Classification Of Bases On The Basis Of Degree Of Ionisation

CLASSIFICATION OF BASES ON THE BASIS OF DEGREE OF IONIZATION

The bases are classified into two categories on the basis of degree of ionization as follows:

• Strong bases
• Weak bases

Strong base: A base contains one or more hydroxyl (OH) groups which it releases in aqueous solution upon ionization. Bases which are almost completely ionised in water, are known as strong bases.

e.g. Sodium hydroxide (NaOH), potassium hydroxide (OH) groups which it releases in aqueous solution upon ionization. Bases which are almost completely ionised in water, are known as strong bases.

NaOH(s) + Water → Na+ (aq) + OH– (aq)

KOH(s) + Water → K+ (aq) + OH– (aq)

Both NaOH and KOH are deliquescent in nature which means that they absorb moisture from air and get liquefied.

Single arrows pointing towards ions indicate complete ionization of bases.

Weak bases: Bases that are feebly ionised on dissolving in water and reduce a low concentration of hydroxyl ions are called weak bases.

eg. Ca(OH)₂, NH₄OH

(i)

Ammonium hydroxide Water Ammonium ion hydroxide ion

or

(ii)

calcium hydroxide Water calcium ion hydroxide ion

(lime water)

or

(iii)

magnesium hydroxide Water magnesium ion hydroxide ion

or

Conclusion:

Greater the degree of ionization, greater is the amount of OH– (aq)  ions produced in the solution and thus stronger is the base (or greater is the strength of base).

Smaller the degree of ionization, smaller is the amount of OH– (aq)  ions produced in the solution and thus weaker is the base (or lesser is the strength of base). Thus, strength of base is directly proportional to the degree of ionization.

Strength of base ∝ Degree of ionization.

DILUTION OF BASE: AN EXOTHERMIC REACTION

Like acids, dilution of bases with water or mixing of bases with water is an exothermic process e.g. if we dissolve bases like NaOH, KOH in water, the solution is found to be hotter. This shows that dissolution of bases in water is an exothermic process.

EFFECT OF DILUTION ON STRENGTH OF A BASE:

Like acids, on dilution of base with water, [OH– ] in the solution decrease and thus, solution becomes less basic (or strength of base decrease)

Important Note: Basic strength of a base is affected by two factors:

(i) Degree of ionization of a base i.e. strength of base ∝ degree of ionization. Greater the degree of ionization, greater will be [OH– ] in the solution and thus greater will be the strength of a base (or stronger will be a base).

Similarly, smaller the degree of ionization, smaller will be [OH– ] in the solution and thus, lesser will be the strength of the base (or weaker will be the base).

(ii) Dilution of a base: Strength of a base ∝ 1/dilution of a base. Greater the dilution of a base, lesser will be [OH^-] in the solution and thus, lesser will be the strength of the base (or weaker will be the base).

Similarly, smaller the dilution of a base, greater will [OH– ] in the solution and thus, greater will be the strength of the base (or stronger will be the base).

COMPARISON BETWEEN PROPERTIES OF ACIDS AND BASES: