Autoionization, water

Every aqueous solution contains hydronium (H30+) and hydroxide (OH-) ions as a result of the water autoionization process,... 

The most common hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water autoionizes into negative hydroxyl ions and hydrogen ions. The salt breaks down into positive and... 

Unless conditions require the use of the exact solution, approximate equations are preferable because they are easier to apply and provide greater physical insight. If a calculation (ignoring water autoionization) of the ionization of a weak acid gives a concentration of H30 smaller than 10 M or if a calculation of base ionization gives a concentration of OH smaller than M, then we have to use the more exact treatment. For buffer solutions, a pH near 7 does not necessarily mean that water ionization is important, unless the acid or base concentration becomes very small. 

When a strong base, such as NaOH, is added to the CH3COOH-NaCH3COO buffer solution, it is consumed by the acidic component, CH3COOH. This occurs in the following way. The additional OH causes the water autoionization reaction to proceed to the left. 

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

The Cao model was used to predict the spatial and temporal distributions of four ionic species—Pb", NO3, H+, and OH—in kaolinite clay. Four one-dimensional partial differential equations (Equation 2.11) are developed, one for each ionic species. Chemical reactions, including production of lead hydroxide, water autoionization, and lead sorption, were described in a set of algebraic equations. Electrical potential, conductivity, and field strength were evaluated using previously presented equations. The boundary conditions applied at the inlet and outlet of the soil column maintained the equality between the fiux of solute at the inside and immediately outside of the column. 

The one-dimensional mass transport equation of Off includes two additional reaction terms, where denotes the concentration of Off" participating in water autoionization reaction, and represents the part that is consumed in reaction with lead. Equation 2.29 presents lead hydroxide precipitation/dissolution reaction in aqueous phase, pff is the major factor that influences the reaction direction and production rate. The equilibrium requires 2 mol of Off- per 1 mol of Pb" to generate 1 mol of Pb(Off)2. This means that the change in hydroxyl molar concentration due to the precipitation reaction is twice as the change in Pb+ s molar concentration. 

Pure water autoionizes to a small extent In a process whose equilibrium constant is the ion-product constant for water, K,/ (1.0x10 at 25°C). 

In pure water, autoionization is the only source of H30 and OH , and the stoichiometiy of the reaction tells us that their concentrations are equal. At 25 °C, the concentrations of hydronium and hydroxide ions in pure water are [H30 ] = [OH ] = 1.0 X 10 M. Using the equilibrium expression, we can calculate the value of at 25°C as follows ... 

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

Kw is also widely called the autoionization constant and sometimes the ion product constant of water. 

The OH from the NaOH has caused the autoionization reaction of water to shift to the left, yielding only a tiny fraction of the H, 04 that is present in pure water. 

Kw ionization constant for the autoionization of water. kelvln the unit of the Kelvin temperature scale. 

Following Platzman (1967), Magee and Mozumder (1973) estimate the total ionization yield in water vapor as 3.48. The yield of superexcited states that do not autoionize in the gas phase is 0.92. Assuming that all of these did autoion-ize in the liquid, we would get 4.4 as the total ionization yield. This figure is within the experimental limits of eh yield at 100 ps, but it is less than the total experimental ionization yield by about 1. The assumption of lower ionization potential in the liquid does not remove this difficulty, as the total yield of excited states in the gas phase below the ionization limit is only 0.54. 

It has long been known that some autoionization occurs in water, and it was presumed that nonaqueous solvents behaved in a similar way. Although the reaction of sodium hydride with 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 conductivity of liquid ammonia is sufficiently high to indicate a very slight degree of autoionization. In order for ions to be produced, something must be transferred from one molecule to another, and in solvents such as water or ammonia it is proton transfer that occurs. Accordingly, the ionization of liquid ammonia can be shown as... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

A solvent that resembles water in many ways is liquid hydrogen fluoride. The molecule is polar, there is some autoionization, and it is a fairly good solvent for numerous ionic solids. Although the boiling point of liquid HF is rather low (19.5 °C), it has a liquid range that is comparable to that of water, partially as a result of extensive hydrogen bonding. One of the problems associated with the use of liquid... 

Be(OH)2 produces less hydroxide ion than the autoionization of water. 

Water undergoes autoionization to a small extent the ionization constant at 25°C is 1.008x10-14 ... 

The positive ion is a hydrated hydrogen ion and the negative ion is a water molecule minus one hydrogen ion. The water molecules act equally as acids and bases this type of behaviour is termed amphiprotic. The extent of the autoionization is very slight. The autoprotolysis constant,... 

If water molecules interact with each other as an acid and a base, autoprotolysis (autoionization) occurs, as in Eq. (3.7) ... 

The autoprotolysis constant (autoionization constant) of water, Kw = q(l l jO )q(OI1 ) ss [H30+][0H, is often called the ion product constant of water. The value of -logKw varies with temperature as in Table 3.2. In water, there exists the following relation between K l of an acid and Kb of its conjugate base ... 

Many solvents autoionize with the formation of a cationic and an anionic species as does water... 

The physical properties of sulfuric acid are listed in Table 10.3. The dielectric constant is even higher than that of water, making it a good solvent for ionic substances and leading to extensive autoionization. The high viscosity, some 25 times that of water, introduces experimental difficulties Solutes are slow to dissolve and slow to crystallize. It is also difficult to remove adhering solvent from crystallized materials. Furthermore, solvent that has not drained from prepared crystals is not reudily removed by evaporation because of the very low vapor pressure of sulfuric acid... 

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