Historical views on acids
• Oxygen (e.g. H2SO4) was originally thought to cause
acidic properties. Later, Hydrogen was implicated,
but it was still not clear why CH4 was neutral.
• Arrhenius made the revolutionary suggestion that
some solutions contain ions & that acids produce H+
ions in solution.
Ionization
Cl H
+
H
+
Cl
• The more recent Bronsted-Lowry concept is
that acids are H+ (proton) donors and bases
are proton acceptors
The Bronsted-Lowry concept
• In this idea, the ionization of an acid by water
is just one example of an acid-base reaction.
H
Cl H
acid
+
O
H
base
+
H
HO
+
Cl
H
conjugate acid conjugate base
conjugate acid-base pairs
• Acids and bases are identified based on
whether they donate or accept H+.
• “Conjugate” acids and bases are found on the
products side of the equation. A conjugate
base is the same as the starting acid minus H+.
Practice problems
Identify the acid, base, conjugate acid,
conjugate base, and conjugate acid-base pairs:
HC2H3O2(aq) + H2O(l)  C2H3O2–(aq) + H3O+(aq)
acid
base
conjugate base conjugate acid
conjugate acid-base pairs
OH –(aq) + HCO3–(aq)  CO32–(aq) + H2O(l)
base
acid
conjugate base conjugate acid
conjugate acid-base pairs
Amphiprotic
• Sometimes a molecule can donate a proton (act as
an acid) and sometimes it can accept a proton (act
as a base).
• Molecules that have this ability to act as both an
acid and a base are called amphoteric or
amphiprotic.
• Water is the most common example of an
amphoteric substance.
Practice
For each of the following reactions identify any BronstedLowry acids and bases.
• HNO3 + H2O  H3O+ + NO3• HNO3 + NH3  NH4+ + NO3• S2+ H2O  HS- + OH• HS+ OH-  S2- + H-OH
• HS+ HCl  H2S + ClAre any of the substances above amphoteric?
• Reference: pg. 488-491
• Try questions 1,2 on page 492