| << Chapter < Page | Chapter >> Page > |
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.
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 defined as:
| C 0 (M) | [OH – ] | K b | pH |
|---|---|---|---|
| 0.50 | 3.2 × 10 –3 | 2.0 × 10 –5 | 11.5 |
| 0.20 | 2.0 × 10 –3 | 2.0 × 10 –5 | 11.3 |
| 0.10 | 1.4 × 10 –3 | 2.0 × 10 –5 | 11.1 |
| 0.050 | 9.7 × 10 –4 | 1.9 × 10 –5 | 11.0 |
| 0.020 | 6.0 × 10 –4 | 1.9 × 10 –5 | 10.8 |
| 0.010 | 4.2 × 10 –4 | 1.9 × 10 –5 | 10.6 |
| 0.005 | 3.0 × 10 –4 | 1.9 × 10 –5 | 10.5 |
| 0.001 | 1.3 × 10 –4 | 1.8 × 10 –5 > | 10.1 |
| 0.0005 | 8.7 × 10 –5 | 1.8 × 10 –5 | 9.9 |
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.
Notification Switch
Would you like to follow the 'Ucd bis2a intro to biology v1.2' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|