Advantages of the Bronsted – Lowry theory

Advantages of the Bronsted – Lowry theory

Advantages of the Bronsted – Lowry theory:

In 1923, J. N. Bronsted and J. M. Bjerrum in Denmark and T. M. Lowry in England independently proposed a theory known as ‘the proton theory of acids and bases’. According to this theory-

  • An acid is a substance – a molecule or an ion – which can donate a proton.
  • A base is a substance – a molecule or an ion – which can accept a proton.

Advantages of the Bronsted – Lowry theory:

(a) It includes all substances described as acids and bases by Arrhenius.

(b) The acid – base reactions are not limited to aqueous solutions only.

(c) The theory provides a method of comparing the strengths of the ‘strong’ acids and ‘strong’ bases. The usefulness of the Bronsted-Lowry concept in determining the relative strengths of acids and bases can be shown by the following considerations.

This concept is not limited to molecules but also covers even the ionic species to act as acids or bases. It can also explain the acid-base reactions in the non aqueous medium.

The strength of an acid is a measure of its tendency to give up a proton and the strength of a base is a measure of its tendency to accept a proton. It may be noted that since a strong acid dissociates completely to give protons its conjugate base cannot keep the proton and hence it must be very weak. Similarly, the conjugate acid of a strong base must be very weak.

(i) A strong acid has a weak conjugate base

(ii) A strong base has a weak conjugate acid.

The relative strengths of acids like- HCl, HNO3, HClO4, HBr, H2SO4 cannot be determined in aqueous solutions as they appear to be equally strong in water. This is because the acids are completely dissociated in water which acts as a strong base. Water here is said to exert a ‘Levelling effect;. If, however, these acids are dissolved in a solvent like glacial ethanoic acid, which is not a good proton acceptor, the relative proton donating tendencies become apparent and can he determined by measuring the molar conductance. The strengths of the above mentioned acids have been found to be in the order:

HClO4 > HBr > H2SO4 > HNO > HCl > HNO3

Limitations of the Bronsted-Lowery concept:

  • It cannot explain the reactions between acidic oxides like CO2, SO2, SO3 and basic oxides like CaO, BaO, MgO, etc. which take place even in the absence of the solvent.
  • Substances like BF3, AlCl3, etc. behave as acids but they do not have protons to loose or donate.

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