Three Major Types of Acids

Usually acids can be divided into three major types. First one is binary acid, second one is oxyacid, and the last one is carboxylic acid. Binary acids are all written in “H-A” form, which means hydrogen bond to a nonmetal atom. The general trend of acidity for binary acid is increasing from top to button in a group and increasing from left to right in a period. That is why HF is weaker than HCl, while HBr is stronger than HCl. (Notice that F, Cl and Br are in the same group).

For oxyacid, it has to have one or more O-H bonds. For example, Sulfuric Acid H2SO4, Nitric Acid HNO3, and so on. Only the hydrogen atom that is bonded to oxygen contributes to the acidity.  There are always questions asking to circle the hydrogen atom that contributes to the molecule’s acidity. If you are asked to arrange acid strength of a series of oxyacids with the same number of OH groups and O atoms, usually the one with the strongest electronegativity in central atom will produce the highest acidity. And if you are asked to arrange acid strength of oxyacids with same central atom but different numbers of oxygen atoms, the one with the most oxygen atoms will be the strongest acid. For example, H2SO4 is stronger than H2SO3. (Both acid has sulfur atom as central atom, while H2SO4 has one more oxygen atom.)

For Carboxylic Acid, only the hydrogen atom in the carboxyl group can be ionized and contribute to acidity. This type of acid comes up a lot in organic chemistry and biology. For example, Benzoic acid is one of the carboxylic acids. Benzene has a ring structure, and the carboxyl group is represented as -COOH. Putting these two things together will become the structure of benzoic acid. In exams it is usually not required to compared the acidity of carboxylic acids since there are too many factors that affect their acidity.


  • Subject : Science
  • Topic : Chemistry
  • Posted By : Jason
  • Created on : 03-10-2011

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