Worksheet 18 - Acids and Bases
The Brønsted-Lowry definition of an acid is a substance capable of donating a proton
(H+), and a base is a substance capable of accepting a proton. For example, the
weak acid, HF, can be dissolved in water, giving the reaction:
HF (aq) + H2O (l) ' H3O+ (aq) + F- (aq)
acid conjugate base
base conjugate acid
In this reaction, HF is the species losing the proton (H+), making it the acid. Water is
the species accepting the proton, to form the hydronium ion, H3O+, making it the base.
The F- (aq) is called the conjugate base of HF. It can gain a proton in the reverse
reaction. H3O+ is the conjugate acid of H2O, since it can lose a proton in the reverse
reaction. The stronger an acid, the weaker its conjugate base will be and the stronger
the base, the weaker its conjugate acid.
The equilibrium concentrations of these species will be determined by the relative
strengths of the acids and bases. The strongest acid will dissociate to the greatest
extent. H3O+ (H+) is the strongest acid that can exist in an aqueous system. So, the
equilibrium in this system will favor the reactants, HF and H2O, the weaker acid and
base. The equilibrium state is described by an equilibrium constant, Ka, in the case of
acids, and Kb in the case of bases. These are related by the expression Kw = Ka x Kb =
1 x 10-14.
1. Classify the following as Brønsted acids, bases or both.
If the compound has a hydrogen, it can be an acid; if it has any lone
pairs, it can be a base
a) H2O b) OH- c) NH3
both both both
d) NH4+ e) NH2- f) CO32-
acid both base
2. What is the conjugate base of the following acids?
a) HClO4 b) NH4+ c) H2O d) HCO3-
ClO4- NH
3 OH
- CO
32-
(Remove one hydrogen and reduce the charge by 1)
3. What is the conjugate acid of the following bases?
a) CN- b) SO42- c) H2O d) HCO3-
HCN HSO4- H
3O+ H
2CO3
(Add one hydrogen and increase the charge by 1)
