Priyam Study Centre

A Man would do nothing, if he waited until he could do it so well that no one would find fault with what he has done.

Nov 5, 2018

Acids and Bases

Acids and Bases:

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.

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 is developed one after another tend to make the definitions more and more broad based.
We need to familiar with the following five concepts :
1. The Arrhenius Concept.
2. The Solvent System Concept.
3. The Protonic Concept.
4. The electronic Theory.

1. The Arrhenius Concept :

Arrhenius was one of the early exponent 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 where as bases when dissolved in water dissociate into hydroxyl ions and cations. Thus HCl is an 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.
Acid :   HCl H⁺ + Cl⁻ 
Base :   NaOH Na⁺ + OH⁻
Acid Base Equilibrium: H⁺ + OH⁻  H₂O

Acid base neutralization reaction in aqueous medium :

The Arrhenius concept was highly useful to explaining acid base reaction in aqueous medium.
Arrhenius concept of acid and base and Solvent System concept of Acids and Bases
Representation of Acid Base Neutralization Reaction
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 explain as follows.Let, 
HA(aq) + BOH(aq)  ⇆ BA(aq) + H₂O
Where, HA and BOH are strongly electrolytes and completely dissociate in the Solution. Hence, ( H⁺ + A⁻ ) + ( B⁺ + OH⁻ ) ⇆ ( B⁺ + A⁻ ) + H₂O (liquid )
Cancelling likes ions, we have H⁺ + OH⁻    H₂O (liquid)
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 same.

Limitation of Arrhenius Concept:

(i) 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.
(ii) It cannot account for the acidic and basic character of the materials in non aqueous solvents, as for example, NH₄NO₃, in liq NH₃ acts as an acid, through 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.
(iii) The neutralization process limited to those reactions which can occurs in aqueous solutions only, although the reactions involving salt formation do occur in many other solvents and even in the absence of solvents.
(iv) It cannot explain the acidic character of certain salts such as AlCl₃ in aqueous solution.

2. The Solvent system Concept:

The non- aqueous solvent molecules may also dissociate into two oppositely charged ions.
Examples:
(i)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 a 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 an acids and all those compounds which can give OH⁻ ions in H₂O will behave as bases.
(ii) Similarly liquid NH₃ can written as 
NH₃ + NH₃ NH₄⁺ + NH₂⁻
Thus those compounds which give NH₄⁺ ions in liquid NH₃ will act as an acids and all those compounds which can give NH₂⁻ ions in liquid NH₃ acts as bases.
Thus the dissociation (or autoionisation) of non-aqueous solvents is directly responsible for the nature of the chemical reactions that can be initiated in such solvents.
According to 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.
Arrhenius concept of acid and base and Solvent System concept of Acids and Bases
Acid-Base Neutralisation Reaction According to Solvent System Concept
The auto-ionisation of some protonic and non-protonic solvents are, Protonic Solvents:
H₂O(Acid) + H₂O(Base) H₃O⁺(Acid) + OH⁻(Base)
NH₃(Acid) + NH₃(Base)  NH₄⁺(Acid) + NH₂⁻(Base)
CH₃COOH(Acid) + CH₃COOH(Base)  CH₃COOH₂⁺(Acid) + CH₃COO⁻(Base)
Non-Protonic Solvents:
SO₂(Acid) + SO₂(Base)  SO⁺²(Acid) + SO₃⁻²(Base)
BrF₃(Acid) + BrF₃(Base)  BrF₂⁺(Acid) + BrF₄⁻(Base)
N₂O₄(Acid) + N₂O₄(Base)  2NO⁺(Acid) + 2NO₃⁻(Base)
Neutralization Reactions according to Solvent System Concept: 
Just as with the Arrhenius definition, neutralisation is a reaction between an acid and a base to produce salt and solvent. 
Neutralisation of some non-aqueous solvents are, 
(i) Neutralisation reaction in liquid NH₃: 
Dissociation of Solvent:    NH₃ + NH₃  NH₄⁺ + NH₂⁻
Dissociation of Acid:         NH₄Cl NH₄⁺ + Cl⁻ 
Dissociation of Base:         KNH₂  K⁺ + NH₂⁻
Acid Base Neutralization reaction: 
NH₄Cl(Acid) + KNH₂(Base)  KCl(Salt) + 2NH₃(Solvent)
(ii) Neutralization reaction in liquid SO₂:
Dissociation of Solvent:    SO₂SO₂  SO⁺² + SO₃⁻²
Dissociation of Acid:         SOBr₂ SO⁺² + 2Br⁻ 
Dissociation of Base:         K₂SO₃ 2K⁺ + SO₃⁻²
Acid Base Neutralization reaction:
SOBr₂(Acid) + K₂SO₃(Base) 2KBr(Salt) + 2SO₂(Solvent)
Arrhenius concept of acid and base and Solvent System concept of Acids and Bases
Neutralization Reactions according to Solvent System Concept

Utility of the Concept:

Evidently this concept of 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:
(i) This theory does not consider a number of acid-base reactions included in the protonic definition.
(ii) It limits acid-base phenomena to solvent system only. Thus it does not explain the acid base reactions which may occurs in the absence of solvent.
(iii) It can not explain the neutralisation reactions occurring without the presence of ions.