Acids and Bases  Elmhurst College
Acids Neutralization & Salts Acid-Base Strength  Chemistry Department
Bases pH  Indicators  Virtual ChemBook

 
Click for larger image

Acids

Introduction and Definitions:

At the simplest definition, an acid is any compound that yields hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water. Hydronium ions are really the combination of H+ ions and H2O molecules, i.e.

H+ + H2O === H3O+.

The hydrogen ion with no electrons bonds to water through what is called a coordinate covalent bond where the oxygen donates both electrons for the bond. The molecule is still short an electron so the positive charge is retained. This is reaction is shown on the left graphic. The hydronium ion also has a trigonal pyramid geometry.

Hydronium ion - Chime in new window

Even though in solution a "bare" hydrogen ion is surrounded by up to 20 water molecules and is the best representation, most of the time just a simple hydrogen ion is used to represent the acid. In the chime model, notice the close contact of the water molecules as they engage in hydrogen bonding.

Hydronium ion cluster - Chime in new window

Table of Common Acids - Chime in new window
 Hydrochloric -  HCl Muriatic Acid - common name; Toilet bowl cleaners, masonry cleaners, gastric juice
 Carbonic -  H2CO3 Carbonated water; club soda - soft drinks, blood buffer
 Sulfuric -  H2SO4 Battery Acid; lead battery, air pollutant mist, acid rain
 Acetic -  HC2H3O2 Vinegar; cooking, salads
 Nitric -  HNO3 Remove warts, causes yellow stains on skin
 Phosphoric -  H3PO4 Acidify soft drinks, in ATP and DNA
 Citric -  H3C6H8O6 Fruit juices
 Lactic -  HC3H5O3 Builds up in muscles during exercise

 
 

 Self Ionization or Dissociation:

An acid ionizes or dissociates (breaks apart) in water to form an equilibrium mixture of its individual ions. This is how the hydrogen ions are formed.

The following are some examples of dissociations reactions:

 Acid
 

 Molecules

Ions

 HCl -->
H+ + Cl-

H2CO3 --> 
H+   + HCO3-
 HCO3- -->  H+  + CO32-
 HC2H3O2  H+  C2H3O2-