Identifying Acids, Bases, and Salts.

You can't get anywhere in an acid/base problems if you can't identify what any of the compounds are. Which are weak acids, which are basic salts, etc...

Here some helpful guidelines to work through in identifying compounds. Rather than simply memorize them, use these ideas to help you think through the identification process.

First, you need to memorize the strong acids and bases. This is key. It will not only allow you to identify the strong acids and bases, but it will help you figure out which ions are simply spectator ions.

Second, you should realize that history has played a role in naming compounds. Therefore there are some compounds we typically think of as acids and some compounds we think of bases. However, every acid has a conjugate base and every base a conjugate acid. So we really have two forms of each compound: one protonated (the acid) and one deprontonated (the base).

How can they be identified based on their names and the chemical structures?

Names of acids are typically "something"-acid. As such they are readily identified. Weaks acids are the acids that are not strong. Memorize the strong acids and you'll know everything else is weak. For example, the following are all weak acids: lactic acid, benzoic acid, oxalic acid, trichloroacetic acid, ...The chemical formula for the acids depends on the type of acid (hydroacid, oxoacid, or carboxylic acid). In particular you want to be able to recognize the carboxylic acid group that is found in many organic molecules. It is an oxygen double-bonded to a carbon with an OH group on the same carbon. This is denoted by RCOOH, where the R is a generic representation of the rest of the molecule. For example, formic acid is HCOOH (R = H), acetic acid is CH3COOH (R = CH3).

These weak acids are all compounds that are "uncharged" in their protonated state. If we neutralize them with an strong base we will end up with a salt composed of a cation and an anion. The anion is the conjugate base of the acid. This is an important clue to the identity of the species. If the acidic compound is not charged once you remove H+ from the compound you will form an anion. Where there is an anion there is always a cation to balance the charge. The cation will be a spectator. Thus the basic salts will be composed of a spectator ion like sodium, Na+, potassium, K+ or something like that and an anion that is the conjugate base of the acid. They can be identified as they are the same compound as the weak acid but they are "missing" a H+. Thus F- is the conjugate base of HF. NaF is a basic salt. K(CH3COO), potassium acetate, is a basic salt that contains the acetate anion that is the deprotantated form of acetic acid. Their names are also helpful in identification. The names of the anions often end in "ate". The benzoate ion is deprotonated benzoic acid. The formate ion is the conjugate base of formic acid.

Weak bases are typically compounds that are like substituted ammonia, NH3. In these compounds the H's have been replaced with something else (like a carbon chain). For example, methyl amine, is CH3NH2. One H has been replaced with a CH3. There is a near endless list of such compounds. As a class, they are called "amines". Thus "something something" amine is identifiable as a weak base. What about their conjugate acids? Since the amines have no charge, if they accept a proton they will add H+. This means their conjugated acids will be positively charged and will form salts with spectator anions. Therefore something like methylammonium chloride (CH3NH3)Cl can be identified as an acidic salt.

What about salts that combine a basic anion and an acidic cation? Consider for example, ammoniumfluoride (NH4F). Here the ammonium ion (NH4+) is an acid and the fluoride ion (F-) is a base. These "mixed" salts are best thought of simply as either acids or bases.

There are also compounds with more than one acidic proton (polyprotic species). These will form anions that can be both acids and bases. These species are called, amphiprotic. They can be identified as anions that still have acidic protons. For example, HCO3-. This is carbonic acid, H2CO3 that has lost one proton. It is both an acid and base. This will be covered more in detail with polyprotic acids.

Finally, there are a few exceptions to these ideas. First there are neutral salts, like NaCl. In this case, both the cation and the anion are the conjugate pair of a strong acid and strong base. As such, a solution of these ions will be neutral. If you have memorized the strong acids and strong bases, these can quickly be identified. Second, there are a number of insoluble hydroxides, like Al(OH)3 that are weak simply since they don't dissolve in water. They don't really have a conjugate acid. They are weak simply as a result of solubility.

What is the best way to learn to identify compounds? Practice. There are two worksheets on identifying acids and bases and salts on the worksheet page.

Concept Question

The salt NaCN is
(mouse over choices to get answer)

  1. acidic
  2. basic
  3. neutral

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