Strong and Weak Acids and Bases


Strong Acids

A strong acid is an acid which is fully ionized in solution.
Strong acids are so fully ionized in solution (close to 100%, but not quite) that the equilibrium constant has very large values much greater than one million.
e.g. HCl(aq) + H2O(l) ↔H3O+(aq) + Cl-(aq) Greater than 99.99% ionized
There aren't very many strong acids, which makes life very easy for you.

Strong Acids
The Formulae
Sulphuric acid
Hydrochloric acid
Hybrobromic acid
Hydroiodic acid
Nitric acid
Perchloric acid
H2SO4
HCl
HBr
HI
HNO3
HClO4

Weak Acids
A weak acid is an acid which is only partially ionized in solution. This also means that it has a small value for the equilibrium constant for the ionization reaction. The value tends to be less than one, and quite a lot less than one.
e.g. CH3COOH(aq) + H2O(l) ↔H3O+(aq) + CH3COO-(aq) Less than 0.4% ionized.

Strong Bases

Similarly, a strong base is one which is fully ionized in solution.
It also has a high value for the equilibrium constant for ionization.
Again there are not many of them, and again you can memories them all.

Strong Bases
The Formulae
Lithium hydroxide
Sodium hydroxide
Potassium hydroxide
Rubidium hydroxide
Caesium hydroxide
Barium hydroxide
Calcium hydroxide
Strontium hydroxide
LiOH
NaOH
KOH
RbOH
CsOH
Ba(OH)2
Ca(OH)2
Sr(OH)2

Weak Bases
Again, anything else is automatically a weak base.
As before, this means that they have a small value for the equilibrium constant for ionization.
Again, there is no reference to the concentration of the solution, or how corrosive it is.

Three concept of acids and bases are:
(1)   Arrhenius concept
(2)   Bronsted-Lowry concept
(3)   Lewis concept
Arrhenius concept (1884):
An acid is a compound that releases H+ ions in H2O.
A base is a compound that releases OH- ions in H2O.
HCl   +   H2O  ®  H3O+  +   Cl-
NaOH   +   H2O  ®  Na+  +   OH-
Usefulness: This concept is useful in the study of chemical reactions.
Limitations:
(a)    Free H+ and OH- ions do not exist in water.
 
(b)    Limited to water only.
These definitions are applicable to water only.
(c)   Some bases do not contain OH-. Example: NH3, CaO
Bronsted-Lowry concept (1923):
An acid is any molecule or ion that can donate a proton (H+).
A base is any molecule or ion that can accept a proton (H+).



Bronsted-Lowry concept is superior to Arrhenius concept:
(a)    Much wider scope.
(b)   Not limited to aqueous solutions.
NH3  +  HCl      ↔      NH4+   +   Cl-
(c)    Release of OH- not necessary to qualify as a base.
NH3  +  H+     ↔       NH4+  
base                          acid
“An acid is a proton donor, while a base is a proton acceptor.”

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