1. The Bronsted-Lowry definition
There are many definitions of acids and bases in existence, but the most useful one is the Bronsted-Lowry definition:
An acid is a substance which can behave as a proton (re presented as a hydrogen ion, H+) donor. Any substance which contains hydrogen bonded to a more electronegative element can thus behave as an acid:
HCl == H+ + Cl-
H2SO4 == H+ + HSO4-
A base is a substance which can behave as a proton acceptor. Any species which has a lone pair of electrons can thus behave as a base:
NH3 + H+ == NH4+
OH- + H+ == H2O
2. Conjugate acid-base pairs
Since all the above reactions are reversible, the species formed when an acid gives up a proton can accept a proton and thus behave as a base. The species formed when a base accepts a proton can give up a proton and behave as an acid:
Cl- + H+ == HCl
HSO4- + H+ == H2SO4
NH4+ == NH3 + H+
H2O == H+ + OH-
Acids and bases thus come in pairs; every acid can lose a proton to become a base and every base can accept a proton to become an acid. Species related in this way are known as conjugate acid-base pairs. Eg Cl- is the conjugate base of HCl; HCl is the conjugate acid of Cl- Eg NH3 is the conjugate base of NH4+; NH4+ is the conjugate acid of NH3
Not all acids are equally good proton donors; in fact some give up their protons very reluctantly; conversely, some bases accept protons readily whereas others accept protons very reluctantly.
The better an acid is at losing protons, the worse its conjugate base will be at accepting them. Thus a strong acid will have a weak conjugate base. The better a base is at accepting protons, the worse its conjugate acid will be at losing them. Thus a weak acid will have a strong conjugate base.
Acids and bases can be ranked in order of decreasing strength; a list of common acids and their conjugate bases ranked in such a way is shown below:
|ACID | |BASE | | |H2NO3+ (NO2+ + H2O) |== |HNO3 |+ H+ | |H2SO4 |== |HSO4- |+ H+ | |HNO3 |== |NO3- |+ H+ | |HCl |== |Cl- |+ H+ | |H3O+ |== |H2O |+ H+ | |H2SO3 (SO2 + H2O) |== |HSO3- |+ H+ | |HSO4- |== |SO42- |+ H+ | |H3PO4 |== |H2PO4- |+ H+ | |HF |== |F- |+ H+ | |HNO2 |== |NO2- |+ H+ | |CH3COOH |== |CH3COO- |+ H+ | |H2PO4- |== |HPO42- |+ H+ | |H2CO3 (CO2 + H2O) |== |HCO3- |+ H+ | |HSO3- |== |SO32- |+ H+ | |NH4+ |== |NH3 |+ H+ | |HCN |== |CN- |+ H+ | |HCO3- |== |CO32- |+ H+ | |HPO42- |== |PO43- |+ H+...