Consider the compounds H2O, NaOH, and CH3CO2H. All of them contain an OH group, yet one of them is an acid, one of them is a base, and the other is neither. How can you tell what the acid/base properties of a molecule that contains an OH group are going to be?
Differentiating acids from bases is easy. Everyone knows that sodium hydroxide, NaOH, is a base. If a compound that contains an OH group also contains a metal ion, then it is a base. The presence of the metal ion indicates that the compound contains ionic bonds. Thus, to whatever extent this compound dissolves in water, the metal ion will dissociate from the OH group. The formation of "free" OH groups, i.e. hydroxide ions, results in a solution that is basic. Compounds that contain "pseudo metal" cations such as ammonium ion, NH4+, bonded to an OH group are also basic.
If the compound in question does not contain a metal ion, i.e. if it contains only covalent bonds, then it is either acidic or non-acidic. There are no molecules that contain only covalent bonds that dissociate in water to yield hydroxide ions! As you know, water, HO-H, is non-acidic. It is the standard by which we measure acidity: aqueous solutions that contain concentrations of hydroxide ion greater than that in water are basic, while solutions in which the concentration of hydronium ion is larger than that in water are acidic. The first thing that you have to be able to do to answer this question is to draw a correct Lewis structure for the compound in question. Once you've done that, you determine if the oxygen atom of the OH group is bonded to an atom that is doubly bonded to another atom. In other words, you look for this fragment:
The following table gives one example of a neutral compound, one of a basic compound, and two examples of acidic compounds. It also lists the melting and boiling points of these materials for comparison purposes. Do you see any correlation between the m.p./b.p. data and the statements in boldface type above?
|
Formula |
Lewis Structure |
m.p./b.p. |
Comments |
This compound is... |
|---|---|---|---|---|
|
H2O |
 |
0/100 |
No metals, no multiple bond |
neutral |
|
NaOH |
 |
318/1390 |
Contains a metal, ionic bonds |
basic |
|
CH3CO2H |
 |
16/117 |
No metals, (1) C=O double bond |
acidic |
|
H3PO4 |
 |
42/213 |
No metals, (1) P=O double bond |
acidic |
The next page shows a similar table that you can use to practice recognizing an acid or a base when you see one.