0.9 Bis2a 02.appendix ii detailed look at acid-base equilibrium  (Page 5/8)

We consider a final note on the autoionization of water. The pH of pure water is 7 at 25°C. Adding any acidto pure water, no matter how weak the acid, must increase [H ₃ O ⁺ ],thus producing a pH below 7. As such, we can conclude that, for all acid solutions, pH is less than 7, or on the other hand, anysolution with pH less than 7 is acidic.

Observation 4: base ionization, neutralization and hydrolysis of salts

We have not yet examined the behavior of base molecules in solution, nor have we compared the relative strengthsof bases. We have defined a base molecule as one which accepts a positive hydrogen ion from another molecule. One of the most commonexamples is ammonia, NH ₃ . When ammonia is dissolved in aqueous solution, the followingreaction occurs:

Due to the lone pair of electrons on the highly electronegative N atom,NH ₃ molecules will readily attach a free hydrogen ion forming theammonium ion NH ₄ ⁺ . When we measure the concentration ofOH ^– for various initial concentration of NH ₃ in water, we observe the results in [link] . We should anticipate that a base ionization equilibrium constant might exist comparable to the acidionization equilibrium constant, and in [link] , we have also calculated the value of the function $K_b$ defined as:

Given that we have dissolved a base in pure water, we might be surprised to discover the presence of positivehydrogen ions, H ₃ O ⁺ , in solution, but a measurement of the pH for each of the solutionsreveals small amounts. The pH for each solution is also listed in [link] . The source of these H ₃ O ⁺ ions must be the autoionization of water. Note, however, that in each case in basic solution, the concentration ofH ₃ O ⁺ ions is less than that in pure water. Hence, the presence of the base in solution has suppressed the autoionization. Because ofthis, in each case the pH of a basic solution is greater than 7.

Base ionization is therefore quite analogous to acid ionization observed earlier. We now consider a comparisonof the strength of an acid to the strength of a base. To do so, we consider a class of reactions called "neutralizationreactions" which occur when we mix an acid solution with a base solution. Since the acid donates protons and the base acceptsprotons, we might expect, when mixing acid and base, to achieve a solution which is no longer acidic or basic. For example, if we mixtogether equal volumes of 0.1M HCl(aq)and 0.1M NaOH(aq),the following reaction occurs:

The resultant solution is simply a salt solution withNaCl dissolved in water. This solution has neither acidic nor basicproperties, and the pH is 7; hence the acid and base have neutralized each other. In this case, we have mixed together astrong acid with a strong base. Since both are strong and since we mixed equal molar quantities of each, the neutralization reactionis essentially complete.