An acid is a chemical that has the ability to donate protons, or protonate other compounds. According to Lewis, acids are acceptors of electron pairs.
Strong and weak acids
Strong acids are those that dissociate completely in aqueous solution. Those that do not are called weak acids.
Note that there's a number of additional acid strength gradations.
- A mid-strength acid is between the strong and weak ones. It does not dissociate completely, but has a high ratio of dissociation. Examples include phosphoric acid, oxalic acid and hydrofluoric acid.
- A superacid is one that is more acidic than pure sulfuric acid. Another definition has superacids as acids that react with water to completion, as if water was a base, forming stable, isolable hydronium salts. Examples include perchloric, triflic and fluoroantimonic acids.
- A very weak acid (a mostly informal category) is an acid that is weaker than carbonic acid, thus its salts react with carbon dioxide in air. Examples include phenol, hydrogen cyanide and hydrogen peroxide.
- Finally, there are compounds that aren't acids in aqueous solution but can be deprotonated and forced to display some acidic properties in exotic conditions. Such compounds are said to be weaker acids than water. Examples include ammonia, hydrocarbons and molecular hydrogen. The conjugate bases of these "acids" are superbases, and their "salts" (amides, hydrides, carbides) are ones as well and react with water irreversibly.
The definition of acid by Arrhenius is that an acid is a compound that dissociates into hydrogen ion(s) and another, negatively charged ion. The release of the H+ ion is what makes an acid an acid.
Broensted and Lowry proved that the H+ ion does not exist as such, it associates with molecules to form other ions such as hydronium (H3O+). Thus the protonation of molecules (transformation into such ions) is what makes acid an acid. Strong acids can protonate themselves, forming cations such as H3SO4+, which are themselves potent acids. The anion left from the protonation is called the conjugate base, and the stronger the acid, the less basic is the conjugate base. Conjugate bases of weak acids are strong bases (the anions peroxide, cyanide, phenolate are nearly as basic as the hydroxide anion), and conjugate bases of strong acids are weak bases (the anions sulfate, perchlorate, iodide are barely basic at all).
According to Lewis, the action of acids is accepting free electron pairs. This can be done by compounds that do not contain hydrogen and do not protonate anything. All Broensted-Lowry acids are Lewis acids because they stick the proton on any free electron pairs possible.