Comparing two acids or two bases
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+.
Each acid-base conjugate pair has a characteristic Ka.
Ka is the equilibrium constant for proton-transfer from the acid to water to give the conjugate base.
H3O+ + conjugate base
pKa = -log Ka
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.
| acid | conjugate base |
| CH3NH3+ | CH3NH2 |
| HPO42– | PO43– |
| CH3CO2H | CH3CO2– |
Each acid-base conjugate pair has a characteristic Ka.
Ka is the equilibrium constant for proton-transfer from the acid to water to give the conjugate base.
Usually Ka is quoted for the acid, such as Ka(HF), because the formula of the conjugate base follows from the acid. As shown below Ka is a characteristic of the acid-base conjugate pair.
acid + H2O H3O+ + conjugate base| Ka = | [conjugate base][H3O+] | |
| [acid] |
Example:
HF + H2O H3O+ + F–
pKa = -log (7.2 × 10–4)
pKa = 3.14
HF + H2O
H3O+ + F–| Ka = | [F–][H3O+] |
| [HF] |
pKa = 3.14
For stronger acids Ka is higher (more products) and pKa is lower.
For stronger bases Ka(conjugate acid) is lower and pKa (conjugate acid) is higher.
For stronger bases Ka(conjugate acid) is lower and pKa (conjugate acid) is higher.