What does the Arrhenius theory of acid actions states?
Arrhenius’ theory was put forward by the Swedish chemist, Arrhenius in the nineteenth century, it is one of the earliest theory of acid-base action that is still useful.
Arrhenius defined an acid as a substance which dissociates in water to produce hydrogen ions, H+. For example:
HCl ——–> Na+ (aq) + OH-(aq)
However, chemists realized that the hydrogen ion which is simply a proton (with a diameter of 3 X 10-15m) could not exist independently in aqueous solution. Studies showed that each proton associates with a water molecule to form the hydroxonium (or oxonium) ion, H3O+. The dissociation of an acid in water is now represented more accurately as follows:
HCl(aq) + H2O(l) ——> H3O+(aq) + Cl-(aq)
NOTE: Although hydrogen ions exist in aqueous solution as hydroxonium ions, we shall, for convenience, use H+ or H+(aq). In equations and using H3O+(aq) only when it is more appropriate.
With these refinements, the Arrhenius theory recognized the role of the ‘solvent’ water molecules in the dissociation of acids. It was able to explain why substances such as hydrogen chloride and ethanoic acid exhibit acidic properties only in the presence of water. Pure dry ethanoic acid and pure dry hydrogen chloride dissolved in dry methylbenzene do not affect litmus paper or react with metals to liberate hydrogen.
