Dissociation Reaction Definition and Examples

What It Means When a Compound Dissociates

antacid tablet in water
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A dissociation reaction is a chemical reaction in which a compound breaks apart into two or more parts.

The general formula for a dissociation reaction follows the form:

  • AB → A + B

Dissociation reactions are usually reversible chemical reactions. One way to recognize a dissociation reaction is when there is only one reactant but multiple products.

Key Takeaways

When writing out an equation, be sure to include the ionic charge if there is one. This is important. For example, K (metallic potassium) is very different from K+ (potassium ion).

Don't include water as a reactant when compounds dissociate into their ions while dissolving in water. While there are a few exceptions to this rule, for most situations you should use (aq) to indicate an aqueous solution.

Dissociation Reaction Examples

When you write a dissociation reaction in which a compound breaks into its component ions, you place charges above the ion symbols and balance the equation for both mass and charge. The reaction in which water breaks into hydrogen and hydroxide ions is a dissociation reaction. When a molecular compound undergoes dissociation into ions, the reaction may also be called ionization.

  • H2O → H+ + OH-

When acids undergo dissociation, they produce hydrogen ions. For example, consider the ionization of hydrochloric acid:

  • HCl → H+(aq) + Cl-(aq)

While some molecular compounds (such as water and acids) form electrolytic solutions, most dissociation reactions involve ionic compounds in water (aqueous solutions). When ionic compounds dissociate, water molecules break apart the ionic crystal. This occurs because of the attraction between the positive and negative ions in the crystal and the negative and positive polarity of water. In a written equation, you'll usually see the state of matter of the species listed in parentheses following the chemical formula: s for solid, l for liquid, g for gas, and aq for aqueous solution. Examples include:

  • NaCl(s) → Na+(aq) + Cl-(aq)
    Fe2(SO4)3(s) → 2Fe3+(aq) + 3SO42-(aq)