Acids and Bases

What are the acids?

    Any of the class of substances whose aqueous solutions are characterized by the sour taste, the ability to turns blue litmus red, and the ability to react with bases and certain metals to form salts.

What are the bases?

    In chemistry, a base is a substance that in aqueous solution is slippery to touch, that's bitter, changed the color of the indicator, (turns red litmus to blue) react with acid to form salts.
    The concepts of acid and base are developed one after another tend to make the definitions more and more broad-based. We need to familiar with the following five concepts :
  1. Arrhenius Concept.
  2. Solvent System Concept.
  3. Protonic Concept.
  4. Electronic Theory.

Arrhenius concept

    Arrhenius was one of the early exponents of electrolytic dissociation theory to define acids and bases. His classification of acids and bases was based on the theory that, acids, when dissolved in water, dissociate hydrogen ions and anions whereas bases when dissolved in water dissociate into hydroxyl ions and cations.
    Thus HCl is acid and NaOH is a base and the neutralization process can be represented by a reaction involving the combination of H⁺ and OH⁻ ions to form H₂O.
HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
H⁺ + OH⁻ ⇆ H₂O

Acids and bases neutralization reaction

    The Arrhenius concept was highly useful in explaining the acid-base reaction in an aqueous medium.
    The heat liberated during neutralization of strong acid (HCl, HClO₄, HNO₃, HBr, HI, and H₂SO₄) and strong base (NaOH, KOH, RbOH, and Ca(OH)₂) is the same, namely 13.4 kcal/mole (56 KJ/mole) indicating that the ion participating in the reaction must be the same (that is H⁺ and OH⁻) and that other ions (Na⁺, K⁺, Rb⁺, Ca⁺², Cl⁻, Br⁻, I⁻, ClO₄⁻ or SO₄⁻²) take no part. This can be explained as follows.
HA(aq) + BOH(aq) ⇆ BA(aq) + H₂O
    Where HA and BOH are strong electrolytes and completely dissociate in the Solution.
(H⁺+A⁻)+( B⁺+OH⁻)⇆(B⁺+A⁻)+ H₂O
Canceling likes ions, we have,
H⁺ + OH⁻ ⇆ H₂O
    Since neutralization of all strong acids and bases reduced to the formation of 1-mole water from H⁺ ion and OH⁻ ion. The enthalpy change (ΔH) will also be the same.

Limitation of Arrhenius concept

  1. According to this concept, HCl is regarded as an acid only when dissolved in water and not in some other solvent such as benzene or when it exists in the gaseous state.
  2. It cannot account for the acidic and basic character of the materials in non-aqueous solvents, as for example, NH₄NO₃, in liquid NH₃ acts as an acid, though it does not give H⁺ ions. Similarly, many organic materials in NH₃, which does not give OH⁻ ions at all, are actually known to show basic character.
  3. The neutralization process limited to those reactions which can occur in aqueous solutions only, although the reactions involving salt formation do occur in many other solvents and even in the absence of solvents.
  4. It cannot explain the acidic character of certain salts such as AlCl₃ in aqueous solution.

Solvent system concept

    The non- aqueous solvent molecules may also dissociate into two oppositely charged ions. We consider the solvent H₂O, its characteristic cation and anion are H⁺ and OH⁻ respectively are,
H₂O ⇆ H⁺ + OH⁻
    Since we know that a bare proton will readily polarize other anions or molecules we write an H⁺ as H₃O⁺ indicating that it is a solvated proton that exists in the solution. So that the overall dissociation of solvent will be:
H₂O + H₂O ⇆ H₃O⁺ + OH⁻
    Thus all those compounds which can give H₃O⁺ ions in H₂O will act as acids and all those compounds which can give OH⁻ ions in H₂O will behave as bases.
NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻
    Thus those compounds which give NH₄⁺ ions in liquid NH₃ will act as acids and all those compounds which can give NH₂⁻ ions in liquid NH₃ acts as bases.
    Thus the dissociation (or autoionization) of non-aqueous solvents is directly responsible for the nature of the chemical reactions that can be initiated in such solvents. According to the solvent system concept,
    An acid is a substance which by dissociation in the solvent forms the same cation as does the solvent itself due to auto-ionization.
    A base is one that, gives on dissociation in the solvent the same anion as does the solvent itself on its ionization.

Auto-ionization of solvents

    Auto-ionization of some protonic and non-protonic solvents are given below

Protonic solvent

H₂O + H₂O H₃O⁺ + OH⁻
Acid Base Acid Base
NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻
CH₃COOH + CH₃COOH ⇆ CH₃COOH₂⁺ + CH₃COO⁻

Non-protonic solvents

SO₂ + SO₂ SO⁺² + SO₃⁻²
Acid Base Acid Base
BrF₃ + BrF₃ ⇆ BrF₂⁺ + BrF₄⁻
N₂O₄ + N₂O₄ ⇆ 2NO⁺ + 2NO₃⁻

Neutralization reactions

    Just as with the Arrhenius definition, neutralization is a reaction between an acid and a base to produce salt and solvent. Neutralization of some non-aqueous solvents are,

Neutralization reaction in liquid NH₃

Dissociation of solvent
NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻

Dissociation of acid
NH₄Cl ⇆ NH₄⁺ + Cl⁻

Dissociation of base
KNH₂ ⇆ K⁺ + NH₂⁻

Thus the acid-base neutralization reaction is
NH₄Cl + KNH₂ KCl + 2NH₃
Acid Base Salt Solvent

Neutralization reaction in liquid SO₂

Dissociation of solvent
SO₂ + SO₂ ⇆ SO⁺² + SO₃⁻²

Dissociation of acid
SOBr₂ ⇆ SO⁺² + 2Br⁻

Dissociation of base
K₂SO₃ ⇆ 2K⁺ + SO₃⁻²

Thus the acid-base neutralization reaction
SOBr₂ + K₂SO₃ 2KBr + 2SO₂
Acid Base Salt Solvent
Solvent System concept of Acids and Bases
Solvent System Concept

Utility of the solvent system concept

    Evidently, this concept of the solvent system can be used to explain the acid-base reactions occurring in aqueous and non-aqueous solvents(protonic or non-protonic both). Thus this theory can simply be said to be an extension of the Arrhenius Theory.

Limitations of the solvent system concept

  1. This theory does not consider a number of acid-base reactions included in the protonic definition.
  2. It limits acid-base phenomena to the solvent system only. Thus it does not explain the acid-base reactions which may occur in the absence of solvent.
  3. It can not explain the neutralization reactions occurring without the presence of ions.

Arrhenius and Solvent System concept of Acids and Bases with appropriate examples and Utility of these Concept with related problems and its solutions

Inorganic Chemistry

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