Self-ionization equilibria

Little is known about the thermodynamics of self-ionization equilibria. It appears that the extent of self-ionization is primarily related to the strength of the bridge bonds. For example, self-ionization constants for HF, HgO, and NHg are decreasing in the order HF K X > HgO K X 10 ) NHg (K x 10 ) which cor-... 

After water, liquid HF is one of the most generally useful of solvents. Indeed in some respects it surpasses water as a solvent for both inorganic and organic compounds, which often give conducting solutions as noted above it can also be used for cryoscopic measurements.333 The self-ionization equilibria in liquid HF are ... 

Unlike other self-ionization equilibria that we shall discuss, reaction 9.14 requires the separation of doubly charged ions, and on these grounds alone, the establishment of this equilibrium must be considered improbable. Its viability is also questioned by the fact that thionyl chloride, SOCI2 (the only reported acid in the solvent), does not exchange or O with the liquid SO2 solvent. Selected properties of SO2 are given in Table 9.3, and its liquid range is compared with those of other solvents in Figure 9.2. 

Association in HF, H2O and NH3 is also maintained when solvent-ions are produced such as may occur by the self-ionization equilibria, usually represented according to the equations ... 

CH3CN(HC1)5 and CH3CN(HC1)7 have been described. Since water is a stronger proton acceptor than the hydrogen halides, it forms H30+X, which are insoluble in the liquid hydrogen halides as are most other ionic compounds. The presence of self-ionization equilibria of the following type has been postulated32j... 

Self-ionization equilibria have been assumed to exist in the pure liquids. Davies and BaughanI have shown that the self-ionization in molten antimony (III) chloride is hkely to be principally due to the presence of small amounts of impurities, but the results are still at least partially in accord with the following equations, which have been regarded as due to chloride ion transfer reactions between solvent molecules ... 

The self-ionization equilibria which have been assumed to exist in the pure liquids have been considered as bromide ion transfer reactions ... 

The self-ionization equilibria of the pure solvents may be described as involving transfer of chloride ions . 

An alternative explanation has been offered as most of the reactions found in the solution can be explained without assuming the presence of the self-ionization equilibria ... 

Self ionization equilibria are known to exist in various other solvents and it may be proposed that their presence should, in principle, be admitted for any liquid system even if such ions were not detectable. The extent of the self-ionization equilibrium appears related to the strength of solvent-solvent interactions and hence to their amphoteric properties in order to mediate the cooperative effects, to the ease of heterolysis of bonds within solvent molecules and to the value of the dielectric constant. In a liquid with unfavourable conditions for the production of SMM-centres by self-ionization such as carbon tetrachloride, SMM-centres must be made available in other ways in order to provide for the existence of the molecular liquid. 

Initially, it was proposed that SO2 underwent self-ioniza-tion according to eq. 9.14. However, this equilibrium requires the separation of doubly charged ions, in contrast to the singly charged ions involved in other self-ionization equilibria described in Sections 9.6-9.11. Two observables suggest that no self-ionization occurs (i) conductance data are not consistent with the presence of ions in liquid SO2 (ii) when labelled SOCI2 is dissolved in liquid SO2, neither... 

Ionized sulfates Self-ionization equilibria and ionic 9... 

GermannIS found that aluminium trichloride reacts with carbonyl chloride to give a solution which yields KAICI4 on addition of potassium chloride. He considered aluminium chloride as a solvo-acid and potassium chloride as a solvobase, according to the following self-ionization equilibrium of the pure solvent (which is unlikely to be correct) ... 

For the pure liquid a self-ionization equilibrium has been assumed to exist according to the equation ... 

Arsenic(III) fluoride is similar in its solvent properties to iodine(V) fluoride. Association in the liquid state seems to be due to fluorine bridging. The occurrence of F-exchange has been concluded to take place from the results of n.m.r. measurementsi05. A self-ionization equilibrium is assumed to be present in the pure liquid due to autofluoridolysis ... 

Carbonyl chloride is of historical interest since it was the first oxyhalide known to behave as an ionizing solvent " . Complex formation between aluminium chloride and calcium chloride was explained by assuming a self-ionization equilibrium of the solvent molecules. 

Although you normally ignore the self-ionization of water in calculating the HsO concentration in a solution of a strong acid, the self-ionization equilibrium still exists and is responsible for a small concentration of OH ion. You can use the ion-product constant for water to calculate this concentration. As an example, calculate the concentration of OH ion in 0.10 M HCl. You substitute [H30 ] = 0.10 M into the equilibrium equation for (for 25°C). 

Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

Molten I2CI6 has been much less studied as an ionizing solvent because of the high dissociation pressure of CI2 above the melt. The appreciable electrical conductivity may well indicate an ionic self-dissociation equilibrium such as... 

We fitted the combined data with model spectra based on the atomic data compiled by Raymond and Smith (1977). The model spectra employed here are both for collisional ionization equilibrium (CIE) and non-equilibrium ionization (NEI) models with cosmic abundances (Allen, 1977). Single Te spectrum for both models can not fit the data. Two components of different Te models can reproduce the data well for both models. The physical parameters obtained with CIE models are self inconsistent because the ionization parameter r ( the electron density n X the elapsed time t the after shock heating ) is about 1011cm 3sec which is too short by an order of magnitude for the CIE condition to be reached. 

