problem of species in solution, consider the case of a solution made by dissolving some potassium chrome alum, KCrfSO s-12H20, in water. On testing, the solution is distinctly acidic. A currently accepted explanation of the observed acidity is based upon the assumption that, in water solution, chromic ion is associated with six H20 molecules in the complex ion, Cr(H20) a. This complex ion can act as a weak acid, dissociating to give a proton (or hydronium ion). Schematically, the dissociation can be represented as the transfer of a proton from one water molecule in the Cr(H20) 3 complex to a neighboring H20 to form a hydronium ion, H30+. Note that removal of a proton from an H20 bound to a Cr+3 leaves an OH- group at that position. The reaction is reversible and comes to equilibrium ... 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Even though the equilibrium constant for the formation of Au3- from gold is very unfavorable, the reaction proceeds because any Au3+ ions formed are immediately complexed by Cl- ions and removed from the equilibrium. In a process widely used in the refining of the metal, gold also reacts with sodium cyanide in an aerated aqueous solution to form the complex ion [Au(CN)2] ... 

In Section 16-6, we describe how metal cations in aqueous solution can form bonds to anions or neutral molecules that have lone pairs of electrons. This leads to formation of complex ions and to chemical equilibria involving complexation. The complexation equilibrium between Ag and NH3 is an example ... 

Now we write the equilibrium constant expression for formation of the complex ion ... 

Simple or complexed ions in solution in equilibrium with exchangeable and other phases. 

Metals, mainly Mg and Fe, in simple reversible equilibrium chemistry (complex ion chemistry) also linked to controls... 

For this equilibrium, the ion product constant has a value of approximately 2.7 X 1CP4. However, the discussion is complex and other species are present in sulfuric acid as a result of equilibria that can be written as... 

M NH3(aq). Because of the large value of Kf = 1.6 x 107, we start by having the reagents form as much complex ion as possible, and approach equilibrium from that point. 

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

We can treat other types of equilibria in much the same way as the ones previously discussed. For example, there is an equilibrium constant associated with the formation of complex ions. This equilibrium constant is the formation constant, Kt. 

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