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Dissociation constants of acids and Bases

(1) Dissociation constant for weak acid: Consider an acid HA which, when dissolved in water ionizes as, 

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

Applying the law of mass action, 
Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

Where, Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced is the dissociation constant of the acid, Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced. It has constant value at definite temperature and does not change with the change of concentration.

Dissociation Constant for polybasic acid: Polybasic acids ionise stepwise as, for example, orthophosphoric acid ionises in three steps and each step has its own ionisation constant.

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced ⇌ Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced (I step)

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced ⇌ Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced (II step)

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced ⇌ Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced (III step)

Let Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced and Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced be the ionization constants of first, second and third steps respectively. Thus,

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

In general, Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

The overall dissociation constantIonization of Polybasic Acids | Chemistry for JEE Main & Advanced is given by the relation,

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

(2) Dissociation constant for weak base: The equilibrium of Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced (a weak base) can be represented as,

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced ⇌ Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

Applying the law of mass action,

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced

Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced is constant at a definite temperature and does not change with the change of concentration.

The document Ionization of Polybasic Acids | Chemistry for JEE Main & Advanced is a part of the JEE Course Chemistry for JEE Main & Advanced.
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FAQs on Ionization of Polybasic Acids - Chemistry for JEE Main & Advanced

1. What is the definition of ionization of polybasic acids?
Ans. Ionization of polybasic acids refers to the process in which a polybasic acid releases multiple hydrogen ions (H+) when dissolved in water, forming corresponding polyvalent anions.
2. How does the ionization of polybasic acids differ from monobasic acids?
Ans. The key difference between the ionization of polybasic acids and monobasic acids lies in the number of hydrogen ions released. Polybasic acids, as the name suggests, have multiple ionizable hydrogen atoms and can release more than one H+ ion per acid molecule, whereas monobasic acids release only one H+ ion per acid molecule.
3. What factors affect the degree of ionization of polybasic acids?
Ans. Several factors influence the degree of ionization of polybasic acids. These include the concentration of the polybasic acid, the strength of the acid, temperature, and the presence of other substances that may react with the polybasic acid.
4. Can you provide an example of a polybasic acid and its ionization reaction?
Ans. Yes, an example of a polybasic acid is phosphoric acid (H3PO4). When dissolved in water, it undergoes ionization as follows: H3PO4(aq) ⇌ H+(aq) + H2PO4^-(aq) H2PO4^-(aq) ⇌ H+(aq) + HPO4^2-(aq) HPO4^2-(aq) ⇌ H+(aq) + PO4^3-(aq) In this example, phosphoric acid releases three hydrogen ions successively, forming three corresponding polyatomic anions.
5. How does the degree of ionization of polybasic acids affect their acidity?
Ans. The degree of ionization of polybasic acids directly affects their acidity. Higher degrees of ionization correspond to stronger acids as they release more hydrogen ions into the solution, resulting in a higher concentration of H+ ions. Therefore, a polybasic acid with a higher degree of ionization will have a greater acidity compared to one with a lower degree of ionization.
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