Definition of Acid

What is an Acid?

There are two main definitions of acid: the Bronsted-Lowry acid and the Lewis acid.

Bronsted-Lowry Acids

Bronsted-Lowry acids are species that donate a proton (H+). For example hydrogen chloride acts as an acid in water, donating a proton to water:

HCl(g)  +   H2O(aq)   ⇌   H3O+(aq)   +   Cl-(aq)

The strength of a Bronsted-Lowry acid is measured using the pH scale.

Lewis Acids

Lewis acids are electron pair acceptors. They have an unoccupied low-energy atomic or molecular orbitals.

The Lewis definition is more flexible than the traditional Bronsted-Lowry definition.

The Lewis definition includes compounds that are Bronsted-Lowry acids but also encompasses compounds that do not donate protons, but still exhibit acid behavior.

Lewis Acid Example 1
In the traditional acid/base neutralization reaction of H+ and OH-, H+ is a Lewis acid because it accepts an electron pair from the OH-.

Lewis Acid Example 2
Boron trifluoride acts as a Lewis acid when it accepts an electron pair from ammonia to form a co-ordinate covalent bond - i.e. a bond in which both bonding electrons come from the same atom:

BF3   +   NH3    →    BF3NH3

The co-ordinate bond may be drawn as a double-headed arrow to represent the fact that both electrons come from one atom, in this case the nitrogen atom:


The reactions of Lewis acids with bases are interpreted with the HSAB (hard and soft acids and bases) concept.

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