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12.6 Dissociation

Degree of dissociation

It is a fraction of the total number of moles of an acid or base or electrolyte that dissociates into ions in an aqueous solution when equilibrium is reached. It is represented by a

Thus, greater the degree of dissociation (a) stronger the acids or bases and vice versa.

Dissociation constants

Strong acids and strong bases dissociate in a solution almost completely. Hence the dissociation studies are important only with respect to weak acids and weak bases.

Dissociation of weak acids : Consider a weak acid HA which dissociates in water as

HA + H2O    H3O+ + A-

For simplicity it can be written as HA H+ + A-

Applying the law of mass action

Where Ka is known as acid ionization constant (or acid constant). Higher the value of Ka greater the degree of dissociation.

\ Acid with high Ka value are relatively stronger than acid with lower Ka value.


Dissociation of weak bases : Consider a weak base BOH which dissociates as

BOH    B+ + OH-

Applying the law of mass action

Where Kb is known as base ionization constant (or base constant). Higher the value of Kb greater the degree of dissociation.

\A base with high Kb value is relatively stronger than a base with lower Kb value.

An acid or base that has Ka or Kb values greater than 10 are considered to be strong acids or bases. Values between 10- 2 to 10- 3 are regarded as moderately strong, while those between 10- 4 to 10- 7 as weak and those with values less than that are regarded very weak acids or bases.


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Index

12.1 - Lowry and Bronsted Concept
12.2 - Conjugate Acid Base Pairs
12.3 - Amphoteric Substance
12.4 - Lewis Acids and Bases
12.5 - Strong and Weak Acids and Bases
12.6 - Dissociation
12.7 - Ostwald's Dilution Law
12.8 - Hydrogen Ion Concentration : pH
12.9 - Polyprotic Acids
12.10 - Salts
12.11 - Methods of Preparation of Salts
12.12 - Properties of Salts

Chapter 13





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