Dissociation (Chemistry) – Definition and Examples

In chemistry, dissociation is a chemical reaction in which a molecule or compound breaks into smaller pieces, such as ions, atoms, or radicals. For example, hydrochloric acid (HCl) dissociates in water, forming the H⁺ and Cl^– ions. Usually, dissociation is a reversible process. The reverse of dissociation is association or recombination.

Dissociation Formula

The general formula for a dissociation is reaction is:

AB ⇄ A + B

Note the reaction arrow, indicating the reaction is reversible. Most of the time, the reaction forms cations and anions:

AB ⇄ A⁺ + B^–

Breaking into more than two pieces occurs in some compounds.

Dissociation Reaction Examples

Here are examples of dissociation reactions:

• NaCl(s) ⇄ Na⁺(aq) + Cl^–(aq)
• HCl ⇄ H⁺(aq) + Cl^–(aq)
• H₂SO₄ ⇄ 2H⁺(aq) + SO₄^2-(aq)
• Fe₂(SO₄)₃(s) ⇄ 2Fe³⁺(aq) + 3SO₄^2-(aq)
• 2 H₂O ⇄ H₃O+ + OH^–

When a molecular dissociation forms ionic products, another name for the reaction is an ionization. By convention, water is not listed as a product for reactions in aqueous solution. Instead, the abbreviation (aq) follows the product formula.

Gases also undergo dissociation. For example, dinitrogen tetroxide (N₂O₄) dissociates in nitrogen dioxide (NO₂), forming an electrically neutral product:

N₂O₄ ⇄ 2 NO₂

Some compounds have options regarding dissociation, and can form a variety of products. For example azo-t-butane undergoes three possible dissociation reactions, forming ionized butene, the t-butyl cation plus a neutral radical, or a different cation plus a neutral t-butyl radical.

Dissociation Constant

If all of a compound dissociates, this is complete dissociation. Strong electrolytes undergo complete dissociation in water. Examples of strong electrolytes include strong acids, strong bases, and salts. However, incomplete dissociation is common. Incomplete dissociation results in mixture that contains some of the original compound, as well as some smaller particles. Incomplete dissociation occurs with weak electrolytes. Weak electrolytes include weak acids and bases and more organic compounds.

Solubility affects the amount of dissociated product. For example, some salts a nearly insoluble, yet are strong electrolytes. For example, silver chloride (AgCl) is a strong electrolyte, yet it is practically insoluble in water. A saturated AgCl solution only contains around 1.3 x 10^-5 M of Ag⁺ and Cl^– ions!

The degree of dissociation (α) is the fraction of a compound that breaks into its components. So, for example, complete dissociation has a degree of dissociation of 1. If half of a compound dissociation, the degree of dissociation is 0.5. If no dissociation occurs, the degree of dissociation is 0.

The van’t Hoff factor (i describes the relationship between the degree of dissociation (α) and the number of moles of particles formed (n):

i = 1 + α(n – 1)

For example, consider the dissociation of sodium chloride:

NaCl ⇄ Na⁺ + Cl^–

One mole of NaCl yields two moles of products (1 mole Na⁺ plus 1 mole Cl^–). So, n = 2.

i = 1 + α(n – 1) = 1 + α(2 – 1) = 1 + α
α = i – 1

Dissociation Constant

For a reversible dissociation at chemical equilibrium, the dissociation constant (K_d) is the ratio of dissociated compound too undissociated compound. The brackets indicate the equilibrium concentration of each chemical species.

AB ⇄ A + B

K_d = [A][B] / [AB]

References

• Atkins, P.; de Paula, J. (2006). Physical Chemistry (8th ed.). W.H. Freeman. ISBN 978-0-7167-8759-4.
• Laidler, K.J. (1978). Physical Chemistry with Biological Applications. Benjamin/Cummings. ISBN 978-0-8053-5680-9.
• Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry: Principles and Modern Applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. ISBN 978-0-13-014329-7.