Acids and bases properties

A+ A-

Definition of acids and bases

Acids and bases posses in a sense some opposite properties. Any of the classes of substances whose aqueous solutions are characterized by the sour taste, the ability to turns blue litmus, and the ability to react with bases and certain metals to form salts.

In chemistry, a base is a substance that in aqueous solution is slippery to touch, that's bitter, changed the color of the indicator, (litmus to blue) reacts with an 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. Lewis concept

Acid-base neutralization reaction

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.
Bases when dissolved in water dissociate into hydroxyl ions and cations.

Thus sodium hydroxide neutralizes hydrochloric acid can be represented by a reaction involving the combination of hydrogen and hydroxyl ions to form water.

HCl → H⁺ + Cl⁻

NaOH → Na⁺ + OH⁻

H⁺ + OH⁻ ⇆ H₂O

What is an example of an acid-base neutralization reaction?

Arrhenius concept of acids and bases was highly useful in explaining the acid-base neutralization process in an aqueous medium.

The heat liberated during neutralization 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)
This 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 the neutralization reaction of all strong acids and bases used to the formation of 1-mole water from H⁺ ion and OH⁻ ion. The enthalpy change (ΔH) will also be the same.

Limitations of Arrhenius acid and base

  1. According to this concept, HCl is 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 ammonia is 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 of acids and bases

Protic solvent molecules may also dissociate into two oppositely charged ions. We consider protic solvent water, its characteristic cation and anion are H⁺ and OH⁻.

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 act 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.

Autoionization of solvents

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

Protic solvent examples

H₂O + H₂O H₃O⁺ + OH⁻
Acid Base Acid Base

NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻

CH₃COOH + CH₃COOH ⇆ CH₃COOH₂⁺ + CH₃COO⁻

Aprotic solvent examples

SO₂ + SO₂ SO⁺² + SO₃⁻²
Acid Base Acid Base

BrF₃ + BrF₃ ⇆ BrF₂⁺ + BrF₄⁻

N₂O₄ + N₂O₄ ⇆ 2NO⁺ + 2NO₃⁻

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

Neutralize process for liquid ammonia

Solvent: NH₃ + NH₃ ⇆ NH₄⁺ + NH₂⁻

Acid: NH₄Cl ⇆ NH₄⁺ + Cl⁻

Base: KNH₂ ⇆ K⁺ + NH₂⁻

Acid-base neutralization reaction

NH₄Cl + KNH₂ KCl + 2NH₃
Acid Base Salt Solvent

Autoionization of SO₂

Solvent: SO₂ + SO₂ ⇆ SO⁺² + SO₃⁻²

Acid: SOBr₂ ⇆ SO⁺² + 2Br⁻

Base: K₂SO₃ ⇆ 2K⁺ + SO₃⁻²

Acid-base neutralization reaction
SOBr₂ + K₂SO₃ 2KBr + 2SO₂
Acid Base Salt Solvent
Online college chemistry courses
Acid base neutralization reaction example
Acid-base neutralization

Uses of solvent system theory

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

Limitations of solvent system theory

  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 acid-base neutralization reactions occurring without the presence of ions.

Acids and bases properties, and definition for online college chemistry courses, neutralization reaction examples, autoionization of liquid ammonia

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