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Chemistry - The Bronsted-Lowry Theory

The Bronsted-Lowery theory attempts to overcome the limitations of the Arrhenius Theory by defining an acid as a proton (H+) donor and a base as a proton (H+) acceptor.

The base accepts the H+ by furnishing a lone pair of electrons for a coordinate-covalent bond, which is a covalent bond in which one atom furnishes both of the electrons for the bond. Normally, one atom furnishes one electron for the bond and the other atom furnishes the second electron. In the coordinate-covalent bond, one atom furnishes both bonding electrons.

With the Arrhenius Theory acid-base reactions are neutralization reactions.

With the Bronsted-Lowery Theory, acid-base reactions are a competition for a proton.

For example:

This is the reaction o ammonia with water:

NH 3(g) + H 2O (l)<--> NH 4+ + OH-

Ammonia is a base – it accepts the proton; water is an acid – it donates the proton, in the forward reaction. In the reverse reaction, the ammonium ion is an acid and the hydroxide ion is a base.

If water is a stronger acid then the ammonium ion, then there is a relatively large concentration of ammonium and hydroxide ions at equilibrium. If, however, the ammonium ion is a stronger acid, more ammonia is present at equilibrium than ammonium ion.

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