Arrhenius Theory of Acids and Bases

Science > Chemistry > Physical Chemistry > Ionic Equilibria > Arrhenius Theory

In this article, we shall study the Arrhenius theory of acids and bases, its advantages and limitations.

Classical or Functional Definitions of Acid and Base:

• Acid: An acid is defined as a substance whose water solution has a sour taste, turns blue litmus to red, can neutralize base and evolves hydrogen gas when treated with active metals like Zn, Mg, Na ., e.g. HCl, HNO₃, H₂SO₄, CH₃COOH
• Base: A base is defined as a substance whose water solution has a bitter taste, has soapy touch, turns red litmus to blue and can neutralize an acid e.g. NaOH, KOH, NH₄OH. The word alkali is used to water-soluble bases.

Need for Conceptual Definitions of Acids and Bases:

• The classical definitions of acids and bases were based on some observed properties of acids and bases.
• These definitions were unable to explain the structure responsible for their properties. Hence there was a need for conceptual definitions of acids and bases.

Arrhenius concept of Acids and Bases:

In 1887, Arrhenius, the Swedish chemist, proposed the theory of ionization to account for the properties of the aqueous solution of electrolytes. According to this concept,

• Acid: An acid is defined as a hydrogen-containing compound which gives hydrogen ions  (H⁺ ) in its aqueous solution.

HCl_(aq)     ⇌   H⁺_(aq)  +      Cl^–_(aq)

CH₃COOH_(aq)   ⇌  CH₃COO^–_(aq)    + H⁺_(aq)

In general, an equilibrium for all acids exists as,

HA_(aq)   ⇌     H⁺_(aq)  +    A^–_(aq)

• Base: A base is defined as a hydroxide compound which gives hydroxyl (OH^–) ions in its aqueous solution.

NaOH_(aq)     ⇌   Na⁺_(aq)  +      OH^–_(aq)

NH₄OH_(aq)     ⇌   NH₄⁺_(aq)  +      OH^–_(aq)

In general, an equilibrium for all bases exist as,

BOH_(aq)   ⇌     B⁺_(aq)  +    OH^–_(aq)

• Neutralization: Neutralization reaction is the reaction in which the acid and base react together to produce salt and water. Consider a reaction between strong acid like HCI and a strong base like NaOH.

HCl   +    NaOH    → NaCl  +  H 2 O

(Acid)    (base)         (salt)        (water)

By ionic (Arrhenius) theory, HCI, NaOH, and NaCI dissociate into their ions in an aqueous medium.

H⁺_(aq) + Cl^–_(aq)  + Na⁺_(aq)  + OH^–_(aq)   →  Na⁺_(aq) +  Cl^–_(aq) +   H₂O

Canceling the common ions of both sides, net equation is,

H⁺_(aq)   +    OH^–_(aq)    →        H₂O

Thus in neutralization, H⁺ ions of acid combine with OH^– ions of the base forming a unionized water molecule. Thus by Arrhenius theory, A process in which H+ ions of an acid combine with OH- ions of alkali to form unionized water molecule is called neutralization.

Notes:

• Properties of acid are due to properties of H⁺ ions present in the solution.
• Strong acids are highly ionized aqueous solution producing a large number of H⁺ ions or protons.
• Weak acids are very little ionized and produce a very small number of protons or H⁺ ions.
• Properties of bases are due to the presence of OH^– ions present in the solution.
• A strong base is highly ionized and gives a large number of OH^– ions.
• A weak base is very little ionized and gives very few OH^– ions.
• Bases which are highly soluble in water are known as alkalies.

Advantages of Arrhenius Theory:

Arrhenius concept is used to explain,

• acid-base properties of substances in an aqueous medium
• neutralization, hydrolysis and
• the strength of acids and bases.

Limitations of Arrhenius Theory:

• Acids and bases are defined in terms of their aqueous solution and not in terms of the substances themselves. Hence this theory is applicable to aqueous solutions only and not applicable to non-aqueous and gaseous reactions.
• It is applicable only to compounds having formula HA for acids or BOH for bases. Thus the theory is unable to explain acidic properties of CuSO₄, AlCl₃, CO₂, SO₂ as they cannot be represented by the formula HA. Similarly, the theory is unable to explain the basic properties of Na₂CO₃, amines, pyridine, NH₃ as they cannot be represented by the formula BOH.
• The theory does not consider the role of solvent in deciding the nature of acid and base. Thus HCl is strong acid when dissolved in water but it is weak acid when dissolved in benzene.
• This theory doesn’t explain the acidic property of HCl and basic property of NH3 in a nonaqueous medium like benzene, acetone or in the gaseous state.
• According to Arrhenius theory, proton (H+) exist free in aqueous solution.  However, in aqueous solution, H+ ion is always hydrated and exist as hydronium ion (H3O+).
• By Arrhenius theory neutralization process in which H+ ions of an acid combine with OH- ions of alkali to form unionized water. Thus the theory is unable to explain the neutralization reaction between HCl(g) and NH3(g)  not involving a combination of H⁺ and OH^– ions

HCl_(g) +    NH_3(g)   →     NH₄Cl_(g)

Science > Chemistry > Physical Chemistry > Ionic Equilibria > Arrhenius Theory