Self-ionization water

The neutral pH of 7 is based on the fact that water self-ionizes to form hydronium (hydrogen) ions and hydroxide ions in equal amounts. The concentration of these ions is equal to 1.0 X 10 7, which corresponds to a pH of 7. 

A Bronsted acid is a proton donor, and a Bronsted base accepts protons. In aqueous solution, [H3O] is formed and in bulk water, self-ionization corresponds to the transfer of a proton from one solvent molecule to another (equation 8.11) illustrating amphoteric behaviour (see Section 6.8). 

Pure water self-ionizes slightly to form HgO and OH ions, as shown in this equation. 

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... 

In contrast to this, consider next a solution of sodium acetate. From vSec. 09 we know that in such a solution the thermal agitation raises a certain number of protons from the solvent molecules to the vacant proton levels of the (CH GOO) ions. In the aqueous solution of such a salt, this process is known as the hydrolysis of the salt and is traditionally regarded as a result of the self-ionization of the water. In Fig. 36, however, it is clear that in the proton transfer... 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

The activity of the solvent molecule HS in a single-component solvent is constant and is included in Kus. The concentration of ions is mostly quite low. For example, self-ionization occurs in water according to the equation 2H20— H30+ + OH". The conductivity of pure water at 18°C is only 3.8 X 10"8 Q"1 cm-1, yielding a degree of self-ionization of 1.4xl0"19. Thus, one H30+ or OH" ion is present for every 7.2 x 108 molecules of water. Some values of Kus are listed in Table 1.5 and the temperature dependence of the ion product of water Kw is given in Table 1.6. 

The evidence for this self-ionization is the conductivity of the anhydride which, though low, exceeds that of acetic acid.204 208 The ionization of acetic anhydride into acetylium and acetate ions is analogous to the ionization of water molecules into protons and... 

Kv is the self-ionization constant for water (Table 3.2) and equation (3.18) reflects the not surprising inverse relation between Ka and Kh. It is only when Ka and Kv for a compound are of different magnitudes that it may be classified as an acid or a base. An example which is difficult to classify is hypoiodous acid (HOI) where K = 2.5 x lO11 mol dm 3 andKh = 3.2 x 10 10 mol dm3. Although Kb has been widely used in the past, it is a quantity which is largely redundant, for Ka (or pKa) may be used to express the strength of bases as well as acids, see Table 3.3. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

Acting as an acid pKa of H2O Water is a very weak acid and can undergo self-ionization as follows ... 

Pure water spontaneously self-ionizes to a small degree, leading to equal concentrations of hydrogen and hydroxide ions (1 x 10 mol/L, or 1 x 10 M) ... 

That the self-ionization of water is slight and is consistent with the poor electrical conductance of the liquid... 

As an illustration of the effects of temperature, pressure, and ionic strength on ionic equilibria in solution, we can consider the self-ionization (autoprotolysis) of water ... 

Water undergoes self-ionization, called autoprotolysis, in which it acts as both an acid and a base ... 

The solvents that are leveling to both acids and bases are self-ionized solvents, e.g., water, ammonia, alcohols, carboxylic acids, nitric... 

So, the pH of pure water is 7.00 because that is the negative logarithm of the hydrogen ion concentration that is generated by the self ionization of water. It is a value based on the inherent acid-base properties of water. 

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. 

What role does the self-ionization of water play in acid-base chemistry ... 

The p-Function 232 Self-Ionization of Water 234 Relationship Between pH and pOH 235... 

A sample of pure water will contain a small quantity of ions (H+ and OH-) produced from the self-ionization of water. These ions exist only for a brief time period before rejoining to form water molecules. At any given moment, only a very small proportion of the sample of water exists as ions (only about one of every billion particles). The following equation describes this process ... 

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