In 1923, Bronsted and Lowry independently defined acid as Proton donor and bases as proton acceptor. For aqueous solution,this definition does not differ. From the Arrhenius theory; such substances that increases the concentration of hydroxonium ion in aqueous solution are acids and those that decreases it are bases.
The Bronsted lowry theory is useful in that it extends the scope of acid base system to cover solvent such as liquid ammonia; concentrated acid, anhydrous sulphate acid and all hydrogen containing solvents. As such in this case BASES ACCEPT PROTON and there is no need for them to contain hydroxyl ion e.g in NH3(liquid).
NH4Cl(acid) + NaNH2(base) – NaCl(salt) + 2NH3(solvent)
It should be noted that substances that reduced the H3O^+ in all aqueous solution above 1*10^-7 mol/L from the self ionization of water are ACIDS and those that decreases it are BASES.
CONJUGATE PAIRS
Chemical species that differ in composition by a proton are called a CONJUGATE PAIR. Thus,every acid has a conjugate base which is formed by the acid and donating a proton. Similarly,every base has a conjugate acid. Example
HCl(acid) + H2O(base) = H3O(conjugate acid) + Cl^-(conjugate base)
In the above reaction;HCl is an acid since it donates proton and in doing so forms chloride ion (conjugate base).
Since water accepts proton,its a base and it forms the hydroxonium ion which is its conjugate acid.
A strong acid has a weak conjugate base and vice versa. A strong base has a weak tacos and vice versa.
LIMITATIONS OF BRONSTED LOWRY THEORY.
- The limitations of this theory is that the extent to which a dissolved substance can act as an acid or base depends largely on the solvent. A solute can only show acidic properties if its proton donating properties exceed those of the solvents. As an example,HF dissolves in perchlonic acid ( HClO4) an extremely strong proton donor.