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

Acids and Bases

Introduction and Definitions:

Acids and bases are encountered frequently both in chemistry and in everyday living. They have opposite properties and have the ability to cancel or neutralize each other. Acids and bases are carefully regulated in the body by the lungs, blood, and kidneys through equilibrium processes.

What are acids and bases?

Observational definitions:
 Acids:  Bases:
Taste sour.  Taste bitter.
Give sharp stinging pain in a cut or wound.  Feels slippery
Turn blue litmus paper red.  Turn red litmus paper blue.
Turn phenolphthalein colorless.  Turn phenolphthalein pink.
React with metals to produce hydrogen gas.  
React with carbonates or bicarbonates to produce carbon dioxide gas.  

See the graphic on the left as a base is wiped over the paper which already has the indicator phenolphthalein on it. The drawing changes to pink.

What are Acids and Bases - on a molecular level?

 Acids:  Bases:
Arrhenius Definition (1887) - covers the dissociation of acids and bases in water.
Produce hydrogen ions (H+)  Produce hydroxide ions (OH-)

Example: HCl

HCl ---> H+) + Cl-

 NaOH

NaOH ---> Na+ + OH-

 In the graphic, the first reaction shows citric acid producing hydrogen ions, therefore it is an acid.

 
 

 Bronsted-Lowery Definition (1923) - The definition of acids and bases involving hydrogen and hydroxide ions, respectively is much too limiting. A broader definition was proposed by Bronsted and Lowry in 1923. The main effect of the definition is to increase the number of substances that act as bases.

 Acid

 Base
 

 Donates hydrogen ions

 Accepts hydrogen ions.
 

 HCl +

 HOH --->
H3O+ + Cl-

  HOH +

  NH3--->
 NH4+ + OH-

The determination of a substance as a Bronsted-Lowery acid or base can only be done by observing the reaction. In the case of the HOH it is a base in the first case and an acid in the second case.

Link to Chime animation of ammonium ion to water transfer - Jeremy Harvey, University of Bristol, England

See the graphic on the left for an example:

To determine whether a substance is an acid or a base, count the hydrogens on each substance before and after the reaction. If the number of hydrogens has decreased that substance is the acid (donates hydrogen ions). If the number of hydrogens has increased that substance is the base (accepts hydrogen ions). These definitions are normally applied to the reactants on the left.

If the reaction is viewed in reverse a new acid and base can be identified. The substances on the right side of the equation are called conjugate acid and conjugate base compared to those on the left.

Also note that the original acid turns in the conjugate base after the reaction is over.