Bronsted-Lowry theory defined an acid to be a substance which donates a proton, while a base is a molecule or ion which accepts proton. This definition of acid and a base could be extended to include acid-base reactions in non-aqueous solutions.
Arrhenius theory can only be applied to reactions which occur in aqueous solution. By now chemists knew that acidity is not an inherent property of a substance under certain conditions. The first theory that had this view was put together by Bronsted and Lowry as stated above.
An example of a Bronsted-Lowry acid-base reaction is that between ethanoic acid and the hydroxide ion.
CH3COOH(aq) + OH-(aq) ↔ CH3COO-(aq) + H2O(l)
In the forward reaction, an ethanoic acid molecule donates a proton to a hydroxide ion which accepts it.
The ethanoic acid while the hydroxide ion behaves as a base (proton acceptor) while the water molecule acts as an acid (proton donor).
In the above reaction, the ethanoic acid and the ethanoic ion are a conjugate acid-base pair. Similarly, the water molecule and the hydroxide ion are also a conjugate acid-base pair.
Thus, a conjugate acid-base pair is an acid-base combination in which one is related to the other by a gain or loss of a proton.
Other examples of Bronsted-Lowry acid-Base reactions are as follows:
- When ammonia reacts with hydrogen chloride, ammonia acts as a base by accepting the proton from hydrogen chloride, in turn acts as an acid by donating the proton. The equilibrium of this reaction lies to the far right because hydrogen chloride is a stronger acid than the ammonium ion.
HCl(g) + NH3(g) ↔ NH4+Cl-(s)
- In the dissociation of hydrogen chloride in water, the hydrogen chloride molecule which accepts it. The hydrogen chloride is acting as an acid and water as a base. The equilibrium of this reaction is also to the far right because hydrogen chloride is a stronger acid than the hydroxonium ion.
HCl(aQ) + H2O(l) ↔ Cl-(aq) + H3O+(aq)
- When water dissociates, it serves as both the acid and the base. Thus, one water molecule donates a proton, which the other accepts to form hydroxonium ion.
H2O(l) + H2O(l) ↔ H3O+(aq) + OH-(aq)