Chemistry: Ionic Compounds: Henderson – Hassel Balch Equation and Solubility Equilibrium (For CBSE, ICSE, IAS, NET, NRA 2022)

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Henderson – Hassel Balch Equation

  • The Henderson – Hassel Balch equation relates the pH of a given buffer solution to the concentrations of its components which can be a weak acid/salt or weak base/salt.
  • In acetic acid – sodium acetate buffer the central equilibrium is CH3COO- (aq) + H2O H3O + (aq) + CH3COO- (aq)

it is characterized by the acid dissociation constant,

rearranging them .

The concentration of undissociated acetic acid can be taken as total acid concentration and that of sodium acetate as the total salt concentration. It is written as

Taking logarithm and multiplying throughout by (-1) we get, -log [H + ] =-log Ka – log

We know that pH =- log [H3O + ] and pKa =-log Ka.

This gives the desired equation which is known as Henderson-Hasselbalch equation pH = pKa – log = pKa + log

Salt Hydrolysis

  • The aqueous solutions of certain salts behave as acids or bases because of the hydrolysis of the cation or anion or both.
  • The hydrolysis reaction here is with water.
  • Depending on the behaviors towards hydrolysis there are four different types of salts they are:
    • Salt of strong acid + strong base (e. g. HCl + NaOH) NaCl
    • Salt of strong acid + weak base (e. g. HCl + NH4OH) NH4Cl
    • Salt of weak acid + strong base (e. g. CH3COOH + NaOH) CH3COONa
    • Salt of weak Acid + weak Base (e. g. CH3COOH + NH4OH) CH3COONH4

Salt of Strong Acid + Strong Base

The cations and anions of the strong bases and acids do not get hydrolyzed. Therefore, the salts of this category do not show any acid-base behaviors and are neutral.

Salt of Strong Acid + Weak Base

The salts of this type dissociate in aqueous solutions to give a cation of a weak base and the anion belonging to strong acid.

Salt of Weak Acid and Strong Base

The salts of this type dissociate in aqueous solutions to give an anion of a weak acid and the cation belonging to strong base.

Salt of Weak Acid and Weak Base

The salts of this type dissociate in aqueous solutions to give an anion of a weak acid and the cation belonging to a weak base.

Solubility Equilibrium

When we dissolve a solid into water, there are way for three possibilities:

Solubility Equilibrium
  • The solid is a non-electrolyte and it dissolves as neutral molecules.
  • The solid is a highly soluble electrolyte; it dissolves almost completely.
  • The solid is a sparingly soluble electrolyte; it dissolves to a limited extent.
  • Let us take the example of dissolution of AgCl to understand the equilibria. When silver chloride is added to water, the following equilibrium is established.

AgCl Ag + (aq) + Cl- (aq) . It is a heterogenous equilibrium because it involves both a solid and solution and it is called as solubility equilibrium.

K = [Ag + ] [Cl-] ÷ [AgCl] = Ksp = [Ag + ] [Cl-] . It is the solubility product constant or solubility product.

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