Question

Define "dissociation" as it applies to ionic compounds that dissolve in water. Why don't strong bases "ionize"?

Short answer

Dissociation is the separation of an ionic compound into its individual ions in water. Strong bases do not 'ionize' because they are already ionic compounds and thus simply dissociate into their constituent ions.

Step-by-step solution

Understanding 'Dissociation'

Dissociation refers to the process in which an ionic compound separates into its individual ions when it dissolves in water. This occurs because the polar water molecules are attracted to the charged ions, pulling them apart from the crystalline structure of the compound.

Differentiating between 'Dissociation' and 'Ionization'

Ionization usually refers to the formation of ions from molecules that are not ionic compounds, like the ionization of acids in water. However, strong bases are composed of ionic compounds that are already made up of ions. When they dissolve in water, they dissociate, rather than ionizing. Strong bases release their hydroxide ions (OH-) into the solution directly upon dissociation.

Clarifying Why Strong Bases Do Not Ionize

Strong bases are typically made from metal hydroxides such as NaOH, which are already composed of sodium ions (Na+) and hydroxide ions (OH-). Since these ions are already present in the structure of the solid, they merely separate from each other in water, hence the process is called 'dissociation' rather than 'ionization'.

Key concepts

Ionic Compounds

Ionic compounds play a fundamental role in chemistry and are essential to understanding various chemical processes. These compounds are formed by the combination of positively charged ions, or cations, and negatively charged ions, or anions. The electrostatic attraction between these oppositely charged ions results in a stable ionic lattice structure. Common examples include table salt (sodium chloride, NaCl) and calcium fluoride (CaF₂).

When ionic compounds dissolve in water, the polar nature of water molecules interacts with the ions, essentially 'pulling' them apart in a process known as dissociation. This is because water molecules have a slightly positive part that is attracted to anions and a slightly negative part that is attracted to cations. The outcome is a solution containing free-moving ions, which can conduct electricity, a characteristic property of ionic solutions.

Ionization vs Dissociation

Ionization and dissociation are often confused, but they refer to distinct processes in chemistry. Ionization is the process by which neutral molecules gain or lose electrons to form ions. This commonly occurs in the ionization of acids or non-ionic compounds where a molecule, upon reaction with water, splits to produce ions that were not originally present.

On the other hand, dissociation is the separation of ions that already exist within a compound. Ionic compounds when dissolved in water undergo dissociation, not ionization. The ions are already present in the solid state and become 'liberated' when the compound dissolves. The term dissociation is specifically chosen to avoid the implication that new ions are being created, which is the essence of ionization.

Example: Salt in Water

When common table salt (NaCl) dissolves in water, it dissociates into sodium (Na⁺) and chloride (Cl^-) ions but does not ionize because no new ions are created.

Strong Bases

Strong bases are a group of substances known for their ability to fully dissociate in aqueous solutions, completely releasing their hydroxide ions (OH^-). Examples of strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂). They are characterized by the presence of ionic compounds, where the metal cations and hydroxide anions are already inherently present within their crystalline structure.

Therefore, when strong bases dissolve in water, they simply dissociate rather than ionize. The term 'ionization' is not used because the ions do not originate from a neutral molecule but from the disassembly of the pre-existing ionic lattice of the solid base. This results in a solution where the hydroxide ions are readily available to react, classifying them as strong bases due to the complete dissociation and high pH of the resultant solution.