Self-Ionization of Water and the pH Scale

Is it appropriate to describe each of the following as a conjugate acid-base pair Explain, (a) HCO3- and COs - (b) HSOs and SO42- (c) H2CO3 and H2C2O4  

In Section 16-1, we learned that the H2O molecule can act as either an acid (reaction 16.2) or a base (reaction 16.1) it is amphiprotic. It should come as no surprise that amongst themselves water molecules can produce HgO and OH ions via the following self-ionization reaction or autoionization reaction  

In this reaction, one water molecule acts as an acid and the other acts as a base. 

even when it is pure, water contains H3O and OH ions, although the concentrations are very low. The presence of these ions can be detect, however, by using precise electrical conductivity measurements. The fact that the self-ionization of water produces very low concentrations of HjO and OH ions is an indication that the equilibrium constant for reaction (16.3) is small, ranging from about 1.14 X 10 at 0 °C to about 5.45 X 10 at 100 °C. The thermodynamic equilibrium constant for reaction (16.3) is defined in terms of activities (see Section 15-1). It is given the symbol and is called the ion product of water. Of course, equilibrium values of [H3O ] and [OH ] must be used in the expression for K.  

In simplifying the expression above, we used the fact that a = 1 for H20(l) and c° = 1 mol/L = 1 M. It is common practice to write the expression for without the units included, [H30 ][0H ], and to substitute only the numerical values of [HgO ] and [OH ] without units into this expression. 

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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. 

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