What Is Bronsted–Lowry Theory?

The Bronsted-Lowry theory (Proton theory of acid and base) is an acid-base reaction theory, introduced by Johannes Nicolaus Bronsted (Danish Chemist) and Thomas Martin Lowry (English Chemist) in 1923. According to the theory, acid and base react with each other and by an exchange of proton acid, forms its conjugate base and the base forms its conjugated acid.

The Bronsted-Lowry theory is an extended version of an Arrhenius theory of acid-base.

According to the Arrhenius theory, in aqueous solution, acid increases the concentration of H+ ions and base increases the concentration of OH ions. The limitations of Arrhenius theory was, it identifies the reaction of an acid and base only in the aqueous medium.

Bronsted-Lowry Theory of Acid and Base

According to Bronsted-Lowry theory, acid is a substance which donates an H+ ion or a proton and forms its conjugate base and the base is a substance which accepts an H+ ion or a proton and forms its conjugate acid.

Bronsted-Lowry Acid

The Bronsted-Lowry acid is a substance which donates a proton or H+ ion to the other compound.

AcidProton+ConjugatebaseAcid \rightleftharpoons Proton + Conjugate base

A conjugate base can accept a proton and acid reforms.

HFH++FHF \rightleftharpoons H^{+} + F^{-} H2SO4H++HSO4H_{2}SO_{4} \rightleftharpoons H^{+} + HSO_{4}^{-} NH4+H++NH3NH_{4}^{+} \rightleftharpoons H^{+} + NH_{3} H2OH++OHH_{2}O \rightleftharpoons H^{+} + OH^{-} F+H+HFF^{-} + H^{+} \rightleftharpoons HF

Bronsted-Lowry Base

The Bronsted-Lowry base is a substance which accepts a proton or H+ ion from other compounds.

Base+ProtonCongatedacidBase + Proton \rightleftharpoons Congated acid

A conjugated acid can donate a proton and base reforms.

OH+H+H2OOH^{-} + H^{+} \rightleftharpoons H_{2}O S2+H+HSS^{2-} + H^{+} \rightleftharpoons HS^{-} CO32+H+HCO3CO_{3}^{2-} + H^{+} \rightleftharpoons HCO_{3}^{-}

The Bronsted-Lowry theory of an acid-base reaction involves the transfer of protons or H+ ions between the acid and base.

Example 1:

Consider a reaction in which ammonia (base) is dissolved in water (acid). Ammonia takes a proton from water and the reaction is as follows,

H2O(l))+NH3(Aq))OH(Aq)+NH4+H_{2}O(l)) + NH_{3}(Aq)) \rightleftharpoons OH^{-}(Aq) + NH_{4}^{+} Acid+BaseConjugatebase+conjugateacidAcid + Base \rightleftharpoons Conjugate base + conjugate acid

Example 2:

A reaction between acetic acid and water, acetic acid donates a proton and acts like acid and water molecule (base) takes a proton.

CH3CO2H(aq)+H2OCH3CO2+H3O+CH_{3}CO_{2}H (aq) + H_{2}O \rightleftharpoons CH_{3}CO_{2}^{-} + H_{3}O^{+} Acid+Baseconjugatebase+conjugateacidAcid + Base \rightleftharpoons conjugate base + conjugate acid

Note:

  • Strong Bronsted-Lowry acids are those which have a strong tendency to give a proton and their corresponding conjugate base is weak.
  • Weak Bronsted-Lowry acids will have a little tendency to donate a proton and their corresponding conjugated base is strong.

The Bronsted-Lowry acids and their Conjugated Bases

The strength of the acid decreases as it descends and the strength of their corresponding conjugate base increases.

Acids

Conjugated base

Perchloric acid (HCIO4)

Perchlorate ion (CIO4)

Hydroiodic acid (HI)

Iodide ion (I)

Hydrobromic acid (HBr)

Bromide ion (Br)

Hydrochloric acid (HCl)

Chloride ion (Cl)

Sulfuric acid (H2SO4)

Hydrogen sulphate ion (HSO4)

Nitric acid (HNO3)

Nitrate ion (NO3)

Hydronium ion (H3O+)

Water (H2O)

Hydrogen sulfate ion (HSO4)

Sulfate ion (SO42-)

Nitrous acid (HNO2)

Nitrite ion (NO2)

Acetic acid (CH3COOH)

Acetate ion (CH3COO)

Carbonic acid (H2CO3)

Hydrogen carbonate ion (HCO3)

Ammonium ion (NH4+)

Ammonia (NH3)

Hydrogen carbonate ion (HCO3)

Carbonate ion (CO32-)

Water (H2O)

Hydroxide ion (OH)

Methanol (CH3OH)

Methoxide ion (CH3O)

Ammonia (NH3)

Amide ion (NH2)

Summary:

  • A Bronsted-Lowry acid is a substance which donates a proton or H+ ion to the other compound and forms a conjugated base.
  • A Bronsted-Lowry base is a substance which accepts a proton or H+ ion from the other compound and forms conjugated acid.
  • Strong acids and bases ionize completely in an aqueous solution, whereas weak acids and bases are partially ionized in aqueous solution.
  • Water molecule is amphoteric in nature, which means it can act as Bronsted-Lowry acid as well as Bronsted-Lowry base.