Degree of ionization

There are two categories of chemical substances that dissolve in water: non-electrolytes and electrolytes. Non-electrolytes are polar substances that do not form ions in solutions or the molten state. Like glucose, benzene, and alcohol, they do not break into ions and do not conduct electricity.

Electrolytes are chemical compounds that create ions in a solution or a molten state. They dissociate into ions such as HCl, NaCl, and other ions, making them good conductors of electricity.

A strong electrolyte undergoes complete ionisation, whereas a weak electrolyte undergoes partial ionisation in an aqueous solution and remains partly in undissociated molecules. The fraction of neutral particles in a gas or aqueous ionised solution is referred to as the degree of ionisation.

Degree of Ionisation

The degree of ionisation (also known as dissociation) is a measurement of an acid’s potency. The number of ionised molecules to the number of molecules dissolved in water is defined as the ratio. It can be expressed as a decimal value or a percentage. Partially ionised (also weakly ionised) refers to a low degree of ionisation, while fully ionised refers to a high degree of ionisation. On the other hand, Fully ionised might also signify that an ion has lost all of its electrons. Strong acids have ionisation degrees greater than 30% at a given molar concentration, weak acids have less than 30%, and the rest are moderate acids. 

Or in other words, the ability of an acid or base to generate ions in a solution is defined by its degree of ionisation. Weak acids and bases ionise partially in solution, whereas strong acids and bases entirely ionise. A dynamic equilibrium between chemicals is achieved in a weak acid where the chemical equilibrium constant can be expressed.

Degree of Ionisation formula

At equilibrium, it’s important to know what fraction of the reactants’ initial amount is changed into products. The degree of dissociation/ionisation refers to the percentage of the starting molecules transformed at equilibrium.

The degree of ionisation formula is given by –

Degree of ionisation (𝝰)= (Number of reactant molecules ionised at the start)/(Number of reactant molecules at the start)

% Dissociation or ionisation (𝝰) = (Number of reactant molecules dissociated or ionised at the start)/(Number of reactant molecules at the start) × 100

Remember that in water, strong acids break down into ions.

HA(g or l) + H₂O(l)↔H3O+ (aq) + A– (aq) Kc>>1

In water, weak acids break down into ions to a small extent.

HA(aq) + H2O(l) ↔ H3O+ (aq) + A– (aq) Kc<<1

In this equation, Kc denotes the equilibrium constant, which can define the acid dissociation constant, Kₐ.

Kc = [H₃O⁺] [A⁻]/[H₂O] [HA] 

Kₐ = Kc [H₂O] = [H₃O⁺] [A⁻]/[HA] 

Factors affecting the degree of ionisation

• Higher the  solvent’s dielectric constant, the greater will be its ionising power.
• The value of the degree of ionisation for strong electrolytes is about one at normal dilution, while it is significantly less than 1 for weak electrolytes.
• The degree of ionisation is inversely proportional to the concentration of the solution and its weight, and it is proportional to the dilution of the solution and the amount of solvent.
• The degree of ionisation in a solution rises as the temperature rises.

Effect of Common Ions on Dissociation Degree

The common ion effect is a phenomenon in which a tiny amount of strong electrolyte containing a common ion suppresses the degree of dissociation of any weak electrolyte.

1. NH₄OH is a base with low strength. When ammonium ions from NH₄Cl (a salt) are added to the hydroxide, they join with the hydroxide to generate unionised ammonium hydroxide.

NH₄Cl → NH₄⁺ + Cl⁻

NH₄OH ⇌ NH₄⁺ + OH⁻

1. The sodium salt of the fatty acid (soap) is dissolved during the base hydrolysis of oil. The concentration of Na+ ions rises dramatically when sodium chloride salt is introduced. NaCl comprises two elements: sodium (Na+) and chlorine (Cl⁻).

NaCl ⇌ Na+ + Cl⁻

Degree of Ionisation of a Strong Acid

The amount of neutral particles that are ionised into charged particles, such as those in gas or aqueous solution, is referred to as the degree of ionisation. It should be regarded as an acid’s or a base’s ability to ionise electrolytes. According to the Arrhenius theory, acids dissociate in an aqueous media to produce hydrogen ions.

Weak acids have varying degrees of ionisation, but it is usually less than 10%. A 0.10 M solution of acetic acid is only around 1.3 per cent ionised, indicating that the equilibrium is quite favourable to the reactants. Weak acids, including fatty acids, ionise to produce the H+ ion and a conjugate base.

Weak Electrolyte Ionisation

All electrolytes are ionised in infinite dilution. Weak electrolytes exist in equilibrium with their unionised molecules in a concentrated solution. Ion concentrations are relevant in many practical scenarios, such as acid-base solubility and solution conductivity.

Conclusion

There are two categories of chemical substances that dissolve in water: non-electrolytes and electrolytes. A strong electrolyte undergoes complete ionisation, whereas a weak electrolyte undergoes partial ionisation in an aqueous solution and remains partly in undissociated molecules. The fraction of neutral particles in a gas or aqueous ionised solution is referred to as the degree of ionisation. The ability of an acid or base to generate ions in a solution is defined by its degree of ionisation. Weak acids and bases ionise partially in solution, whereas strong acids and bases entirely ionise. We have also learnt the various factors affecting the degree of ionisation.