Bronsted-Lowry Theory
In 1923 Bronsted and Lowry proposed the proton transfer theory of acids and bases. Bronsted-Lowry theory is based on proton transfer. Therefore it is called as protonic theory.
The substance that donates a proton is bronsted acid.
The substance which shows a tendency to lose or donate a proton is called an acid. So a Bronsted-Lowry acid is a proton donor.
- HCl H+ + Cl-
The substance that accepts a proton is bronsted base.
The substance which shows a tendency to gain a proton is called as base. So a Bronsted-Lowry base is a proton acceptor.
The substance which when added to water produces hydroxide ions OH- is called base.
- NH3 + H+ NH4+
Neutralisation:
The transfer of proton from an acid to base is called neutralisation.
- acid + base hydronium ion + chloride ion
- HCl + H2O H3O+ + Cl-
Conjugate acid-base pairs:
A pair of acid-base which differs by a single proton is known as conjugate acid-base pair.
The removal of proton from a bronsted acid gives its conjugate base.
- acid - proton conjugate base
- HCl + H2O H3O+ + Cl-
The addition of proton to a bronsted base gives its conjugate acid.
- base + proton conjugate acid
- NH3 + H2O NH4+ + OH-
The conjugate base of strong acid is weak base.
HCl is strong acid. Cl- is weak base (conjugate).
The conjugate base of weak acid is strong base.
CH3COOH is week acid. CH3COO- is strong base (conjugate).
The conjugate acid of strong base is weak acid.
OH- is strong base. H2O is weak acid (conjugate).
The conjugate acid of weak base is strong acid.
C2H5OH is week base. C2H5OH2+ is strong acid (conjugate).
According to Bronsted-Lowry theory:
An acid which shows a greater tendency to donate a proton is a strong acid. Ex. H2SO4, HNO3
An acid which shows a less tendency to donate a proton is a weak acid. Ex. CH3COOH, H2S
A base which shows a greater tendency to accept a proton is a strong base. Ex. CH3COO-
A base which shows a less tendency to accept a proton is a weak base. Ex. Cl-
Amphiprotic substances:
A substance that shows tendency to donate or accept protons is called amphiprotic substances.
- acid conjugate base
- H2O H+ + OH-
- base conjugate acid
- H2O H+ H3O+
Leveling effect of water:
In water all the strong acids with different strengths like H2SO4, HCl produce equal concentrations of H+ ions since they are 100% ionized in water. Similarly in water all the strong bases with different strengths like NaOH, KOH produce equal concentrations of OH- ions since they are 100% ionized in water. This is known as the leveling effect of water.
Limitations of Bronsted-Lowry theory:
Proton acceptance or donation only happens in the presence of another substance.
Bronsted-Lowry theory could not explain the acidic nature of electron deficient compounds like AlCl3, BCl3, etc.