Acids and Bases : Bronsted-Lowry concept of acids and bases


In 1923, the Two scientists Danish N.Bronsted and Thomas M.Lowry pointed out that acid-base reactions can be seen as proton-transfer reactions and that acids and bases can be defined in terms of this proton (H+) transfer.

With the Brønsted-Lowry concept we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion.

Brønsted and Lowry independently came up with the idea that an acid is an acid because it provides or donates a proton to something else. When an acid reacts, the proton is transferred from one chemical to another.

And a base is a species that accepts a proton in a proton-transfer reaction.

For example:

the reaction of HCl with NH3 , writing its ionic equation:

H3o+ (aq) + Cl- (aq)   + NH3 (aq) --------> H2O(l) + NH4(aq)  + Cl- (aq)

after cancelling the spectator ions,you obtain net ionic equation

H3O+ (aq)   +   NH3 (aq)  ---------> H2O(l)  +  NH4+ (aq)

acid                     base

in this reaction in aqueous solution a proton (H+) is transferred from the H3O+ ion to the NH3 molecule, giving H2O and NH4+ .Here H3O+ ion is the proton donar or acid and NH3 is the proton acceptor or base.

In any reversible reaction,both forward and reverse reactions involve proton transfers.Consider the reaction of NH3 with H2O.

    NH3 +  H2O <----------> NH4+  +  OH-

 in  the forward reaction NH3 accepts a proton from H2O.Thus,NH3 is a base and H2O is an acid.In the reverse reaction,NH4+ donates a proton to OH- .NH4 ion is the acid and OH- is the base.

NH3 +  H2O <----------> NH4+  +  OH

acid      base                     acid       base

Now notice , that NH3 and NH4+ differ by a proton, which means that NH3 becomes NH4+ ion by gaining a proton, whereas the NH4+ ion becomes the NH3 molecule by losing a proton.
The species NH4+ and NH3 are a conjugate acid-base pair.

A conjugate acid-base consists of two species in an acid-base reaction, one acid and one base,that differ by the loss or gain of a proton. 

Lets see an example:

Sample probelm:

HCO3 +  HF   <--------------------->  H2CO3  + F-


Step 1: Recalling bronsted-lowry concept acid is a proton donar and base is a proton acceptoe so examine the equation to find proton donor on each side.

Step 2: On the left HF is the proton donor , on the right H2CO3 is the proton donor.

Step 3: The proton acceptors are HCO3- and F- .

now equation can be labeled;

HCO3 +  HF   <--------------------->  H2CO3    +       F-

base          acid                                       acid               base