Properties of Acids and Bases

Properties of an acid and a base

Properties of an acid and a base some sense opposite. Acids are the substances whose aqueous solution is sour teste and turns blue litmus to red. But the properties bases are the substance whose aqueous solution is slippery to touch, bitter teste, changed the color of the litmus to blue and reacts with acids to form salts.

The properties of acids and bases are developed one after another tends to make the acid and base definitions to more broad-based.

To describe the properties of acids and bases we need to know about five theory

  1. Arrhenius acid and base theory
  2. Acids and bases in a solution
  3. Protonic concept
  4. Lewis concept
  5. The hard-soft acid-base theory
Properties of acids and bases solution
Acids and bases

Properties of Arrhenius acids and bases

Arrhenius define acids and bases in water as a solution. According to his classification, an acid when dissolved in water, dissociate hydrogen ions and anions but a base when dissolved in water dissociate into hydroxyl ions and cations.

Thus NaOH neutralizes HCl, but this acid-base neutralization reaction involving the combination of hydrogen and hydroxyl ions to form water.

HCl → H+ + Cl
NaOH → Na+ + OH
H+ + OH+ ⇆ H2O

Acid-base neutralization

Acids and bases reactions are highly useful in explaining the acid-base neutralization in an aqueous solution.

HA + BOH BA + H2O
Acid Base Salt   Water

where HA and BOH are strong acids and bases and completely dissociate in the solution.

H+ + A + B+ + OHB+ + A + H2O

Thus canceling by likes ions
H+ + OH ⇆ H2O

The heat of neutralization reaction

The neutralization reaction of all strong acids and bases used to the formation of 1-mole water from H+ ion and OH ion. Thus the thermodynamics enthalpy change (ΔH) will also be the same for all the reactions between strong acids and bases.

H+ + OH ⇆ H2O + 13.6 kcal

So heat release 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 or 56 KJ/mole.

Limitations of Arrhenius acid and base

  1. According to Arrhenius’s theory, HCl is an acid only when it dissolved in water. But if we use solvent such as benzene or when it exists in the gaseous state, it does not consider as an acid.
  2. This theory cannot account for the acidic and basic properties of the materials in non-aqueous solvents.
    For example, NH4NO3 in liquid ammonia is an acid, though it does not give H+ ions.
    Similarly, many organic compounds in NH3, which do not give OH ions at all, 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. This theory cannot explain the acidic character of certain salts such as AlCl3 in aqueous solution.

The acidic and basic solution

Protic solvent dissociates into two oppositely charged ions. If we consider water as the protic solvent, it dissociates H+ and OH ions.

H2O ⇆ H+ + OH

But H+ ion readily polarizes other anion or molecule which exists in the solution. So overall dissociation reaction of the solvent

H2O + H2O ⇆ H3O+ + OH

Thus according to solvent chemistry, 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.

Properties of acids and bases in liquid ammonia

NH3 dissociates into two oppositely charged ions which are NH4+ and NH2.

NH3 + NH3 ⇆ NH4+ + NH2

Thus those compounds which give NH4+ ions in liquid NH3 will act as acids but all those compounds which can give NH₂⁻ ions in liquid NH3 act as bases.

Definition of acid and base in a solvent

The dissociation of non-aqueous solvents directly responsible for the nature of the chemical reactions that can be initiated in such solvents. Thus according to the solvent system concept

  1. An acid is a substance which by dissociation in the solvent forms the same cation as does the solvent itself due to auto-ionization.
  2. A base is one that, gives on dissociation in the solvent the same anion as does the solvent itself on its ionization.

Protic and aprotic solvents examples

Solvent Autoionization of solvent
Protic Solvent H2O + H2OH3O+ + OH
NH3 + NH3NH4+ + NH2
Aprotic Solvent SO2 + SO2SO+ + SO3
BrF3 + BrF3BrF2+ + BrF4
N2O4 + N2O42NO+ + 2NO3

Examples of acid and base neutralization

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
Solvent Acid and base reaction
NH3 NH4Cl + KNH2 ⇆ KCl + NH3
SO2 SOBr2 + K2SO3 ⇆ 2KBr + 2SO2
NOCl NOCl + AgNO3 ⇆ AgCl + N2O4
H2O HBr + KOH ⇆ KBr + H2O

Limitations of acid and base theory

  1. The acidic and basic solution does not consider a number of acid-base reactions included in the protonic definition.
  2. Acids and bases phenomena in solution explain the reaction in the presence of a solvent. Thus it does not explain the acid and base reactions that may occur in the absence of solvent.
  3. It can not explain acid-base neutralization properties, occurring without the presence of ions.