Introduction to Acids and Bases

Acids

Description:

According to the Arrhenius theory, acids are substances that react with water to form an aqueous hydronium ion, H⁺¹(aq). These compounds have characteristics in common with both ionic and molecular compounds. They are studied extensively in General Chemistry, in part, because they are useful compounds and, in part, because they are very good examples of equilibria, the topic studied for about a third of the year of a typical General Chemistry course. Acids are water soluble, they are electrolytes, and concentrated solutions are corrosive.

Identification:

There is more than one way to define an acid. Unfortunately, compounds that fit one definition might not be considered acids by another definition. In the broadest and most general sense, acids are compounds similar to ionic compounds but with H⁺¹ as their positive ions. This will be written first in the formula. Acid names end with the word ‘acid’ making them easy to identify from the name. In the laboratory, acids are identified by their pH (lower than 7) using a pH meter or a chemical indicator such as litmus paper or phenolphthalein.

Nomenclature:

There are two categories of acids: binary acids and oxyacids.

Binary Acids: these compounds contain hydrogen and one other non-metal. To name a binary acid, identify the other element. Use that element’s name as the root in the format ‘hydro____ic acid.’ For example, HBr is hydrobromic acid and H₂Se is hydroselenic acid.

Oxyacids: these compounds contain hydrogen, oxygen, and at least one other element. The non-hydrogen part of the compound will be an anion, usually from the list in the ionic nomenclature section above. To name an oxyacid, identify the name of the anion. If the ending of that ion is –ate, change it to –ic. If the ending of that ion is ite, change it to –ous. End the name with the word ‘acid.’ For example, H₂SO₄ is sulfuric acid, and HNO₂ is nitrous acid. Note that these names do not include the prefix ‘hydro’ like the binary acids.

Formulae:

When given the name of an acid, you must first identify the negative ion. If the acid name ends in ‘-ic’ the negative ion is a simple, monatomic ion and you can determine its charge from its location on the periodic table. If the acid name ends in –ic, it contains a negative ion whose name ends in –ate. If the acid name ends in –ous, it contains a negative ion whose name ends in –ite. Write H⁺¹ as the positive ion followed by the negative ion. Use subscripts to make the compound neutral. For example, hydrochloric acid is HCl, chromic acid is H₂CrO₄, and hypoclorous acid is HClO. Note that this last example contains the hypochlorite ion rather than having a misspelled binary acid prefix.

Concept Check: What is the name of the acid with the formula H₃PO₄? What is the formula for acetic acid?

Answer: Phosphoric acid is the name of the acid with the formula H₃PO₄. Acetic acid has a formula of HC₂H₃O₂.

Bases

Description

Bases, also called alkali, are substances that chemically combine with acids to cancel their acidity, most often by converting them to water. A base is the chemical opposite of an acid. Like acids, bases are water soluble, electrolytes, and concentrated solutions of strong bases are corrosive, especially to eye tissue.

Identification

In the laboratory, bases are identified by pH (greater than 7).

The most common laboratory bases are aqueous solutions of ionic compounds that have a group 1 or 2 cation and hydroxide, carbonate or hydrogen carbonate as the anion. These are named the same as any ionic compound. Examples of common bases are sodium hydroxide, NaOH, and calcium carbonate, CaCO₃. Another common base is an aqueous solution of the binary molecular compound ammonia, NH₃, also called ammonium hydroxide.

Concept Check: What is the name of the base with the formula KHCO₃? What is the formula for magnesium hydroxide?

Answer: Potassium hydrogen carbonate is the name of the base with the formula KHCO₃. Magnesium hydroxide has a formula of Mg(OH)₂.

This site was last updated 08/03/06