Indian River State College
10/09, rev 7/12, 4/13, 5/13, 6/13, 11/13, 3/14 mds/cs
Conjugate Acids and Bases
A. The heart of the Bronsted-Lowey concept of acids and bases is that acid-base reactions are proton
transfer reactions.
An acid is a proton donor.
A base is a proton acceptor.
Example: HCl + H2O (aq) → H3+O + Cl¯ (aq)
HCl is an acid, it donates a H+ and H2O is a base, it accepts the H+.
Example: HCHO2 (aq) + H2O (aq) H3+O (aq) + CHO2¯ (aq)
The formic acid molecule is donating a proton to a water molecule and behaves as an acid.
Water accepts the proton and behaves as a base (proton acceptor).
B. The reverse reaction:
CHO2¯ (aq) + H3+O (aq) → HCHO2 (aq) + H2O (aq)
H3+O behaves as in acid, as it donates a proton to the CHO2¯ ion. The CHO2¯ ion behaves as a base
accepting the proton. Accordingly, there are two acids and two bases in this reaction. Substances
that differ from each other by a proton are referred to as a conjugate acid base pair (CJ Pair).
CJ Pair
HCHO2 + H2O H3O+ + CHO2−
Acid Base Acid Base
CJ Pair
C. One method of solving these problems involves identifying the acid in the left side of the equation
as the other component will be the base on the left side of the equation. On the right side of this
equation look for the material that has a difference of a proton from the left side. Therefore H3+O is
a CJ acid of H2O and CHO2¯ is conjugate base of HCHO2, since it cannot provide another hydrogen
for H2O.
D. Determining the conjugate acid or conjugate base of existing compound.
Question 1: Find the conjugate base of HNO3 and conjugate acid of HSO4¯.
1. The first step is to recognize that members of any conjugate acid-base pair differ by one H+. The
member having the greater number of hydrogen ions is the acid.
