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.
Donates hydrogen ions
Accepts hydrogen ions.
||H3O+ + Cl-
|| 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
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
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.