Self-ionization of water

Before leaving a discussion of the properties of water and aqueous solutions, we should discuss its self-ionization. Recall that the Bronsted-Lowry definition of acids and bases is in terms of proton donors and acceptors, respectively. Water is an amphoteric substance that is, it can act as either an acid or a base as shown in Equation (11.7)  

Equation (11.7) has an equilibrium constant (often represented as K ) of approximately 1.0 X 10 (at 298 K), and a simple acid-base equilibrium calculation yields the result that, in pure water, [H30 ] = [OH ] = 1.0 X lO M. It follows that the pH (-log[H30+] = -log[H ]) ofwater is 7.00 at room temperature. Equation (11.7), then, is the basis of the pH scale as commonly presented in general chemistry. Any substance that raises the concentration of H30 ions produces a pH less than 7 and is an acid. Any substance that lowers the concentration of the hydronium ion or raises the concentration of the hydroxide ion produces a pH greater than 7 and is a base. Note that the small value of indicates that the self-ionization process occurs only to a very small extent. Another way to appreciate just how few hydronium ions there are in pure water is to realize that the concentration of water molecules in pure water is 55.6 M therefore, for every hydronium ion, there are (55.6/1.0 X 10 =) 556 X 10, or 556 million water molecules. 

You may have seen Equation (11.7) in a slightly different form. Sometimes the self-ionization ofwater is represented as shown in Equation (11.8)  

The water molecules labeled (i) can sometimes be isolated with the hydronium ion to form cations of formula H9OJ 

Consider the adds where Y is Cl. The Cl atom becomes effectively more electronegative as more 0 atoms are attached to it. As a result, the 0 atom bonded to the H atom becomes effectively more electronegative, so attracts electrons more easily the H atom then becomes more acidic.Thus, HCIO4 has the greatest acid strength of this series of oxoacids. 

We conclude that the acid strength of a polyprotic acid and its anions decreases with increasing negative charge (see Table 16.2). 

In aqueous solutions, two ions have dominant roles. These ions, the hydronium ion, H30 (or hydrogen ion, H ), and the hydroxide ion, (OH ), are available in any aqueous solution as a result of the self-ionization of water, a reaction of water with itself, which we will describe in the next section. This will also give us some background to acid-base equilibrium calculations, which we will discuss in Chapter 17. 

Although pure water is often considered a nonelectrolyte (nonconductor of electricity), precise measurements do show a very small conduction. This conduction results from self-ionization (or autoionization), a reaction in which two like molecules react to give ions. In the case of water, a proton from one H2O molecule is transferred to another H2O molecule, leaving behind an OH ion and forming a hydronium ion, H30 ( 3 ). 

You can see the slight extent to which the self-ionization of water occurs by noting the small value of its equilibrium constant Kc. 

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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... 

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 ... 

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

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 ... 

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. 

Perhaps you recognize this equation from the beginning of the chapter. If you don t, it is the equation for the self-ionization of water, from which Kw was derived. What you ve now seen are three equations. The first is an equation that would be used to calculate the Ka of acetic acid. The second is the equation used to calculate Kh for the conjugate base. The third, which is derived from these two, is the formula for calculating Kw. There is a very clear relationship between Ka, Ku, and Kw. The three constants are all related in Equation 14.9, shown below ... 

Pitzer, K. S., Self-ionization of water at high-temperature and the thermodynamic properties of the ions. J. Phys. Chem. 86, 4704 708 (1982). 

Self-ionization of Water In all aqueous solutions the autoprotolysis... 

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. 

Explain the relationship between the self-ionization of water and... 

The hydronium ion is a hydrated hydrogen ion, which means that a water molecule is attached to a hydrogen ion by a covalent bond. However, the symbols H+ and H3O+ can be used interchangeably in chemical equations to represent a hydrogen ion in aqueous solution. Thus, a simplified version of the equation for the self-ionization of water is... 

Water not only serves as the solvent in solutions of acids and bases, it also plays a role in the formation of the ions. In aqueous solutions of acids and bases, water sometimes acts as an acid and sometimes as a base. You can think of the self-ionization of water as an example of water assuming the role of an acid and a base in the same reaction. 

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. 

In this way, two neutral water molecules can react and produce a positively charged hydronium (H30+) ion and a negatively charged hydroxide (OH ) ion. This type of reaction, called the self-ionization of water, is actually a relatively rare occurrence, but it does happen. You may have measured this occurrence in the laboratory, if you ever measured the pH of pure water. Pure water at 25 °C has a pH of 7, which means that the concentration of hydronium ions, [H30+] is 1.0 X 10 7 moles/liter (remember molarity ), or 1.0 X 10 7 M. This pure water, with a pH of 7, is said to be neutral, because it has the same number of hydronium ions and hydroxide ions, which makes sense, because of the chemical equation shown preceding this paragraph. 

This autoionization (self-ionization) of water is an acid-base reaction according to the Bronsted-Lowry theory. One H2O molecule (the acid) donates a proton to another H2O molecule (the base). The H2O molecule that donates a proton becomes an OH ion, the conjugate base of water. The H2O molecule that accepts a proton becomes an H3O+ ion. Examination of the reverse reaction (right to left) shows that H3O+ (an acid) donates a proton to OH (a base) to form two H2O molecules. One H2O molecule behaves as an acid and the other acts as a base in the autoionization of water. Water is said to be amphiprotic that is, H2O molecules can both donate and accept protons. 

Using the equilibrium expression that you reviewed in Chapter 12, the equilibrium constant for the self-ionization of water can be expressed as... 

Storage of reducing power an electron from chlorophyll may be captured by a hydrogen ion (H+), produced from the self-ionization of water, to yield a hydrogen atom, which is immediately taken up by... 

NADP (nicotinamide adenine dinucleotide phosphate), storing the reducing power in the form of NADPH. The chlorophyll ion can regain an electron from a hydroxyl ion (OH ), which is also formed during the self-ionization of water, and the resulting hydroxyl radical (OH-, which has no charge) combines with others to form oxygen and water. 

This process is the autoionization, or self-ionization, of water. Water is therefore a very weak electrolyte and a very poor conductor of electricity. Water has both acid and base properties dissociation produces both the hydronium and hydroxide ion. 

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