1. Acids
and
Bases

2. 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 + H Cl H Cl +
The more recent Bronsted-Lowry concept is that acids are H+ (proton) donors and bases are proton acceptors

3. The Bronsted-Lowry concept
In this idea, the ionization of an acid by water is just one example of an acid-base reaction. + Cl H O
acid
base
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+.

4. 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

5. 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.

6. Practice Reference: pg. 488-491 Try questions 1,2 on page 492
For each of the following reactions identify any Bronsted-Lowry acids and bases.
HNO H2O  H3O NO3-
HNO NH3  NH NO3-
S H2O  HS OH-
HS OH-  S H-OH
HS HCl  H2S Cl-
Are any of the substances above amphoteric?
Reference: pg
Try questions 1,2 on page 492