|Lecture Notes: 3 December||
What are acids and bases? There are three major definitions. We will look at two in which the proton is a major defining component (the third, Lewis definition, is not needed for our study).
(Although I will signify protons in water as H+, you should realize that naked protons do not exist in water - they are always hydrated. At a minimum we see the hydronium ion, H3O+. But hydronium ion is in fact also generally thought to be hydrated, so you will sometimes see hydrogen ion represented as H5O2+, H7O3+, etc.)
- Acids release protons (H+) into water.
- Bases release hydroxide ions (OH-) into water.
- This is very limited - it only deals with acids and bases in water, and many substances which chemists and others think of as bases (such as ammonia) don't fit the definition. Thus we will focus on the Brønsted-Lowry definition (Brønsted definition in abbreviation):
- Acids are proton donors.
- Bases are proton acceptors.
- Note that there is no restriction as to solvent, and many substances besides hydroxide ion can contribute basicity.
- A consequence of the Brønsted definition is that all acids and bases are related to one or more conjugate bases or acids. That is, when an acid dissociates to give a proton, it also generates a conjugate base which can react with (accept) a proton in the reverse reaction. For example, in the case of water:
H2O H+ + OH- acid conj. base H+ + OH- H2O base conj. acid H3O+ H+ + H2O OH- + H+
These terms have nothing to do with concentration, rather they refer to the degree of dissociation of an acid or base:
The concentration of hydronium ion in water is extremely influential on all kinds of chemistry. The range of hydronium ion concentration in water is also vast, with extremes of about 10M to about 10-15M, and commonly ranging from 1M - 10-14M. Imagine plotting [H3O+] vs. volume of acid added to a base solution in a titration. If you had one cm on the graph paper = 10-14M, then you would need a piece of paper 109 km long (greater than the distance from the Sun to Jupiter) to plot this titration! Obviously a more convenient measure is needed. This is easily accomplished by looking instead at the logarithm of [H+] and defining a new term,
Turns out that the concentration of hydrogen ion in water is related to the concentration of hydroxide ion due to the equilibrium dissociation of water:
Let's look at pH a bit:
Note that the "p" has the more general meaning of "-log". Thus pOH is -log [OH-], pCa = -log [Ca2+], etc.
© R A Paselk
Last modified 3 December 2009