Ionic Equilibrium

In Ionic equilibrium, the ionic substance dissociates into its ions in polar solvents, resulting in a dynamic equilibrium between the ions and the undissociated solute in the solution.

⇒ Representation of Ionic Equilibrium: X^a+ Y^b+ ⇌ aX^b+ + bY^a+

Introduction to Ionic Equilibrium

Reactants and products are in equilibrium, so that the conversion of reactants to products is always less than 100%. Equilibrium reactions may involve the decomposition of a covalent (non-polar) reactant or the ionization of ionic compounds into their ions in polar solvents.

In this section, we will learn about the ionic equilibrium in ionic solutions. Substances in Ionic Equilibrium can be classified into two categories based on their ability to conduct electricity:

Non-Electrolytes

Non-electrolytes are substances that consist of molecules that bear no electric charge, do not dissociate into their constituent ions and thus do not conduct electricity in their aqueous solution or molten state. For example, sugar solution.

Electrolytes

These substances are electrolytes, which are substances that dissociate into ions when dissolved in water or melted, allowing them to conduct electricity. Examples of electrolytes include salt solutions, acid solutions, and base solutions.

Electrolytes in ionic equilibrium can be further classified into strong and weak electrolytes.

Strong electrolytes are substances that completely ionize when dissolved in a solution, whereas weak electrolytes only partially dissociate into ions.

For example, NaCl undergoes complete ionization in its aqueous solution, resulting in sodium ions (Na+) and chloride (Cl–) ions. On the other hand, acetic acid undergoes partial ionization, producing some amount of acetate ions (CH3COO–) and hydrogen (H+) ions.

In the case of a strong electrolyte, the dissociation reaction is said to be complete, thus moving only in the forward direction, whereas, in the case of a weak electrolyte, the reaction is said to be reversible.

In the case of a weak electrolyte, ionic equilibrium is established between the ions and the unionized molecules. This concept can be illustrated with the following example.

⇒ Also Read:

Chemical Equilibrium

Equilibrium Constant

Le Chatelier’s Principles

Ostwald’s Dilution Law: Degree of Dissociation

Ostwald’s Dilution Law is the application of the Law of Mass Action to weak electrolytes in solution.

A binary electrolyte AB that dissociates into A+ and B– ions.

A+ + B– ⇌ AB

(i) For very weak electrolytes, since α \ll 1, (1 - α) = 1

(\begin{array}{l}\sqrt{KV} = C\alpha^{2}\end{array} )

(ii) Concentration of any ion, Cα = $\sqrt{CK} = \sqrt{\frac{K}{V}}$

The degree of ionization increases with dilution, meaning the degree of dissociation of a weak electrolyte is proportional to the square root of the dilution.

Limitations of Ostwald’s Dilution Law

The law is only applicable to weak electrolytes and is not valid for strong electrolytes.

Ionic Equilibrium Key Points Discussion - Part 1

Ionic Equilibrium

Ionic Equilibrium Key Points Discussion - Part 2

Ionic Equilibrium

Ionic Equilibrium Formulas

It becomes necessary to know what proportion of the initial amount of reactants is converted into products at equilibrium.

The degree of Dissociation/ionization is the fraction of the initial molecules that are converted at equilibrium.

The Degree of Dissociation or Ionization is equal to α, which is calculated by dividing the number of reactant molecules that have dissociated/ionized at the start by the number of reactant molecules at the start.

The percentage of dissociation of an ionic compound in equilibrium can be expressed as the degree of dissociation.

Degree of Dissociation or Ionization = $\alpha$ = $\frac{(Number\ of\ Reactant\ Molecules\ Dissociated\ or\ Ionized\ at\ the\ Start)}{(Number\ of\ Reactant\ Molecules\ at\ the\ Start)}$ $\times$ 100

Degree of Ionization

The degree of ionization is dependent on

Nature of the electrolyte: Strong, Weak, Insoluble

Nature of the Solvent: High dielectric solvents increase ionization.

Dilution: The higher the dilution, the greater the ionization.

Temperature: The higher the temperature, the greater the ionization.

The ionization of a weak electrolyte is decreased by the presence of common ions.

Dissociation of Ionic Compounds in Polar Solvents

Ionic compounds dissolve in polar solvents, ionizing into cations and anions.

The ionized ions are in equilibrium with the un-dissociated molecules.

A$\times$B $\leftrightarrow$ xA$\times$y + yB$\times$x

Ionic Solids Dissolved in Solutions

Strong electrolytes (α ≈ 100% ionization), Weak electrolytes (α ≈ 10% ionization), Sparingly soluble (α ≈ 100% ionization)

Answer: HCl, NaOH, Salts NH4OH, Organic acids, AgCl, BaSO4

Ionization of Weak Electrolytes

In infinite dilution, all electrolytes are fully ionized. In a concentrated solution, weak electrolytes exist in equilibrium with their unionized molecules. Concentrations of the ions are important in many practical situations like acid-base solubility, and conductance of the solution.

Common Ion Effect on Degree of Dissociation

Weak electrolytes are poorly ionized in an aqueous solution. Their ionization may further be reduced if one of the ions are present from another source. This is called a common ion effect.

i) On addition of ammonium chloride (a salt) to ammonium hydroxide (a weak base), the ammonium ions from the salt will combine with the hydroxide to form unionized ammonium hydroxide.

NH4Cl → NH4+ + Cl–

NH4OH <=> NH4+ + OH-

ii) The hydrolysis of oil results in the sodium salt of the fatty acid (soap) being in a dissolved state.

When sodium chloride salt is added, the concentration of Na+ ions significantly increases.

CnH2n+1 COONa ⇄ CnH2n+1 COO– Na+

NaCl <=> Na+ + Cl-

Therefore, the excess of [CnH2n+1COO–] [Na+] causes the solubility product of soap to be exceeded, resulting in the precipitation of soap from the solution. This is known as salting out of soap.

(iii) Manufacture of Sodium Bicarbonate (Baking Soda):

The NaHCO3 is precipitated out when the CO2 gas is passed through ammonical brine in Solvay’s soda process.

NH4OH + CO2 \rightarrow NH4HCO3

NH4HCO3 + NaCl → NaHCO3 + NH4Cl

NaHCO3 is precipitated first due to its lower solubility product compared to NH4Cl, NH3HCO3, and NaCl.

⇒ Check: This statement needs to be rewritten.

Physical Equilibrium

Dynamic Equilibrium