Limitations of the Arrhenius Concept of Acids & Bases

Limitations of the Arrhenius Concept of Acids & Bases

Despite its successes, the Arrhenius concept/definition of acids and bases proved to be inadequate for several reasons. 

•   It is and was recognized that acid-base reactions also take place in solvents other than water. But the Arrhenius concept is not applicable for non-aqueous solutions,i.e., it cannot explain the properties of acids and beses in abscence of water.

• Compounds which do not contain hydrogen can release H⁺ ion by reacting with water. For instance, 

• Some bases do not contain OH^- ions. For example, NH₃, CaO etc.

• It is also believed that a H⁺ ion cannot exist in free state in any solvent because of its high charge density. A hydrogen ion would combine with one or more molecules of the solvent. In the case of water as solvent H⁺ ion would combine with water molecule to form what is known as a hydronium/hydroxonium/oxonium ion(H₃O⁺).

These facts together with the studies on the catalysis by acids and bases suggested that the ideas of Arrhenius were inadequate and had to be modified.

Arrhenius Concept of Acids & Bases

    Swedish scientist Svante Arrhenius first explained the cause of acidity and basicity in 1884, with his concept known as the ‘Arrhenius Concept of Acids and Bases’.

The Arrhenius concept of acids and bases can be stated as follows:

     "An acid is a hydrogen containing substance which yields H⁺ (i.e. hydronium ion,H₃O⁺) or increases the concentration of H₃O⁺ (aq.) when dissolved in water."

Example:

     HCl is an acid according to the Arrhenius concept, because it produces or yields H₃O⁺ ion in water.

   According to the Arhenius concept, 

     "A base is a substance which contains hydroxide(OH^-) ion and produces/yields OH^- ion dissolved in water."

 Example:

      NaOH is a base according to the Arrhenius concept of acids and bases, because it produces OH^- ion in water.

Carbonyl Compounds

Carbon-Oxygen Double Bond

   Carbonyl compounds, that is, aldehydes and ketones, are classes of organic compounds which contain a carbon atom doubly bonded to oxygen.

   The functional group of carbonyl compounds is the carbonyl group, which determines the chemistry of these compounds and this is why aldehydes and ketones are collectively called carbonyl compounds.

•  In aldehydes, the carbonyl carbon is linked to one hydrogen atom and one alkyl group(Formaldehyde, in which -CO is joined to two H atoms being an exception).

• On the other hand, in ketones, the -CO group is linked to two alkyl groups.

   The carbonyl group is made up of a sigma bond and a pi-bond. It is polar due to oxygen's greater electronegativity.