Equilibrium autoionization

The Bronsted Theory Acid-Base Equilibrium Autoionization of Water pH... 

A water molecule has amphoteric character. This means it can act as both an acid and a base. The autoionization equilibrium process in water. 

One of the primary pieces of evidence that water undergoes some autoionization comes from the conductivity of pure water. The equilibrium can be written as... 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

Investigations on the doubly excited states of two electron systems under weakly coupled plasma have been performed by several authors. Such states usually occur as resonance states in electron atom collisions and are usually autoionizing [225]. Many of these states appear in solar flare and corona [226,227] and contribute significantly to the excitation cross-sections required to determine the rate coefficients for transitions between ionic states in a high temperature plasma. These are particularly important for dielectronic recombination processes which occur in low density high temperature plasma, occurring e.g. in solar corona. Coronal equilibrium is usually guided by the balance between the rates of different ionization and... 

The ion-product constant for water, Kw is the equilibrium constant for the dissociation of H20. Water molecules may donate protons to other water molecules in a process known as autoionization ... 

The pre-dissociation results from a non-adiabatic coupling between electronic states (NA v) and (N A ec) at internuclear distances (usually smaller than the equilibrium distance) that correspond to the vibrational continuum of N A state, or when the N A state is a purely dissociative one. Autoionization and pre-dissociation probabilities for Npa(v) and Npir(v)(N > 4) states have been calculated in [68], while for the states with N < 3 they have been evaluated from experimental measurements [69,70]. Auto-ionization and pre-dissociation rates are very high ( 108s 1 for N = 3,4), and rapidly increase with increasing v. More experimental or theoretical information is needed for these processes. 

This reaction, which is an example of autoionization, proceeds to the right to only a very tiny extent because water is such a weak acid and base. To write an equilibrium constant expression for this reaction, we again consider the concentration of water itself to be a constant, which we incorporate into the value of K. We get another type of constant, called the water ionization constant and denoted K. ... 

But is autoionization an important source of H+ ions In pure water at 2S°C, [H+] is 10-7 M. In 1.0 M HCl the water will produce even less than 10-7 M H+, since by Le Chatelier s principle the H+ from the dissociated HCl will drive the position of the water equilibrium to the left. Thus the amount of H+ contributed by water is negligible compared with the 1.0 M H+ from the dissociation of HCl. Therefore we can say that [H+] in the solution is 1.0 M and that... 

This reaction is responsible for the autoionization of water, which leads to small but measurable concentrations of hydronium and hydroxide ions at equilibrium. The equilibrium expression for this reaction is... 

To calculate the amounts of H30, A, and HA at equilibrium, we use the methods of Chapter 14, with partial pressures replaced by concentrations. A new feature here is that one of the products (H30 ) can also come from a second source, the autoionization of the solvent, water. In most of the applications we study, this second effect is small and can be neglected in the equations. Even so, it is a good idea to verify at the end of each calculation that the [H30 ] from the acid ionization alone exceeds 10 M by at least one order of magnitude. Otherwise, we have to use the more complete method of analysis given in Section 15.8. 

Set up the table of the changes in concentrations that occur as the reaction goes to equilibrium, neglecting the small contribution to [OH ] from the autoionization of water ... 

This is called the autoionization of water. Kw is the equilibrium constant for this reaction. 

In the study of acid-base reactions in aqueous solutions, the hydrogen ion concentration is key, because it indicates the acidity or basicity of the solution. Expressing the proton as rather than H30, we can write the equilibrium constant for the autoionization of water, Equation (15.1), as... 

However, the cases considered are the exceptions rather than the rule. Practical use of the solvosystem concept is laboured since for the systems similar to equation (1.1.19) identification of the ions formed by autoionization and measurement of their equilibrium concentrations are very difficult. Also, the systems mentioned have low dielectric constants that create additional obstacles for the investigations the first being the incomplete dissociation and formation of ionic associates even in diluted solutions. For example, it is known that in liquid sulfur dioxide the following acid-base interaction takes place [26] ... 

In order to generalize the definition of the solvent system for the case of ionic media, we shall analyse Franklin s definition. First, it should be noted that the term auto-ionization in this definition should be substituted by auto-dissociation or intrinsic acid-base equilibrium of the solvent , as a more common case of heterolytic break down of the constituent particles of a liquid. Indeed, for molecular solvents or those which are slightly ionized at room temperature, the terms autoionization and intrinsic acid-base equilibrium of the solvent , relate to the same process, whereas for ionic liquids they differ considerably. For example, although sodium nitrate (NaNOs) is subject to practically... 

EXAMPLE 3.14 In the Study of the equilibrium of a weak acid in water, the ionization of the water is generally neglected. If we cannot do this, we have the equilibrium equation for the autoionization of water... 

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) ... 

Like any equilibrium process, the autoionization of water is described quantitatively by an equilibrium constant ... 

Both ions are present in all aqueous systems. Thus, all acidic solutions contain a low concentration of OH ions, and all basic solutions contain a low concentration of H30 ions. The equilibrium nature of autoionization allows us to define acidic and basic solutions in terms of relative magnitudes of [H30" ] and [OH ] ... 

The sum of the two dissociation reactions is the autoionization of water. Recall from Chapter 17 that, for a reaction that is the sum of two or more reactions, the overall equilibrium constant is the product of the individual equilibrium constants. Therefore, writing the expressions for each reaction gives... 

The equilibrium for the autoionization of water is given by Kw, the ion product constant of water. At 25... 

The added OH from the salt has shifted the water autoionization equilibrium... 

Several different subscripts are used with the equilibrium constant When all reactants or products that appear in the equilibrium expression are gases, and pressures are used instead of molarity, the symbol for the equilibrium constant is Kp. If weak acids and bases are under investigation, the equilibrium constant symbol is written as K or K,. When the equilibrium constant refers to the autoionization of water, the symbol K, is used. Solubility product equilibria express the ion concentrations of partially soluble salts. The equilibrium constant symbol K p is used for solubility product equilibria. ... 

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