Comparing strength of acids and bases
conjugate base
CH3NH2
PO43–
CH3CO2–
CH3NH2
PO43–
CH3CO2–
acid
CH3NH3+
HPO42–
CH3CO2H
CH3NH3+
HPO42–
CH3CO2H
Acids are proton (H+) donors.
Acids can be cations, molecules or anions.
Bases are proton (H+) acceptors.
Bases can be molecules or anions.
The formulae of the members of an acid-conjugate base pair differ by one H+.
The base is always one unit more negative and has one fewer hydrogens.
The acid is always one unit more positive and has one more hydrogen.
The acid is always one unit more positive and has one more hydrogen.
Each acid-base conjugate pair has a characteristic Ka.
Ka is the equilibrium constant for reaction of the acid with water to give the conjugate base.
Usually Ka is quoted for the acid, such as Ka(HF), because the acid determines the formula of the conjugate base.
| Ka = | [conjugate base ][H3O+] | pKa = -log Ka |
| [acid] |
acid + H2O
H3O+ + conjugate baseWhen comparing acid strength the conjugate pair having
the stronger acid has higher Ka (more forward reaction) and lower pKa.
Note that for two solutions, each having one of the acids dissolved,
if the concentrations of the acids are the same, the aqueous solution of the stronger acid also has a lower pH.
if the concentrations of the acids are the same, the aqueous solution of the stronger acid also has a lower pH.
When comparing base strength the conjugate pair having
the stronger base has lower Ka (more reverse reaction) and a higher pKa.
Note that for two solutions each having one of the conjugate bases dissolved,
if the concentrations of the bases are the same, the aqueous solution of the stronger base also has a higher pH.
if the concentrations of the bases are the same, the aqueous solution of the stronger base also has a higher pH.