Let s see, in pure water the hydrogen ion concentration is 1.0 x 10-7 M. Okay, you say. So let s just add the 1.0 x 10"8 M from the HC1 to the 1.0 x 10-7 M from the water. But that doesn t work, because the introduction of H+ from HC1 impacts the self-ionization of water. According to Le Chatelier s principle, the position of the equilibrium will be shifted to the left, because we are adding a product. Many biological systems are coupled equilibria, so if you change one, you change them all. If you want to solve one system, you have to solve all of them simultaneously, because they are all interconnected. 

Using the equilibrium expression that you reviewed in Chapter 13, the equilibrium constant for the self-ionization of water can be expressed as ... 

The second variation is to determine either the pH or the hydrogen ion concentration of a solution when given the hydroxide ion concentration, [OH-], for the solution. To solve these problems you need to utilize the equilibrium constant expression for the self-ionization of water (Kw). This expression will allow you to convert from the hydroxide ion concentration, [OH-], to the hydrogen ion concentration, [H+], The [H+] can then be used to calculate pH if necessary. One of the free-response questions on the 1999 test required this calculation. 

In the calculations, we have omitted the self-ionization of water. Since the equilibrium concentration of hydrogen ion [H+] is so small (1.0 X 10 7), it is negligible compared to the molarity of the acetic acid. 

Ion product — A temperature-dependent constant related to pure substances that can dissociate forming ions and remain in equilibrium with them. It is the product of the ion activities raised to the stoichiometric coefficients of such ionic species in former pure substance. Since the concentration of the pure substance is practically a constant, it is not included in this equilibrium expression. Common pure substances characterized by an ion-product constant are -> amphiprotic solvents, and those salts that are partially dissolved in a given solvent. In the latter case, the ion product is synonymous with solubility product. The following table (Table 1) summarizes self-ionization ionic products and - autoprotolysis constants of some - amphiprotic solvents [i]. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

Spontaneous ionization requires both good leaving groups and that the resulting carbenium ions are sufficiently stable. For example, although primary triflates are very stable covalent species which do not self-ionize, secondary triflates with phenyl substitents are very reactive and spontaneously ionize. The ionization equilibrium of styryl triflate could not be established because of side reactions such as Friedel-Crafts alkylation [56], On the other hand, methoxymethylium triflate is partially ionized with equilibrium constants Kj = 5-10 4 at 10° C and Kt = 210 4 at -70° C in S02 [57]. In this system, ionization is endothermic. Secondary triflates with alkoxy substituents, such as those in polymerizations of vinyl ethers, are apparently more strongly ionized than their primary counterparts [58,59],... 

Increasing basicity or acidity of the solvent displaces the equilibria (3-8) and (3-10) to the right. The addition of these two equations gives a new equilibrium describing the self-ionization (autoprotolysis) of the solvent. 

As catalyst for the polymerization of THF, SbCls works by self-ionization [88] according to the equilibrium... 

As also shown in Figure 9, a pair of water molecules are in equilibrium with two ions—a hydronium ion and a hydroxide ion—in a reaction known as the self-ionization of water. 

Recall that an equilibrium-constant expression relates the concentrations of species involved in an equilibrium. The relationship for the water equilibrium is simply [H30" ][0H ] = K q. This equilibrium constant, called the self-ionization constant of water, is so important that it has a special symbol, Its value can be found from the known concentrations of the hydronium and hydroxide ions in pure water, as follows ... 

The result is a special equilibrium constant expression that applies only to the self-ionization of water. The constant, K, is called the ion product constant for water. The ion product constant for water is the value of the equilibrium constant expression for the self-ionization of water. Experiments show that in pure water at 298 K, [H+] and [OH ] are both equal to 1.0 X 10 M. Therefore, at 298 K, the value of K is 1.0 X 10 ... 

The product of [H+] and [OH ] always equals 1.0 X 10 at 298 K. This means that if the concentration of H+ ion increases, the concentration of OH ion must decrease. Similarly, an increase in the concentration of OH ion causes a decrease in the concentration of H+ ion. You can think about these changes in terms of Le ChStelier s principle, which you learned about in Chapter 18. Adding extra hydrogen ions to the self-ionization of water at equilibrium is a stress on the system. The system reacts in a way to relieve the stress. The added H+ ions react with OH ions to form more water molecules. Thus, the concentration of OH ion decreases. Example Problem 19-1 shows how you can use to calculate the concentration of either the hydrogen ion or the hydroxide ion if you know the concentration of the other ion. 

Water itself is ionized to a very small extent (equation 6.1) and the value of the self-ionization constant, (equation 6.2), shows that the equilibrium lies well to the left-hand side. The self-ionization in equation 6.1 is also called autoprotolysis. 

As we have already mentioned, liquid NH3 undergoes self-ionization (equation 8.12), and the small value of Aijeif (Table 8.4) indicates that the equilibrium lies far over to the left-hand side. The [NH4] and [NH2] ions have ionic mobilities approximately equal to those of alkali metal and halide ions. This contrasts with the situation in water, in which [H30]+ and [OH] are much more mobile than other singly charged ions. 

However, a competing equilibrium is established which arises from the self-ionization process of H2SO4 described in part (a) ... 

Before we discuss the next major definition of acid-base behavior, let s examine a crucial property of water that enables us to quantify [H30 J in any aqueous system water is an extremely weak electrolyte. The electrical conductivity of tap water is due almost entirely to dissolved ions, but even water that has been repeatedly distilled and deionized exhibits a tiny conductance. The reason is that water itself dissociates into ions very slightly in an equilibrium process known as autoionization (or self-ionization) ... 

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